Bonding and lone pair electrons in covalent compounds

The covalent bond

Like ionic bond, covalent bond forms due to the need of the atoms to reach the nearest stable electronic configuration (the nearest electron configuration of noble gas).

A covalent bond, a bond in which two electrons are shared by two atoms. In covalent compounds (contain only covalent bonds), the atoms that form covalent bond approach each other and their outer most energy levels overlap. Only valence electrons are shared in the covalent bond.

Example, the covalent bond formation between two H atoms, each atom has one electron in 1s. The two atoms approach each other, and their 1s orbitals start to overlap. Now each electron can move around the two atoms in the overlapped space between the two atoms, and the electrons are considered to be shared by the two atoms.

The electrons are attracted simultaneously by the positive charges of the two hydrogen nuclei. Because the two hydrogen atoms have the same nuclei size the electron are equally shared by the two atoms.  The shared pair of electrons is often represented by a single line [H-H].

Another example; the formation of covalent bond between two fluorine atoms [F₂ or F-F]. The electron configuration of F is [1s²2s²2p⁵]. The 1s electrons are low in energy and stay near the nucleus most of the time and do not participate in bond formation. For 2s and 2p electrons, they are considered to be the valence electrons. By that, F has 7 valence electrons. There is only one unpaired electron on F, so each F atom comes close to each other, the orbitals overlap, and the two unpaired electrons are shared between two atoms:

Bonding and lone pair electrons

For any atom to be considered a stable atom the electrons need to be paired. Let’s look again at F-F, only one electron in each atom is considered to be unpaired.

The electrons participated in the formation of the covalent bond we name them bonding electrons. The other electrons that are not involved in the formation of covalent bond - nonbonding electrons - are called lone pairs. We count every two electrons together as pair thus, each F in F₂ has three lone pairs and one bonding pair:

Another example; the formation of a bond between H and Cl to produce HCl molecule [H-Cl]:

The hydrogen atom participate with its 1s orbital, for the Cl [1s²2s²2p⁶3s²3p⁶4s¹] the valence electrons are (3p⁶4s¹). 4s¹ is the only unpaired electron, so 4s¹ is the only involved electron in the formation of covalent bond with the H 1s¹ electron to form bonding pair the other valence electrons will be considered as lone pair electrons. Thus, Cl atom has 3 lone pair and one bonding pair.

Oxygen molecule:

Oxygen atom has 8 electrons, the electronic configuration of oxygen atom is [1s²2s²2p⁴]. 1s² is an inner orbital and the valence electrons of oxygen atom are [2s²2p⁴]. The p orbital has 4 electrons p_x² p_y¹ p_z¹, the py and pz are the two unpaired electrons. So, each oxygen atom has two unpaired electrons and two lone pair electrons:

When oxygen molecule forms, the two unpaired electrons in p orbital approach each other forming two bonding pairs. For that, oxygen has two bonding pairs and two lone pairs.