Period 6 begins with the filling of the 6s subshell.

Moseley found that $\sqrt{\nu}$ of the X-rays of an element is a straight-line function of its atomic number $Z$: $\sqrt{\nu} = a(Z - b)$. The modern periodic law: properties are a periodic function of $Z$. Temporary IUPAC names join the roots nil, un, bi, tri, quad, pent, hex, sept, oct and enn for the digits of $Z$ and add ium.