Chemistry

Chemical bonding

Chemical bonding

There are four types of chemical bonds:

- Ionic bond
- Covalent bond
- Metallic bond
- Intermolecular forces of attraction (VDW: Td-id, pd-d / Note: H-bonds are not VDW forces)

Ionic bonds (Electrovalent bonds)

Definition: Ionic bonds are the strong electrostatic forces of attraction between oppositely charged ions.

Ionic bonds are formed between two atoms with a large electronegativity difference. It is between a metal and a non-metal. Metal atoms lose their valence electrons to form cations while non-metals gain electrons to form anions. Both cations and anions attain the stable electronic configuration of a noble gas (octet or duplex configuration).

Exceptions: Cations formed from most transition elements do not follow the octet rule.

The electronegativity of an atom refers to its ability to attract electrons in a bond. For example, in NaCl, chlorine is more electronegative and attracts electrons towards itself, adopting a negative charge.

Structure of ionic compounds

Ionic compounds have giant ionic structures whereby oppositely charged ions are attracted together by strong ionic bonds in a regular three-dimensional lattice.

The coordination number of an ion is the number of oppositely charged ions that surrounds it.

In a crystal structure of an ionic compound, the ionic bonds are non-directional, where the ions arrange themselves in alternating layers of cations and anions in a continuous three-dimensional lattice. The ions are arranged as such in the ionic lattice so as to maximise attractive forces between oppositely charged ions and minimise repulsive forces between similarly charged ions.

Strength of ionic bonds

Ionic bonds are strong electrostatic forces of attraction that require a lot of energy to break. The strength of the bonds is indicated by the lattice energy of the compound.

Strength of ionic bonds:

- increases with increasing magnitude of charges
- decreases with increasing ionic size

|Lattice energy| ∝ (q+q-) / (r+ + r-) ,
where q+ and q- are the charges of the cations and anions respectively and r+ and r- are the inter-ionic distance between the cations and anions respectively.

Lattice energy is defined as the enthalpy change (energy change) when one mole of a solid ionic compound is formed from its constituent gaseous ions under standard conditions of 298k and 1atm.

Physical properties

Melting and boiling point

Large amounts of energy is required to overcome the strong electrostatic forces of attraction (ionic bonds) during a change in physical state. Ionic compounds have high melting and boiling points. Hence, ionic compounds are used as refractory materials, being chemically and physically stable at high temperatures.

Note:
Melting → Overcoming some ionic bonds
Boiling → Overcoming all (breaking) ionic bonds

Hardness and brittleness

Ionic compounds are hard because the ionic bonds are strong and non-directional.

However, they are also brittle. When a force is applied along a cleavage plane, the orderly arrangement of oppositely charged ions in the crystal lattice are disrupted and similarly-charged ions come into contact with each other. This generates a strong repulsive force that shatters the structure and forces the layers apart.