Atoms and Molecules - Properties of the Ionic and the Covalent

Ionic compounds are formed by the transfer of electrons from a metal to a non-metal, resulting in the formation of cations (e.g., $Na^+$) and anions (e.g., $Cl^-$).

Ionic compounds have high melting and boiling points because a large amount of energy is required to break the strong electrostatic forces of attraction between the oppositely charged ions.

Ionic compounds are generally soluble in water but insoluble in organic solvents like kerosene or petrol. They conduct electricity in molten or aqueous states due to the movement of free ions, but not in the solid state.

Covalent compounds are formed by the sharing of one or more pairs of electrons between two atoms, typically non-metals (e.g., $H_2O$, $CH_4$).

Covalent compounds generally have low melting and boiling points because the intermolecular forces (Van der Waals forces) between the molecules are weak.

Covalent compounds are mostly poor conductors of electricity because they do not form ions in solution and consist of neutral molecules.

The chemical formula of a compound is determined by the valencies of the constituent elements. For example, to form Magnesium Chloride, $Mg$ has a valency of $2+$ and $Cl$ has a valency of $1-$, resulting in $MgCl_2$.