Energy is released when the atoms of two elements bond together to form a compound compared to the total potential energy of atoms before bonding the total potental energy of the atoms after bonding is

According to the conservation of energy, energy is neither created nor destroyed, only transformed. In a system, ideally, the energy before the reaction would be the same after the reaction. But in reality, some energy are dissipated into the surrounding. Therefore, the energy after the reaction would always be lesser.