The above figure can be understand in terms of Molecular orbital theory.According to this theory the electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule they are the fundamental constituents of matter involved in bonding,. Electrons are shared among individual atoms in a molecule to form covalent chemical bonds. Generally, up to three bonds can form between atoms in a molecule. Single, or sigma covalent bonds which result from the interaction between the nuclei of two discrete atoms; multiple bonds then can result due to the additional formation of pi bonds between overlapping orbitals of like symmetries.Electrons in sigma bonds are located between the nuclei, while electrons in pi bonds are delocalized in regions above and below the nuclei.This theory uses a linear combination of atomic orbitals (LCAO) to represent molecular orbitals resulting from bonds between atoms. These orbitals are divided into 1.Bonding orbitals
2.Anti-bonding orbitals, and
3.Non-bonding orbitals.
The Schrödinger equation is used to build up the orbital bases of an atom or molecule. A molecular orbital is a Schrödinger orbital that includes two or more nuclei. If this orbital is of the type in which the electron(s) in the orbital have a higher probability of being between nuclei than elsewhere, the orbital will be a bonding orbital, and will tend to hold the nuclei together. A bonding orbital is of lower energy than the two contributing atomic bases. For bonding orbitals, the atomic orbital of lower energy contributes more to the molecular orbital, causing the molecular orbital to more closely resemble it. If the electrons tend to be present in a molecular orbital in which they spend more time elsewhere than between the nuclei, the orbital will function as an anti-bonding orbital and will actually weaken the bond. Conversely, an antibonding orbital will more closely resemble the higher energy atomic basis. Electrons in non-bonding orbitals tend to be in deep orbitals associated almost entirely with one nucleus or the other, and thus they spend equal time within as they do between nuclei. These electrons neither contribute to nor detract from bond strength and thus are reffered to as lone pairs.
Now in the above figure the orbitals are combining resulting in the formation of bonding and anti-bonding orbitals.Like 1s combine to form bonding and anti-bonding orbitals.Similarly for 2pz and 2px.Filling of electrons in these orbitals follow the rules-Aufbau’s principle,Hund’s rule and Pauli’s Exclusion principle.Energy is increasing towards upwards which shows bonding orbitals have lower energy than anti-bonding orbitals.