Out of first $100$ elements no. of elements having electrons in $3\mathrm{d}$ orbital (in their complete electronic configuration) are:-

The element with the electronic configuration ${\mathrm{ns}}^2(\mathrm{n}-1){\mathrm{s}}^2{\mathrm{p}}^6{\mathrm{d}}^0(\mathrm{n}-2){\mathrm{s}}^2{\mathrm{p}}^6{\mathrm{d}}^{10}{\mathrm{f}}^7$ lies in the:-

In the lithium atom screening effect of valence shell electron is caused by-