A chemical bond is an attractive force that holds various constituents (atom, ion etc.) together in different chemical species. On this page, we will study a type of chemical bond known as ionic bond.
What is an Ionic Bond
When a bond is formed as a result of complete transfer of electrons from one atom to another so as to attain their stable electronic configuration, the bond is called an ionic bond or electrovalent bond. The resulting compound is called an ionic compound.
Formation of Ionic Bond
Atoms have equal number of electrons and protons i.e., they are electrically neutral species. On losing an electron, an atom becomes positively charged because now the number of protons are greater than the number of electrons.
On the other hand, when an atom gains an electron, the atom becomes negatively charged because now the number of electrons exceeds the number of protons.
The positively and negatively charged species are called ions. There are two types of ions:
- Anions : Atoms that have gained one or more electrons are called anions. They are negatively charged species.
- Cations : Atoms that have lost one or more electrons are called cations. They are positively charged species.
formation of a cation
formation of an anion
Ionic bond is formed as a result of electrostatic forces of attraction between oppositely charged ions. It should be noted that the ionic compound thus formed must be electrically neutral i.e., the charges of anions and cations must cancel out each other.
Question : Explain the formation of sodium chloride.
Answer : The atomic number of sodium is 11 and its electronic configuration is 2,8,1. By losing an electron from its outermost shell, sodium acquires the inert gas configuration of neon — 2,8. In doing so, sodium atom changes into a cation.
On the other hand, chlorine, with atomic number 17, has electronic configuration 2,8,7. Chlorine accepts the electron released by sodium and changes into an anion in order to attain the inert gas configuration of argon — 2,8,8.
Now, when the positively charged sodium ion and the negatively charged chlorine ion come close to each other, they experience a strong electrostatic force of attraction; As a result of this force of attraction, formation of sodium chloride takes place.
The above steps may be written directly as follows:
Important Points Regarding Ionic Bonds
- No bond exists that is pure ionic in nature; all ionic compounds possess some degree of covalent bonding. Hence, the term "ionic bond" is given when the ionic character exceeds the covalent character.
- In general, the cation is a metal atom and the anion is a nonmetal atom. There are, however, some exceptions such as NH4+, which is a cation made from two non-metallic elements.
Factors Governing the Formation of Ionic Bonds
The formation of ionic bond depends on the following factors :
- Ionization energy : Ionization energy of an element is defined as the amount of energy required
to remove an electron from the outermost shell of an isolated atom in gaseous phase to form a cation.
- Electron affinity : Electron affinity of an element is defined as the amount of energy released or spent when an electron is added to an atom in the gaseous phase to form an anion.
- Lattice energy : Lattice energy is the energy released when ions in gaseous phase combine to form an ionic solid.
Atoms having low ionization energy lose some electrons to form cations so as to achieve a stable electronic configuration, while atoms having higher electron affinity accept the electrons —and become anions— to attain their stable electronic configuration. The higher the lattice energy of the resulting ionic compound, the greater the stability of the compound.
Properties of Ionic Compounds
- Physical State : Ionic compounds are usually solid.
- Structure : Ionic compounds exist as ions —anions and cations— and not as molecules. These anions and cations are arranged in a regular pattern to form lattice.
- Melting and Boiling Point : Ionic compounds have high melting and boiling point because ions in ionic compounds are held together by strong electrostatic forces of attraction; hence, a lot of energy is required to break them.
- Solubility : Ionic compounds are soluble in polar solvents such as water and insoluble in non-polar solvents.
- Electrical conductivity : Ionic compounds conduct electricity in solution or in molten state because ions are free to move in these states.