Explain why the chemical formula for calcium hydroxide , ca(oh)2 is not written as cao2h2

The formula CaO₂H₂ correctly gives the relative number of each type of atom in the compound.
But the OH group usually behaves as a unit. It doesn't fall apart.
So chemists have agreed to write the OH group as a unit: Ca(OH)₂.
This shows that there are 1 Ca atom and 2 OH groups in a formula unit of calcium hydroxide.

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.