Ionic bonds, ionic compounds and characteristics of ionic compounds

When a complete transfer of one or more valence electrons from the atom of a metal to that of a non-metal takes place, an ionic (or electrovalent) bond is formed. As a result of this electron transfer following changes occur in the reacting atoms.

• Both the atoms acquire stable noble gas configurations, one completing its octet by losing its electron(s) and the other by gaining its electron(s).

• The atom that loses its electrons becomes a positively charged ion, called 'cation', whereas the atom which gains these electrons becomes a negatively charged ion called 'anion'.

• The two oppositely charged ions, i.e. the cation and the anion, are then held together by the coulombic forces of attraction to form an ionic bond.

During the formation of an ionic bond a certain amount of energy is released.

Thus, an ionic bond may be defined as: 'The bond formed by the complete transference of electron(s) from one atom to another so as to acquire noble gas configuration'.

The compounds in which the constituent ions are held together by ionic bond (coulombic forces) are called ionic (or electrovalent) compounds.

Some typical ionic compounds are sodium chloride, potassium chloride, potassium sulphate etc. When the transfer of electrons form a compound, the element that loses the electrons is said to be 'oxidized' and the element that gains the electrons is said to be 'reduced'. Oxidation is a process which involves loss of electrons whereas reduction is a process which involves gaining of electrons.

Electrovalency

The number of electrons lost or gained by an atom of any element is termed as its electrovalency. The elements that give up electrons to form positive ions are said to have positive valency, while the elements which accept electrons to form negative ions are said to exhibit negative valency. Thus, the electrovalency of sodium is 1+, and that of chlorine is 1- in NaCl. Similarly, calcium, magnesium in their chloride exhibit an electrovalency of 2+. There are many elements, which show different electrovalencies in different compounds. This phenomenon is called variable electrovalency e.g., iron exist as Fe²⁺ and Fe³⁺ in ferrous sulphate and ferric sulphate respectively.

Formation of sodium chloride

For example, during the formation of an ionic bond between sodium and chlorine atoms, sodium atom (1s² 2s² 2p⁶ 3s¹) has only one electron in its valence shell. By losing this one electron it complete its octet and acquires the noble gas configuration of neon (2, 8). While the chlorine atom (1s² 2s² 2p⁶ 3s²3p⁵) which has seven electrons in its valence shell, gains one electron to complete its octet and acquires the stable electronic configuration of argon.

Formation of magnesium chloride

Magnesium whose atomic number is 12, has 2, 8, 2 configuration. So, it has two electrons in its valence shell. The electronic configuration of chlorine (At. no. 17) is 2,8,7. So it has seven valence electrons. Since, magnesium has two electrons in excess of the neon configuration

(2, 8), and chlorine is one electron short of the argon configuration (2,8,8), hence one atom of magnesium will transfer its two electrons to two atoms of chlorine (one to each) as shown below;

The Mg²⁺ and the two Cl^- so formed, then form ionic bonds between them.

In terms of Lewis dot structure,

Properties of ionic (or electrovalent) bond

An ionic or electrovalent bond has the following characteristics:

• An ionic bond is formed due to the coulombic attraction between the positively and negatively charged ions.

• An ionic bond is non-directional i.e., the strength of interaction between two ions depend upon distance, but not on the direction.

• An ionic bond gets broken when the substance is dissolved in a polar solvent or when the substance is melted.

Stoichiometric (or empirical) Formulae of Ionic Compounds

The electrostatic field of an ion is nondirectional.Each positive ion attracts several negative ions around it depending upon its size or radius and vice-versa ,resulting in a three dimensional solid aggregate called an ionic crystal in which oppositely charged ions alternate in regular,continuous geometrical pattern.The ionic compounds contain ions distributed in such a way that a cation has an anion as it nearest neighbour and vice-versa. There is no discrete molecule in an ionic compound. Therefore it is not correct to assign a molecular formula to an ionic compound. The formula representing the simple ratio of the elements present in an ionic compound may thus be termed as empirical formula or simply as stoichiometric formula. For example, stoichiometric formula of sodium chloride is NaCl or Na⁺Cl^-. The stoichiometric formula of an ionic compound can be easily written, if the charges or the constituent ions are known. Valence of some common monoatomic ions of the main group elements are given below:

Problem

1. Deduce the electronic configuration and formulae for the ionic compound formed by the elements Al and F; draw its Lewis structure. (Al = 13, F = 9)

Solution

The electronic configurations of aluminium (At. no. 13) and fluorine (At. no. 9) atoms are 2,8,3 and 2,7 respectively. Aluminium has 3 electrons in excess of its nearest noble gas (neon) configuration. Fluorine is one electron short of its nearest noble gas (neon) configuration. Therefore, one atom of aluminium will transfer its three electrons to three atoms of fluorine (one of each) as shown below:

The Al³⁺ and three F^- ions then form ionic bonds between them,

In terms of Lewis dot structure,

Energy changes in the formation of Ionic bond

There are three steps involved in the formation of an electrovalent (or ionic) bond.

Removal of electron(s) from one atom

In this step, energy equal to the ionization energy is absorbed.

Gaining of electron(s) by the other atom

In this step, energy equal to the electron affinity is released.

Combination of cations and anions

Coulombic forces of attraction hold these ions together. In this step, energy equal to the lattice energy is released.