A covalent bond is where 2 atomic orbitals overlap to form a molecular bonding and anti-bonding orbital, which allows electrons from each atom to lower their energy with respect to their original atomic orbitals.For example 2 hydrogen atom 1s orbitals, each with 1 electron in, overlap to give a σ bonding orbital and a σ* antibonding orbital. The σ molecular orbital is as much lower in energy than the constituent atomic orbitals as the σ* orbital is higher in energy than them. however, both the electrons in the bond are in the σ bonding orbital, making them lower in energy, so bonding is favorable. Non polar covalent bond is formed between 2 atoms of similar electronegativities and Polar covalent bond is formed between atoms of different electronegativities (electronegativity difference equal or less than 1.7). If a bond is too polarised (electronegativity difference more than 1.7) it is more favorable for electrons to fully transfer and ions are formed which attract electrostatically, with orbitals of the right shape and size to interact & ionic bond is formed.