Nuclear Chemistry

Electrochemistry

Oxidation and Reductions

Acid / Base Chemistry

Chemical Equilibria

Reaction Kinetics

Thermodynamics

Solution Chemistry

Interparticle Forces and Phase Changes

Gases

Bonding and Molecular Geometry

The Elements

Periodic Trends

Atomic Theory

Stoichiometry

Mole Calculations

Chemical Reactions and Equations

Chemical Nomenclature

Binary Ionic Compounds (compounds consisting of two different ions)

Binary ionic compounds are formed between a metal and a nonmetal.  Metallic elements tend to lose electrons (forming positive ions) in order to become isoelectronic with (to have the same number of electrons as) a noble gas.  Nonmetals tend to gain electrons (forming negative ions) to become isoelectronic with a noble gas.  The charge on an ion is also referred to as the oxidation number of that element when it is in that ionic form.

1.The oxidation number of the nonmetallic element in a binary ionic compound is always equal to the number of electrons that it needs to gain to become isoelectronic with a noble gas.  For example, an oxygen atom needs 2 electrons to become isoelectronic with neon, so an oxide ion always has a charge of -2.  A nitride ion always has a charge of -3.  Chloride is always -1.

2.The oxidation number for metals is not always as simple.  Some metals are monovalent, meaning that they tend to form only one type of ion.  For example, the oxidation number of alkali metals (Li, Na, K, etc.) is always +1 in a compound.  The oxidation number of alkaline earth metals (Be, Mg, Ca, etc.) is always +2 in a compound.  Silver ions always have a +1 charge.  Zinc ions always have a +2 charge.  Aluminum ions always have a +3 charge.

3.Most other metals can have more than one oxidation number in a compound.  There are 5 metals, in particular, that usually have one of two different oxidation numbers when they are part of a compound.  These are bivalent metals:
Bivalent Metal     Oxidation Numbers
   Cu+1 or +2
      Hg+1 or +2
   Fe+2 or +3
   Sn+2 or +4
   Pb+2 or +4

Matter

Do you need some examples of polymers? Here is a list of materials that are polymers, plus some examples of materials that are not polymers.

Chemical, Physical, and Nuclear Changes Quiz

Do you understand the differences between the types of changes and the concepts associated with these reactions? Here's a ten question quiz you can take to test your knowledge.

Fluid Mixing - Miscibility

Find out why fluid volumes aren't always additive. Your Guide's introduction to miscibility includes links for further reading.

What Is the Difference between a Molecule and a Compound?

Molecules and compounds are two types of chemical species. Here is a look at the difference between a molecule and a compound.

Molecular Solid

This is an explanation of what a molecular solid is and examples of molecular solids.

What Is a Mixture?

You may have heard the term mixture used in reference to chemistry or cooking. Let's take a look at what a mixture is.

What Is Glass?

Here is an explanation of what glass is with respect to phases of matter.

Is Glass a Liquid or a Solid?

You may have heard different explanations about whether glass should be classified as a solid or as a liquid. Here is a look at the modern answer to this question and the explanation behind it.

Physical & Chemical Properties of Matter - Video

This chemistry video examines the physical and chemical properties of matter. Learn what the physical and chemical properties are, how to tell them apart, and get examples of each type of property.

Is Dissolving Sugar in Water a Chemical or Physical Change?

Is dissolving sugar in water an example of a chemical or physical change? This process is a little trickier to understand than most, but if you look at the definition of chemical and physical changes, you'll see how it works. Here's the answer and an explanation of the process.

Examples of Physical Changes and Chemical Changes

Here are some examples of physical changes and chemical changes, along with an explanation of how you can tell physical and chemical changes apart.

Is Dissolving Salt in Water a Chemical Change or Physical Change?

Is dissolving salt in water an example of a chemical change or a physical change? Here's a look at the process that occurs when you dissolve salt in water and an explanation of the answer to the question.

Chemical Properties

Here is an explanation of what chemical properties are, plus several examples of chemical properties of matter.

Physical Properties

Here is an explanation of what physical properties are and examples of several physical properties.

Physical Properties List

This is an extensive list of physical properties of matter. It's especially useful if you need to cite examples of physical properties.

What Is Plasma?

Here's a look at what plasma is, what plasma is used for and what plasma is made of. Examples of plasma also are provided.

