The lattice is formed because the ions attract each other and form a regular pattern with oppositely charged ions next to each other. Remember that the lattice arrangement is giant - for example, a single grain of salt may contain 1. The lattice arrangement continues in three dimensions. This is why solid ionic compounds form crystals with regular shapes.
An ionic lattice is held together by strong electrostatic forces of attraction between the oppositely charged ions. See the study guide on the three states of matter to see how bonding and structure are related to the properties of substances. Energy must be transferred to a substance to make it melt or boil.
This energy overcomes the strong electrostatic forces of attraction which act in all directions between the oppositely charged ions:. The more energy needed, the higher the melting point or boiling point.
Since the electrostatic forces of attraction between oppositely charged ions are strong, their melting and boiling points are high. Ionic compounds are held together by electrostatic forces between the oppositely charged ions. These forces are usually referred to as ionic bonding. On a macroscopic scale, ionic compounds, such as sodium chloride NaCl , form a crystalline lattice and are solids at normal temperatures and pressures.
The charge on the cations and anions is determined by the number of electrons required to achieve stable noble gas electronic configurations.
The ionic composition is then defined by the requirement that the resulting compound be electrically neutral overall. For example, to combine magnesium Mg and bromine Br to get an ionic compound, we first note the electronic configurations of these atoms valence level in indicated in italics :.
In order to achieve noble gas configurations, the magnesium atom needs to lose its two valence electrons, while the bromine atom, which has 7 valence electrons, requires one additional electron to fill its outer shell. Therefore, for the resulting compound to be neutral, two bromine anions must combine with one magnesium cation to form magnesium bromide MgBr 2.
In addition, though any ratio of 2 bromine atoms to 1 magnesium atom will satisfy the two requirements above, the formula for ionic compounds is typically presented as the empirical formula, or the simplest whole-number ratio of atoms with positive integers. Note that the cation always precedes the anion both in written form and in formulas.
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