Question

Predict the possible atomic number(s) of an atom in which the third shell is incompletely filled and a maximum of 3 more electrons can be added in that shell.

Answer 1

1. The number of electrons that can be found in the nth shell = $2n^2$, where n is the principal quantum number.
2. For 3^rd shell, $n=3$; the maximum number of possible electrons in the third shell is, $2{\mathrm{n}}^2=2\times 3^2=18$.
3. Given that the third shell is incompletely filled and can only hold three electrons, the following configurations are possible:

$1s^{2}2s^{2}2p^{6}3s^{2}3p^3$

4. As a result, the first electronic configuration's probable atomic number is $2+2+6+2+3=15$ and $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^7$;

5. As a result, the second electronic configuration's probable atomic number is $2+2+6+2+6+2+7=27$.

6. Hence, the possible atomic numbers are $15$ and $27$.