What Are Examples of Pure Substances?

A pure substance is a material that is homogeneous and has constant properties throughout the sample. Here are examples of pure substances.

Types of Solids

There are 5 main types of solids, each characterized by specific properties and structures. Here is a look at the 5 types of solids.

Is Air Made of Matter?

You can't see or smell air, so you may be wondering whether or not it is made of matter. Here is the answer, plus how you can tell whether it is matter.

10 Chemical Change Change Examples

This is a list of 10 examples of chemical changes.

10 Physical Change Examples

This is a list of 10 examples of physical changes.

Physical and Chemical Changes Quiz

Practice identifying physical and chemical changes with this chemistry quiz.

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Chemistry Basics

Molecules may be represented in two dimensions by a structural diagram:

Ethylene		Bromo-Chloro-Fluoro-Iodo-Methane

While this representation allows for the derivation of the three dimensional structure of the left molecule, the right molecule requires further description.
Molecules may be represented in three dimensions by a ball and stick configuration like this. This allows examination of the molecule's configuration, in this case an sp3 hybridized central carbon with 4 different atoms attached:

X axis	Y axis	Z axis

Another method used to describe atoms and their bonding is through the location of the electrons in their outermost shell. Because there is no exact location of where the electron may be found, probability distributions (commonly called orbitals) based on quantum mechanical calculations are sketched showing where the electron is most likely to be found. A few of these representations are presented here to illustrate this approach. The first three types of orbitals for individual atoms are:

s		p		d

The p and d orbitals actually change according to how many electrons are in the outer shell. A full p orbital is customarily indicated as shown below:

When two or more atoms are bounded to form a molecule, the orbitals from each atom may be combined into hybrid atomic orbitals, which are useful in describing molecular structural properties such as geometry. A few examples are illustrated below. The hybrid sp combination yields the linear geometry of CO₂; sp² describes the trigonal planar geometry of BF₃; and the sp³ combination fits the familiar tetrahedral geometry of CH₄.

sp		sp2		sp3

Hybrid atomic orbitals are also useful in describing double bond formation. We illustrate this for the case of ethylene, which will be used in the polymer synthesis section as an example of free radical formation in connection with addition polymerization. After the sp² orbitals with the hydrogen are occupied, two electrons in carbon 2s states interact to form a sigma bond along the C-C centerline and the single electron remaining in each of the unhybridized 2p orbitals interact to form a pi bond (a "combined" p orbital appearance). For a more complete discussion of these topics see any recent college chemistry text.

Chirality

The ability of carbon to form four bonds allows for the possibility of two configurations using the same molecular formula. Particularly, a configuration about the central carbon and its mirror image is possible. The mirror image is not superimposible upon the original configuration; thus it represents a new form of the molecule:

Original molecule	Mirror image

Such molecules are said to be chiral, and the central atom known as the chiral center. One of the configurations about the chiral center is designated S and the other R. The pair of molecule configurations due to chirality are known as enantiomers. Enantiomers have identical chemical properties except towards optically active reagents, and identical physical properties except for the direction of rotation of the plane of polarized light. A mixture of equal amounts of both enantiomers is known as a racemic mixture and is not optically active. It is possible for a molecule to contain two chiral centers and thus four possible configurations may result. This may result in chiral molecules which are not mirror images of each other. Such molecules are known as diastereomers, and have similar chemical properties but may have vastly different physical properties.

Geometric Isomers

cis		trans
I		II

Examination of the two molecules above reveals they share the same molecular formula (BrCHCHBr) as well as the location of the double bond. Thus they are isomers; both represent 2-ethylene but their physical properties differ. The difference between the two isomers is hinted by their two dimensional representation but becomes more clear when the examined in three dimensions.
The difference lies in the way the atoms are oriented in space. They are not mirror images of each other, and thus they are termed diastereomers. However, because the rotation about the carbon carbon double bond is hindered (since it would break the pi bond), they are classified as geometric isomers. The two configurations are named by inserting the prefix "cis" (Latin: on this side) for I and "trans" (Latin: across) for II which indicate the location of the bromo groups each on the same or opposite sides of the molecule. Thus, the proper name for I is cis-1,2-dibromo-ethene and II trans-1,2-dibromo-ethene.

