Write a balanced chemical equation for the standard formation reaction of solid mercury(ii) oxide .

The balanced equation of mercury (ii) oxide is:
2Hg(l) + O2(g) → 2 HgO(s)

as the original equation between Oxygen and Mercury to form Mercury(ii)oxide is:
Hg(l) + O2(g) → HgO(s) and we start to compare
first, O atom number on both sides of the equation:
we find 2 atoms on the right side and 1 in the left side so,
we put 2HgO instead of HgO to make the number of O equals on each side
so the equation become Hg + O2 → 2HgO
then we start to compare the Hg atom number on each side:
we find 1 Hg on the right side and 2 Hg on the left side So,
we put 2Hg instead of Hg to make the number of Hg atoms equals in each side.
So we get our final balanced equation is:
2Hg(l) + O2(g) → 2HgO (s)

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.