What makes an ionic difference than a covalent bond

I’m not completely sure what you mean here but if your talking about charges in covalently bonded molecules then here is something:

Charges in covalent bonds are partial because the ions are shared when bonding.

The charges are no longer full and strong they are half charges, they are not as strong.

You might want to just google a little bit. :)

Hope that helps

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.