Question

why the energy of  antibonding molecular orbital is more than bonding molecular orbital

Answer 1

Energy has to do with stability. Please note that, as per molecular orbital theory, the electrons which are in bonding orbitals in a bond are between two nuclei. While the one in antibonding are outside of two nuclei region. So the electrons in bonding are more stable being bounded by two nuclei and thus have lower energy. Other way to understand (stability from energy instead of energy from stability) is that when two atoms combine to form bond they form two orbitals one with energy lower than original atoms(the reason between two nuclei) and one with energy higher( outside of two nuclei region, the antibonding.)

Answer 2

the electrons which are in bonding orbitals in a bond are between two nuclei. While the one in antibonding are outside of two nuclei region. So the electrons in bonding are more stable being bounded by two nuclei and thus have lower energy. Other way to understand (stability from energy instead of energy from stability) is that when two atoms combine to form bond they form two orbitals one with energy lower than original atoms(the reason between two nuclei) and one with energy higher( outside of two nuclei region, the antibonding.)