Covalent bonds form when atoms or ions share electrons such that their outer shells become occupied. Covalent bonds, also called molecular bonds, only form between nonmetal atoms with identical or relatively close electronegativity value. Electronegativity, denoted by the symbol χ, is a chemical property that describes the tendency of an atom to attract a shared pair of electrons (or electron density) towards itself.
The number of covalent bonds an atom can form is called the valence of the atom. This property represents the electrons of an atom that can participate in the formation of chemical bonds with other atoms. They are the furthest electrons from the nucleus.
A prime example of a covalent bond is the hydrogen molecule, which forms from two hydrogen atoms, each with one electron in their outer shell. Bond formation releases heat; therefore, it is exothermic. For the hydrogen molecule, the heat released during its formation, also known as the standard enthalpy change (ΔH°), is −435 kJ per mole. The reverse process, breaking the H—H bond, requires 435 kJ per mole, a quantity called the bond strength.
Another classic example of a covalent bond is hydrogen chloride (HCl), which is a hydrogen halide. The chlorine atom has 7 atoms in its outer shell while hydrogen has 1 electron in its outer shell. Both combine perfectly so each atom fills their valence shells, forming a highly stable molecule. Now, the HCl molecule will not react further with other chlorine or hydrogen atoms.
Differences between ionic and covalent bonds
- Covalent bonds are much more common in organic chemistry than ionic bonds.
- In covalent bonds, atoms share electrons, whereas in ionic bonds atoms transfer electrons.
- The reaction components of covalent bonds are electrically neutral, whereas for ionic bonds they are both charged. This explains why sodium chloride (salt) conducts electricity when dissolved — its components are charged.
- Covalent bonds are easier to break than ionic bonds.
- Covalent bonds are far more common in nature than ionic bonds. Most molecules in living things are covalently bonded, for instance.
- Covalent bonds can form between atoms of the same elements (i.e. H2). However, ionic bonds cannot do this.
- Covalent bonds are formed between two non-metals, whereas ionic bonds are formed between a metal and non-metal.
- Molecules formed by covalent bonds have a low melting point, whereas those with ionic bonds have a high melting point. The same relationship exists for boiling point.
- At room temperature, covalently bonded molecules are in the vast majority of cases liquids or gases, whereas ionic compounds are solid.
Similarities between ionic and covalent bonds
- Both types of bonds lead to the formation of stable chemical compounds.
- It takes exothermic reactions (i.e. that release heat) in order to create ionic and covalent bonds.
- Valence electrons are involved in both bonding processes.
- It doesn’t matter whether a molecule is formed through ionic or covalent bonding as far as its electrical charge is concerned: the result is always electrically neutral.