Nomenclature

Naming large molecules can be a complex task. The IUPAC Nomenclature Committee has established conventions that eliminate confusion. In general, a polymer with an unspecified number of monomers is named by adding the prefix "poly" to the constitutional repeating unit (CRU). Otherwise, the Greek prefix corresponding to the number of monomers is added to the name of the CRU.

#	Prefix
1	Meth
2	Eth
3	Prop
4	But
5	Pent
6	Hex
7	Hept
8	Oct
9	Non
10	Dec
11	Undec

The name of the CRU is formed by naming each of the subunits. If a polymer contains only one subunit, the prefix "poly" is added to the name of the subunit. For example, if a polymer is formed from a combination of ethylene monomers, its name would be polyethylene. Polymers containing more than one subunit are named based on the largest subunits, and are set off from the prefix "poly" with parentheses.
The naming of the subunits is the most difficult part. The subunits are named by the main constituent of the subunit with smaller molecules added on as prefixes. The best way to demonstrate this nomenclature is to use examples. For example, the monomer:

would be called oxy(1-fluoroethylene). To start, we see that there is an oxygen atom attached to a larger subunit. Therefore, the name will have the prefix oxy-. The rest of the monomer can be seen to be an ethylene molecule with a fluorine atom attached. Since the fluorine atom is attached to the first carbon atom, the name of this part of the monomer is 1-fluoroethylene. Thus the name of the complete monomer is oxy(1-fluoroethylene), and the name of the polymer formed from this monomer would be poly[oxy(1-fluoroethylene)].
For monomers with branched chains, the monomer is named for the longest continuous chain. For example:

The molecule above is named for pentane, since the longest continuous chain is five carbons long. However, there are also two extra methyl groups attached to the 2nd and 3rd carbons in the chain. Therefore, this monomer is called 2,3 dimethylpentene, and the name of the polymer formed from these monomers is called poly(2,3 dimethylpentane). It may be tempting to label this 3,4 dimethylpentane, but convention mandates that you use the name with the smallest numbers possible

chemistry lecture

The pages on this website are the chemistry lecture notes, including charts and diagrams, that I have developed over the past several years for teaching chemistry.  I have published these notes to provide chemistry help for high school and  college chemistry students.  These chemistry lecture notes include the entire curriculum that is normally covered in high school chemistry and the first two semesters of college chemistry.  I hope you will use these notes for chemistry help and for developing a better understanding of general chemistry concepts.  They will show you how to solve every type of problem that you are likely to see on any high school chemistry or college chemistry test or exam as well as on the SAT II Subject Test in Chemistry and the College Board AP Chemistry Exam.  To get chemistry help in any subject area of chemistry, just click on one of the buttons on the left, and then click through to each of the screens for that subject area.  The screens are ordered in the sequence that I cover the material in my classes.

In addition to the chemistry help provided on this website, if you want to get great chemistry help for developing a solid understanding of chemistry, and for achieving higher exam scores, I also strongly encourage you to use ChemTutor Excalibur (from Interactive Learning, Inc.) for chemistry help.  ChemTutor Excalibur is an awesome chemistry tutorial software package that makes chemistry easier and more fun to learn, and it provides excellent chemistry help.  The tutorials in ChemTutor Excalibur teach chemistry using a unique approach that students appreciate and enjoy.  The tutorials are concise and to-the-point, yet they contain everything that a chemistry student needs to know.  Combined with the chemistry help provided on this website, ChemTutor Excalibur provides all the chemistry help any high school or college student needs to excel in chemistry.

