Question

The elements with atomic numbers $39$ to $48$ belong to:

• A. forth period
• B. fifth period
• C. sixth period
• D. third period

Answer 1

The correct option is B fifth period

The element with the atomic number 39 has the electronic configaration $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}4d^{1}5s^2$ and the elemet is yttrium.
The element with the atomic number 48 has the electronic configaration $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}4d^{10}5s^2$ and the elemet is cadmium.

In both these elements, the outer electron fall in the 5th shell hence belongs to the 5th period.

Option B is correct.