How are the periods or horizontal rows on a periodic table ordered

Into periods or horizontal rows that share a pattern, into vertical groups of elements that have similar physical and chemical properties,and by increasing order of atomic number

Acovalent bond is formed between the two hydrogen bonds

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.