Ionic compounds are also called electrovalent compounds. An ionic (or electrovalent) compound may also be defined as, The compound formed due to the transfer of electrons from the atom of those to that of another is called an ionic (or electrovalent) compound.
During the formation of ionic (or electrovalent) compounds, the element whose atom loses electrons is said to be oxidized, and the element whose atom gains electrons is said to be reduced.
Ionic compounds formation
Ionic (or electrovalent) compounds are formed when an atom loses one or more electrons and the other gains one or more electrons.
Formation of some typical ionic compounds is described below.
i) Formation of sodium chloride:
The electronic configuration of sodium atom (atomic no 11) is 2, 8, 1. So, it has one electron in its valence shell. The electronic configuration of chlorine atom (atomic no 17) is 2, 8, 7. So, it has been seven valence electrons.
When the two combine, there is a transfer of one electron from sodium atom to chlorine atom. During the transfer of an electron, both the atoms attain the noble gas configuration: sodium that of neon (2, 8) and chlorine that of argon (2, 8, and 8). The two ions, Na+ and Cl– are then held together by the electrostatic attraction to form the ionic compound sodium chloride (Na+Cl–).
The columbic force of an attraction between Na+ and Cl– lowers the energy of the system, and hence makes it a stable compound.
Energy changes in the formation of sodium chloride:
The first three steps are endothermic, whereas the last two steps are exothermic.
Net energy released = (Energy released)- (Energy absorbed)=(787 +349) kj mol-1-(108 +496+121) kj mol-1=411 kj mol-1
Thus, during the formation of one mole of sodium chloride from its elements, 411 kJ energy is released, that is, sodium chloride is more stable than its elements.
ii) Formation of magnesium chloride, MgCl2
The electronic configuration of the two atoms is,
Magnesium (atomic no. 12) Chlorine (atomic no.17)
No. of electrons =12 no. of electrons =17
Electronic configuration: (2, 8, and 2) Electronic configuration: (2, 8, 7)
Thus, magnesium atom has two electrons in its valence shell. The chlorine atom has seven valence electrons. Thus , magnesium has two electrons in excess of the neon configuration(2,8), and chlorine is one electron short of argon configuration(2,8,8), so one atom of magnesium will transfer its two valence electrons to two chlorine atoms,(one to each) as shown below.
iii) Formation of AIF3:
The electronic configuration of aluminum (atomic no 13) and fluorine (atomic no.9) atoms are 2, 8, 3 and 2, 7 respectively. Aluminum has 3 electrons in excess of its nearest noble gas (neon) electronic configuration. Therefore, one atom of aluminum will transfer its three valence electrons to three atoms of fluorine (one to each) as shown below.
Al → Al3+ + 3e–
(2, 8, 3) (2, 8)
3F + 3e– → 3F–
(2, 7) (2, 8)
Al3+ + 3F– → AlF3
iv) Formation of sodium sulphide (Na2S):
The electronic configurations of sodium (Na) and sulpher(S) are,
Na (atomic no. 11), electronic configuration, 2, 8, 1
S (atomic no. 16), electronic configuration 2, 8, 6
Sodium atom has only the valence electron, while sulphur atom has 6 valence electrons. Thus, each sodium atom would lose one electron to attain stable neon electronic configuration. Each sulphur atom would gain two electrons to complete its outermost shell.