What Is Chemistry way it is important

Chemistry is the study of matter and energy and the interactions between them. This is also the definition for physics, by the way. Chemistry and physics are specializations of physical science. Chemistry tends to focus on the properties of substances and the interactions between different types of matter, particularly reactions that involve electrons. Physics tends to focus more on the nuclear part of the atom, as well as the subatomic realm. Really, they are two sides of the same coin.
The formal definition of chemistry is probably what you want to use if you're asked this question on a test.
Why Study Chemistry?
Because understanding chemistry helps you to understand the world around you. Cooking is chemistry. Everything you can touch or taste or smell is a chemical. When you study chemistry, you come to understand a bit about how things work. Chemistry isn't secret knowledge, useless to anyone but a scientist. It's the explanation for everyday things, like why laundry detergent works better in hot water or how baking soda works or why not all pain relievers work equally well on a headache. If you know some chemistry, you can make educated choices about everyday products that you use.
What Fields of Study Use Chemistry?
You could use chemistry in most fields, but it's commonly seen in the sciences and in medicine. Chemists, physicists, biologists, and engineers study chemistry. Doctors, nurses, dentists, pharmacists, physical therapists, and veterinarians all take chemistry courses. Science teachers study chemistry. Fire fighters and people who make fireworks learn about chemistry. So do truck drivers, plumbers, artists, hairdressers, chefs... the list is extensive.
What Do Chemists Do?
Whatever they want. Some chemists work in a lab, in a research environment, asking questions and testing hypotheses with experiments. Other chemists may work on a computer developing theories or models or predicting reactions. Some chemists do field work. Others contribute advice on chemistry for projects. Some chemists write. Some chemists teach. The career options are extensive.
Where Can I Get Help With a Chemistry Science Fair Project?
There are several sources for help. A good starting point is the Science Fair Index on this website. Another excellent resource is your local library. Also, do a search for a topic that interests you using a search engine, such as Google.
Where Can I Find Our More About Chemistry?
Start with the Chemistry 101 Topic Index or list of Questions Chemistry Students Ask. Check out your local library. Ask people about the chemistry involved in their jobs.
Where Can I Get Answers to Chemistry Questions?
The Chemistry Forum is a great place for quick answers from scientists, teachers, students, and other people interested in chemistry.

10 Basic Chemistry Facts

This is a collection of 10 fun and interesting basic chemistry facts.

1. Chemistry is the study of matter and energy and the interactions between them. It is a physical science that is closely related to physics, which often shares the same definition.
2. Chemistry traces its roots back to the ancient study of alchemy. Chemistry and alchemy are separate now, though alchemy still is practiced today.
3. All matter is made up of the chemical elements, which are distinguished from each other by the numbers of protons they possess.
4. The chemical elements are organized in order of increasing atomic number into the periodic table. The first element in the periodic table is hydrogen.
5. Each element in the periodic table has a one or two letter symbol. The only letter in the English alphabet not used on the periodic table is J. The letter q only appears in the symbol for the placeholder name for element 114, ununquadium, which has the symbol Uuq. When element 114 is officially discovered, it will be given a new name.
6. At room temperature, there are only two liquid elements. These are bromine and mercury.
7. The IUPAC name for water, H₂O, is dihydrogen monoxide.
8. Most elements are metals and most metals are silver-colored or gray. The only non-silver metals are gold and copper.
9. The discoverer of an element may give it a name. There are elements named for people (Mendelevium, Einsteinium), places (Californium, Americium) and other things.
10. Although you may consider gold to be rare, there is enough gold in the Earth's crust to cover the land surface of the planet knee-deep.

What is Mole

Question: What Is a Mole and Why Are Moles Used?

Answer: A mole is simply a unit of measurement. Units are invented when existing units are inadequate. Chemical reactions often take place at levels where using grams wouldn't make sense, yet using absolute numbers of atoms/molecules/ions would be confusing, too. Like all units, a mole has to be based on something reproducible. A mole is the quantity of anything that has the same number of particles found in 12.000 grams of carbon-12. That number of particles is Avogadro's Number, which is roughly 6.02x10²³. A mole of carbon atoms is 6.02x10²³ carbon atoms. A mole of chemistry teachers is 6.02x10²³ chemistry teachers. It's a lot easier to write the word 'mole' than to write '6.02x10²³' anytime you want to refer to a large number of things! Basically, that's why this particular unit was invented.
Why don't we simply stick with units like grams (and nanograms and kilograms, etc.)? The answer is that moles give us a consistent method to convert between atoms/molecules and grams. It's simply a convenient unit to use when performing calculations. Okay... you may not find it too convenient when you are first learning how to use it, but once you become familiar with it, a mole will be as normal a unit as, say, a dozen or a byte

Model of the Atom

All matter consists of particles called atoms. This is a list of the basic characteristics of atoms:

