In a heteronuclear molecule a bond is build between different atoms, and the most important difference from the homonuclear case is that molecular orbitals (MOs) are no longer shared exactly between atoms. As like a molecule where each atom has just one valence atomic orbital (AO): As like would be gas-phase LiH with 2s on Li and 1s on H. When MOs are build using the LCAO approximation.
the coefficients c2 and c1 are no longer equal. In LiH the two AOs differ greatly in energy, as H has a bigger ionization energy and higher electronegativity than Li. If is the AO of lower energy (i.e. of higher ionization energy or greater electronegativity;), then the bonding MO has c2>c1.