NaCl and the structure of Salts

Chemical Structures

Ionic compounds are formed due to the electrostatic attraction between oppositely charged ion. Ionic compounds consist of

• A metal (an element on the lower left side of the periodic table), which loses one or more electrons to become a positively charged ion (cation) and
• A non-metal (an element on the upper right side of the periodic table), which gains one or more electrons to become a negatively charged ion (anion).

The charge of many elements can be predicted based on their position on the periodic table.

Ionic compounds differ from molecular compounds in several important ways. For example,

• Ionic compounds typically have higher melting points.
• Ionic compounds are typically found as crystalline materials.
• Ionic compounds often dissolve in water.
• Solutions of ionic compounds in water conduct electricity.
• Ionic compounds do not form discrete molecules (see below).

Table Salt, NaCl

Many of the most important features of ionic compounds can be illustrated using NaCl as an example. This compound contains the metal sodium (Na) and the non-metal chlorine (Cl). In ionic compounds, the charges of Na and Cl can be determined by referring to the periodic table (see above), giving:

• Sodium loses one electron to become Na⁺¹
• Chlorine gains one electron to become Ca^-1

Atoms are incredibly small. In order for a compound to exist as a stable solid, the total charge must be zero. (Otherwise, the incredible number of charges would give rise to a huge electrostatic repulsive force, which would tear the material apart). In sodium chloride, the +1 charge on the sodium ion is exactly balanced by the -1 charge on the chloride.

The simplest picture of NaCl is a single pair of Na⁺¹ and Cl^-1 ions as shown below. However, this diagram suggests that each Na ion is only attached to or surrounded by a single chloride (and vice versa), which is not correct.

A more realistic picture of NaCl can be drawn be imagining that a sodium ion is placed on a flat surface. This ion can then be surrounded by four (4) chloride ions. In turn, each of these chloride ions, is surrounded by four sodium ions. This process can be repeated in two-dimensions as long as ions remain. A small sample of this could look like is shown in the two-dimensional drawing shown below.

However, molecules are not really 'flat' but instead exist in three-dimensions. In ions are placed above and below the layers shown above, a 'three-dimensional' image can be drawn. This image reflects that crystalline structure of sodium chloride.

Note that each layer is 'offset' so that Na⁺¹ ions are not placed on top of each other, but are placed over (or below) Cl^-1 ions. 

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