• Atoms cannot be divided using chemicals. They do consist of parts, which include protons, neutrons, and electrons, but an atom is a basic chemical building block of matter.
• Each electron has a negative electrical charge.
• Each proton has a positive electrical charge. The charge of a proton and an electron are equal in magnitude, yet opposite in sign. Electrons and protons are electrically attracted to each other.
• Each neutron is electrically neutral. In other words, neutrons do not have a charge and are not electrically attracted to either electrons or protons.
• Protons and neutrons are about the same size as each other and are much larger than electrons. The mass of a proton is essentially the same as that of a neutron. The mass of a proton is 1840 times greater than the mass of an electron.
• The nucleus of an atom contains protons and neutrons. The nucleus carries a positive electrical charge.
• Electrons move around outside the nucleus.
• Almost all of the mass of an atom is in its nucleus; almost all of the volume of an atom is occupied by electrons.
• The number of protons (also known as its atomic number) determines the element. Varying the number of neutrons results in isotopes. Varying the number of electrons results in ions. Isotopes and ions of an atom with a constant number of protons are all variations of a single element.
• The particles within an atom are bound together by powerful forces. In general, electrons are easier to add or remove from an atom than a proton or neutron. Chemical reactions largely involve atoms or groups of atoms and the interactions between their electrons.

Does the atomic theory make sense to you? If so, here's a quiz you can take to test your understanding of the concepts.

What is an atom

Question: What Is an Atom?

The building blocks of matter are called atoms. Yet you may be wondering what, exactly, is an atom? Here's a look at what an atom is and some examples of atoms.

Answer: An atom is the basic unit of an element. An atom is a form of matter which may not be further broken down using any chemical means. A typical atom consists of protons, neutrons and electrons.

Examples of Atoms

Any element listed on the periodic table consists of atoms. Hydrogen, helium, oxygen and uranium are examples of types of atoms.

What Are Not Atoms?

Some matter is either smaller or larger than an atom. Examples of chemical species that are not typically considered atoms includes particles that are components of atoms: protons, neutrons and electrons. Molecules and compounds consists of atoms but are not themselves atoms. Examples of molecules and compounds include salt (NaCl), water (H₂O) and ethanol (CH₂OH). Electrically charged atoms are called ions. They are still types of atoms. Monoatomic ions include H⁺ and O^2-. There are also molecular ions, which are not atoms (e.g., ozone, O₃^-).

The Gray Area

Would you consider a single unit of hydrogen to be an example of an atom? Keep in mind, most hydrogen "atoms" do not have a proton, neutron and electron. Given that the number of protons determines the identity of an element, many scientists consider a single proton to be an atom of the element hydrogen.

Intraduction to the Atoms

All matter consists of particles called atoms. This is a list of the basic characteristics of atoms:

• Atoms cannot be divided using chemicals. They do consist of parts, which include protons, neutrons, and electrons, but an atom is a basic chemical building block of matter.
• Each electron has a negative electrical charge.
• Each proton has a positive electrical charge. The charge of a proton and an electron are equal in magnitude, yet opposite in sign. Electrons and protons are electrically attracted to each other.
• Each neutron is electrically neutral. In other words, neutrons do not have a charge and are not electrically attracted to either electrons or protons.
• Protons and neutrons are about the same size as each other and are much larger than electrons. The mass of a proton is essentially the same as that of a neutron. The mass of a proton is 1840 times greater than the mass of an electron.
• The nucleus of an atom contains protons and neutrons. The nucleus carries a positive electrical charge.
• Electrons move around outside the nucleus.
• Almost all of the mass of an atom is in its nucleus; almost all of the volume of an atom is occupied by electrons.
• The number of protons (also known as its atomic number) determines the element. Varying the number of neutrons results in isotopes. Varying the number of electrons results in ions. Isotopes and ions of an atom with a constant number of protons are all variations of a single element.
• The particles within an atom are bound together by powerful forces. In general, electrons are easier to add or remove from an atom than a proton or neutron. Chemical reactions largely involve atoms or groups of atoms and the interactions between their electrons.

Does the atomic theory make sense to you? If so, here's a quiz you can take to test your understanding of the concepts.

