Ion is an atom or group of atoms carrying a positive or negative electric charge resulting in loss or gain of one or more electrons. Positively charged ions are cations and negatively charged ones are anions. Positively charged ions and negatively charged ions attract each other through the electrostatic forces of attraction that exist between them. The forces of attraction that bind these oppositely charged ions are called ionic bonds. Ionic substances are the compounds made up of electrically neutral groups of ions that are joined by electrostatic forces. An ionic compound must have an equal number of cations and anions. A good example of anionic compoundis sodium chloride in which sodium has a single valence electron that can be easily lost in order to attain stability. On the other hand, chlorine requires one electron for it to become stable. Therefore, when sodium and chlorine react to form an ionic compound, sodium will donate its valence electron to a chlorineatom. In other words, sodium and chlorine atoms combine in a one-to-one ratio hence both ions have stable electron configurations.
Electron Transfer in NaCl
In the illustration above, the ionic compound NaCl formed has the lowest whole number ratio of cation Na+ and anion Cl- as seen through the chemical formula. Similarly, the formation of aluminium bromide can be illustrated as shown below. Aluminium has 3 valence electrons while bromine has 7 valence electrons. Each bromine atom readily gains one additional electron to become stable. Therefore, when aluminium and bromine react, 3 bromine atoms combine with each aluminium atom to form aluminium bromide- AlBr2.
Formation of aluminium bromide
Properties of ionic compounds
Ionic compounds are crystalline in nature at room temperature. The component ions in such structures are arranged in 3D patterns that are repetitive. For instance, in sodium chloride each Na+ ion is surrounded by 6 Cl- ions and each Cl- ion is surrounded by 6 Na+ ions. In this structure, each ion is strongly held to each other and the repulsive forces of attraction are minimal. The structure is very stable due to the large attractive forces that exist between the ions.
Ionic bond between Na and Cl
Coordination number refers to the number of oppositely charged ions that surrounds the anion in a crystal. From the above diagram, Na+ has a coordination number of 6 because it is surrounded by 6 chloride ions. Furthermore, each Cl- ion is surrounded by 6 Na+ ions and also has a coordination number of six.
X-ray Diffraction Crystallography
X-ray diffraction crystallography is the technique used to determine the internal structure of crystals. In this process, the x-rays are passed through a crystal and results recorded on a film. The pattern on the exposed film shows how ions are deflected by the x-rays. Generally, the pattern is used to calculate the position of the ions in the crystal and can also be used to define the structure of the crystal.
X-ray Diffraction Crystallography
Ionic compounds can conduct the electric current when melted. Melting of sodium chloride makes the crystal structure to breakdown. If a voltage is applied across the molten mass, positively charged ions will migrate towards the cathode while the negatively charged ions will move towards the anode. For a similar reason, ionic compounds also conduct electricity if they are dissolved in water. If you dissolve sodium chloride in water, the ions are freely moved in the aqueous solution.