An element with atomic number 20 will be placed in which period of the periodic table?

The element with atomic number Z = 20 is calcium (Ca).

Electronic configuration of Ca is $1{\mathrm{s}}^{2}2{\mathrm{s}}^2{\mathrm{sp}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}4{\mathrm{s}}^2$

In the above configuration, the valence shell is $4^{\mathrm{th} }$shell

Therefore, the element calcium belongs to 4th period.