Simply put:
Among the molecular orbitals produced by combination, the atomic orbitals in the energy region are called bonding orbitals; Those above the atomic orbital are called antibonding orbitals.
Definition:
Several atomic orbitals can be combined into several molecular orbitals, half of which are formed by the superposition of two atomic orbitals with the same symbol. The probability density of electrons between two nuclei increases, and its energy is lower than the original atomic orbit, which is beneficial to bonding, and it is called bonding molecular orbit, such as σ and π orbits (axisymmetric orbits). The other half molecular orbital is formed by the superposition of two atomic orbitals with different symbols. The probability density of electrons between two nuclei is very small, and its energy is higher than that of the original atom orbit, which is not conducive to bonding. They are called anti-bond molecular orbitals, such as σ * and π * orbitals (mirror symmetry orbitals, the sign of anti-bond orbitals is constant "*" to distinguish bonding orbitals).
Several simple atomic orbital combinations that conform to the principle of symmetric matching are: s-s, s-px, px-px form σ molecular orbital (for x axis); Py-py and pz-pz (to the xy plane) form π molecular orbitals. When two symmetrically matched atomic orbitals are combined into molecular orbitals, due to the similarities and differences of lobe symbols, there are two ways to combine them: two atomic orbitals with the same lobe symbols (i.e.++overlap or-overlap) are combined into bonded molecular orbitals; Two atomic orbitals with opposite lobes (that is,+-overlap) combine to form an anti-bond molecular orbital.
In the anti-bond orbit, the middle region of the two nuclei of the electron cloud principle is biased towards the external measurement of the two nuclei, thus separating the two atoms.