Ionic Bonding

The chemical bond formed by transfer of electron from a metal to a non-metal is known as ionic or electrovalent bond. For example, when sodium metal and chlorine gas are brought into contact, they react violently and we obtain sodium chloride.

Sodium Chloride

This reaction is:

2Na(s) + Cl₂(g) → 2NaCl(s)

Sodium (Na) has the atomic number 11 and its electronics configuration is 2, 8, 1. It has one electron in its outermost (M) shell. If it loses this electron, it is left with 10 electrons and becomes positively charged. Such a positively charged ion is called a cation. The cation in this case is called sodium cation, Na⁺.

The sodium cation has 11 protons but 10 electrons only. It has 8 electrons in the outermost (L) shell. Thus, sodium atom has attained the noble gas configuration by losing an electron present in its outermost shell. Loss of electron results into formation of an ion and this process is called ionization.

Thus, according to octet rule, sodium atom can acquire stability by changing to sodium ion (Na⁺). The ionization of sodium atom to give sodium ion requires an energy of 496 kJ mol^–1.

Now, chlorine atom having the atomic number 17, has the electronic configuration 2, 8, 7. It completes its octet by gaining one electron from sodium atom with electronic configuration 2, 8, 1. Both sodium ion (Na⁺) and chloride ion (Cl^–) combine together by ionic bond and become solid sodium chloride (NaCl).

The chlorine atom has gained an additional electron hence it has become a negatively charged ion (Cl^–). Such, a negatively charged ion is called an anion. Chloride ion has 8 electrons in its outermost shell and it therefore, has a stable electronic configuration according to the octet rule. The formation of chloride ion from the chlorine atom releases 349 kJ mol^-1 of energy.

Since the cation (Na⁺) and the anion (Cl^–) formed are electrically charged species, they are held together by Coulombic force or electrostatic force of attraction. This electrostatic force of attraction which holds the cation and anion together is known as electrovalent bond or ionic bond.

If you compare the energy required for the formation of sodium ion and that released in the formation of chloride ion, there is a net difference of 147 kJ mol^–1 of energy. If only these two steps are involved, the formation of sodium chloride is not favourable energetically. But sodium chloride exists as a crystalline solid. This is because the energy is released when the sodium ions and the chloride ions come together to form the crystalline structure. The energy so released compensates for the above deficiency of energy.

Each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions in its solid state structure. The force of attraction between sodium and chloride ions is uniformly felt in all directions. Thus, no particular sodium ion is bonded to a particular chloride ion. Hence, there is no species such as NaCl. Here NaCl is empirical formula and shows that there is one Na⁺ for every Cl^–.

Magnesium Chloride

Mg has atomic number 12. Thus, it has 12 protons. The number of electrons present in it is also 12. Hence the electronic configuration of Mg atom is 2, 8, 2. It has 2 electrons in its outermost shell. If it loses these two electrons, then it can achieve the stable configuration of 2, 8 (that of noble gas neon).

Mg → Mg²⁺ + 2e^–

The resulting magnesium ion has only 10 electrons and hence it has 2+ charge. It is a dipositive ion and can be represented as Mg²⁺ ion. The two electrons lost by the magnesium are gained - one each by two chlorine atoms to give two chloride ions.

2Cl(g) + 2e^– → 2Cl^–(g)

Thus, one magnesium ion and two chloride ion join together to give magnesium chloride, MgCl₂.

Magnesium Oxide

The oxygen atom having atomic number eight has 8 electrons. Its electronic configuration is 2, 6. It can attain a stable electronic arrangement (2, 8) of the noble gas neon if it gains two more electrons. The two electrons, which are lost by the magnesium atom, are gained by the oxygen atom. On gaining these two electrons, the oxygen atom gets converted into the oxide anion.

O + 2e^– → O^2–

The oxide has 2 more electrons as compared to the oxygen atom. Hence, it has 2 negative charges on it. Therefore, it can be represented as O^2– ion. The magnesium ion (Mg²⁺) and the oxide ion (O^2–) are held together by electrostatic force of attraction. This leads to the formation of magnesium oxide.

Thus, magnesium oxide is an ionic compound in which a dipositive cation (Mg²⁺) and a dinegative anion (O^2–) are held together by electrostatic force. Similar to the case of sodium chloride, the formation of magnesium oxide is also accompanied by lowering of energy which leads to the stability of magnesium oxide as compared to individual magnesium and oxygen atoms.