What Is Chemistry and way study chemistry

Chemistry is the study of matter and energy and the interactions between them. This is also the definition for physics, by the way. Chemistry and physics are specializations of physical science. Chemistry tends to focus on the properties of substances and the interactions between different types of matter, particularly reactions that involve electrons. Physics tends to focus more on the nuclear part of the atom, as well as the subatomic realm. Really, they are two sides of the same coin.
The formal definition of chemistry is probably what you want to use if you're asked this question on a test.
Because understanding chemistry helps you to understand the world around you. Cooking is chemistry. Everything you can touch or taste or smell is a chemical. When you study chemistry, you come to understand a bit about how things work. Chemistry isn't secret knowledge, useless to anyone but a scientist. It's the explanation for everyday things, like why laundry detergent works better in hot water or how baking soda works or why not all pain relievers work equally well on a headache. If you know some chemistry, you can make educated choices about everyday products that you use.
What Fields of Study Use Chemistry?
You could use chemistry in most fields, but it's commonly seen in the sciences and in medicine. Chemists, physicists, biologists, and engineers study chemistry. Doctors, nurses, dentists, pharmacists, physical therapists, and veterinarians all take chemistry courses. Science teachers study chemistry. Fire fighters and people who make fireworks learn about chemistry. So do truck drivers, plumbers, artists, hairdressers, chefs... the list is extensive.
What Do Chemists Do?
Whatever they want. Some chemists work in a lab, in a research environment, asking questions and testing hypotheses with experiments. Other chemists may work on a computer developing theories or models or predicting reactions. Some chemists do field work. Others contribute advice on chemistry for projects. Some chemists write. Some chemists teach. The career options are extensive.
Where Can I Get Help With a Chemistry Science Fair Project?
There are several sources for help. A good starting point is the Science Fair Index on this website. Another excellent resource is your local library. Also, do a search for a topic that interests you using a search engine, such as Google

Atomic structure

Introduction to the Periodic Table

Scientists use the Periodic Table in order to find out important information about various elements. Created by Dmitri Mendeleev (1834-1907), the periodic table orders all known elements in accordance to their similarities. When Mendeleev began grouping elements, he noticed the Law of Chemical Periodicity. This law states, "the properties of the elements are periodic functions of atomic number." The periodic table is a chart that categorizes elements by "groups" and "periods." All elements are ordered by their atomic number. The atomic number is the number of protons per atom. In a neutral atom, the number of electrons equals the number of protons. The periodic table represents neutral atoms. The atomic number is typically located above the element symbol. Beneath the element symbol is the atomic mass. Atomic mass is measured in Atomic Mass Units where 1 amu = (1/12) mass of carbon measured in grams. The atomic mass number is equal to the number of protons plus neutrons, which provides the average weight of all isotopes of any given element. This number is typically found beneath the element symbol. Atoms with the same atomic number, but different mass numbers are called isotopes. Below is a diagram of a typical cells on the periodic table.
There are two main classifications in the periodic table, "groups" and "periods." Groups are the vertical columns that include elements with similar chemical and physical properties. Periods are the horizontal rows. Going from left to right on the periodic table, you will find metals, then metalloids, and finally nonmetals. The 4th, 5th, and 6th periods are called the transition metals. These elements are all metals and can be found pure in nature. They are known for their beauty and durability. The transition metals include two periods known as the lanthanides and the actinides, which are located at the very bottom of the periodic table. The chart below gives a brief description of each group in the periodic table.

Group 1A	Known as Alkali Metals Very reactive Never found free in nature React readily with water
Group 2A	Known as Alkaline earth elements All are metals Occur only in compounds React with oxygen in the general formula EO (where O is oxygen and E is Group 2A element)
Group 3A	Metalloids Includes Aluminum (the most abundant metal in the earth) Forms oxygen compounds with a X2O3 formula
Group 4A	Includes metals and nonmetals Go from nonmetals at the top of the column to metals at the bottom All oxygen form compounds with a XO2 formula
Group 5A	All elements form an oxygen or sulfur compound with E2O3 or E2S3 formulas
Group 6A	Includes oxygen, one of the most abundant elements. Generally, oxygen compound formulas within this group are EO2 and EO3
Group 7A	Elements combine violently with alkali metals to form salts Called halogens, which mean "salt forming" Are all highly reactive
Group 8A	Least reactive group All elements are gases Not very abundant on earth Given the name noble gas because they are not very reactive

Charges in the Atom

The charges in the atom are crucial in understanding how the atom works. An electron has a negative charge, a proton has a positive charge and a neutron has no charge. Electrons and protons have the same magnitude of charge. Like charges repel, so protons repel one another as do electrons. Opposite charges attract which causes the electrons to be attracted to the protons. As the electrons and protons grow farther apart, the forces they exert on each other decrease.

Atomic Models and the Quantum Numbers

There are different models of the structure of the atom. One of the first models was created by Niels Bohr, a Danish physicist. He proposed a model in which electrons circle the nucleus in "orbits" around the nucleus, much in the same way as planets orbit the sun. Each orbit represents an energy level which can be determined using equations generated by Planck and others discussed in more detail below. The Bohr model was later proven to be incorrect, but provides a useful model for building an explanation. The "accepted" model is the quantum model. In the quantum model, we state that the electron cannot be found precisely, but we can predict the probability, or likelihood, of an electron being at some location in the atom. You should be familiar with quantum numbers, a series of three numbers used to describe the location of some object (like an electron) in three-dimensional space:

1. n: the principal quantum number, an integer value (1, 2, 3...) that is used to describe the quantum level, or shell, in which an electron resides. The principal quantum number is the primary number used to determine the amount of energy in an atom. Using one of the first important equations in atomic structure (developed by Niels Bohr), we can calculate the amount of energy in an atom with an electron at some value of n:

   En = -

   Rhc n2

   where:
   R = Rydberg constant, a value of 1.097 X 10⁷ m^-1
   c = speed of light, 3.00 X 10⁸ m/s
   h = Planck's constant, 6.63 X 10 ^-34 J-s
   n = principal quantum number, no unit For example, how much energy does one electron with a principal quantum number of n= 2 have?
   
   

   En = - Rhc n2

   or

   En = - (1.097x107 m-1 ∗ (6.63x10-34 J•s)∗(3.0x108 m•s-1) 22

   = 5.5x10-19 J

   You might ask, well, who cares? In addition to the importance of knowing how much energy is in an atom (a very important characteristic!), we can also derive, or calculate, other information from this energy value. For example, can we see this energy? The table below suggests that we can. For example, suppose that an electron starts at the n=3 level (we'll call this the excited state) and it falls down to n=1 (the ground state). We can calculate the change in energy using the equation:
   
   

   ΔE = hv = RH

   1 ni2

   -

   1 nf2

   Where:
   ΔE = change in energy (Joules)
   h = Planck's constant with a value of 6.63 x 10^-34 (J-s)
   ν is frequency (s^-1)
   R_H is the Rydberg constant with a value of 2.18 x 10^-18J.
   n_i is the initial quantum number
   n_f is the final quantum number
   Using the equation below, we can calculate the wavelength and the frequency of the energy. The wavelength and the frequency give us information about how we might "see" the energy:
   
   

   vλ = c

   Where:
   ν = the frequency of radiation (s^-1)
   λ = the wavelength (m)
   c = the speed of light with a value of 3.00 x 10⁸ m/s in a vacuum
   
   

   Speed of light =	3.00E+08
   Rydberg constant =	2.18E-18
   Planck's constant =	6.63E-34
   Excited state, n =	3	4	5
   Ground state, n =	2	2	2
   Excited state energy (J)	2.42222E-19	1.363E-19	8.72E-20
   Ground state energy (J)	5.45E-19	5.45E-19	5.45E-19
   ΔE =	-3.02778E-19	-4.09E-19	-4.58E-19
   ν =	4.56678E+14	6.165E+14	6.905E+14
   λ(nm) =	656.92	486.61	434.47

2. l ("el", not the number 1): the azimuthal quantum number, a number that specifies a sublevel, or subshell, in an orbital. The value of the azimuthal quantum number is always one less than the principal quantum number n. For example, if n=1, then "el"=0. If n=3, then l can have three values: 0,1, and 2. The values of l are typically not identified as "0, 1, 2, and 3" but are more commonly called by their historic names, "s, p, d, and f", respectively. Since the quantum numbers were discovered through the study of light and lines on an electromagnetic spectra, chemists identified the lines by their quality: sharp, principal, diffuse and fundamental. The table below shows the relationship:
   

   Value of l	Subshell designation
   0	s
   1	p
   2	d
   3	f

3. m: the magnetic quantum number. Each subshell is composed of one or more orbitals. In the study of light, it was discovered that additional lines appeared in the spectra produced when light was emitted in a magnetic field. The magnetic quantum number has values between -l and +l. When l =1, for example, m can have three values: -1, 0, and +1. Because you know from the chart above that the subshell designation for l =1 is "p", you now know that the p orbital has three components. In your study of chemistry, you will be presented with p_x, p_y, and p_z. Notice how the subscripts are related to a three-dimensional coordinate system, x, y, and z. The chart below shows a summary of the quantum numbers:
   

   Principal Quantum Number (n)	Azimuthal Quantum Number (l)	Subshell Designation	Magnetic Quantum Number (m)	Number of orbitals in subshell
   1	0	1s	0	1
   2	0 1	2s 2p	0 -1 0 +1	1 3
   3	0 1 2	3s 3p 3d	0 -1 0 +1 -2 -1 0 +1 +2	1 3 5
   4	0 1 2 3	4s 4p 4d 4f	0 -1 0 +1 -2 -1 0 +1 +2 -3 -2 -1 0 +1 +2 +3	1 3 5 7

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Chemists care about where electrons are in an atom or a molecule. In the early models, we believed that electrons move like billiard balls, and followed the rules of classical physics. The graphic below attempts to show that earlier models thought that we could identify the exact path, position, velocity, etc. of an electron or electrons in an atom:
A more accurate picture is that the electron(s) reside in a "cloud" that surrounds the nucleus of the atom. This concept is shown in the graphic below:

Chemists are interested in predicting the probability that the electron will be at some particular part of this cloud. The cloud is better known as an orbital, and comes in several different types, or shapes. Atomic orbitals are known as s, p, d, and f orbitals. Each type of atomic orbital has certain characteristics, such as shape. For example, as the graphic below shows, an s orbital is spherical in shape:

On this graph, the horizontal (x) axis represents the distance from the nucleus in units of a₀, or atomic units. The value of a₀ is 0.0529 nanometers (nm). The vertical (y) axis represents the probability density. What you should notice is that as the electron moves farther away from the nucleus, the probability of its being found at that distance decreases. In other words, the electron prefers to hang around close to the nucleus.
The three graphics below show some other orbitals. The first graph (top left) is of a "2s" orbital. Each "s" orbital can hold two electrons in its cloud. Notice how there is a relatively high probability of an electron being near the nucleus, then some space where the probability is close to zero, then the probability increases substantially at some distance from the nucleus. The graphic at the top right shows a "2p" atomic orbital. Orbitals that are "p" orbitals can hold up to six (6) electrons in their cloud. Notice its "dumbbell" or "figure of eight" shape. At the bottom left is a "3s" orbital. Again, notice its spherical shape. Finally, at the bottom right, is a "3p" orbital.

Determining Electron Configuration

One of the skills you will need to learn to succeed in freshman chemistry is being able to determine the electron configuration of an atom. An electron configuration is basically an account of how many electrons there are, and in what orbitals they reside under "normal" conditions. For example, the element hydrogen (H) has one electron. We know this because its atomic number is one (1), and the atomic number tells you the number of electrons. Where does this electron go? The one electron of hydrogen goes into the lowest energy state it possibly can, which means it will start at "level" one and goes into "s" orbitals first. We say that hydrogen has a "[1s¹]" electron configuration. Looking at the next element on the Periodic Table --helium, or He -- we see it has an atomic number of two, so two electrons. Since " s" orbitals can hold up to two electrons, helium has an electron configuration of "[1s²]". What about larger atoms? Let's look at carbon, with an atomic number of 6. Where do its 6 electrons go?

• First two: 1s²
• Next two: 2s²
• Last two: 2p²

We can therefore say that carbon has the electron configuration of "[1s²2s²2p²]".
The table below shows the subshells, the number of orbitals, and the maximum number of electrons allowed:

Subshell	Number of Orbitals	Maximum Number of Electrons
s	1	2
p	3	6
d	5	10
f	7	14

The Abridged (shortened) Periodic Table below shows the electron configurations of the elements. Notice for space reasons we sometimes leave off a portion of the electron configuration. For example, look at argon (Ar), element 18. The table below shows its electron configuration as "[3s²3p⁶]" (remembering that "p" orbitals can hold up to six (6) electrons). Its actual electron configuration is:
Ar = [1s²2s²2p⁶3s²3p⁶] Sometimes you will see the notation: "[Ne]3s²3p⁶", which means to include everything that is in neon (Ne, 10) plus the stuff in the "3"-level orbitals.

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