Write the electronic configurations of the following ions:
(a) $H^{–}$
(b) $N{a}^{+}$
(c) $O^{2–}$
(d) $F^{–}$

(ii) What are the atomic numbers of elements whose outermost electrons are represented by
(a) 3$s^1$
(b) 2$p^3$ and
(c) 3$p^5$ ?

Which atoms are indicated by the following configurations?
(a) [He] 2$s^1$
(b) [Ne] 3$s^{2}3p^3$
(c) [Ar] 4$s^{2}3d^1$

(i).

(a) $H^{–}$
The electronic configuration of H atom is 1$s^1$.
A negative charge on the species indicates the gain of an electron by it. $\therefore$ Electronic configuration of $H^{–}=1s^2$

(b) $N{a}^{+}$
The electronic configuration of Na atom is $1s^{2}2s^{2}2p^{6}3s^1$.
A positive charge on the species indicates the loss of an electron by it. $therefore$ Electronic configuration of $N{a}^{+}=1s^{2}2s^{2}2p^{6}3s^{0}or1s^{2}2s^{2}2p^6$

(c) $O^{2–}$
The electronic configuration of 0 atom is $1s^{2}2s^{2}2p^4$.
A dinegative charge on the species indicates that two electrons are gained by it.
$\therefore$ Electronic configuration of $O^{2–}$ ion = $1s^{2}2s^2{p}^6$

(d) $F^{–}$
The electronic configuration of F atom is $1s^{2}2s^{2}2p^5$.
A negative charge on the species indicates the gain of an electron by it.
$\therefore$ Electronic configuration of $F^{–}$ ion = $1s^{2}2s^{2}2p^6$

(ii).

(a) 3$s^1$
Completing the electron configuration of the element as
$1s^{2}2s^{2}2p^{6}3s^1$.
$\therefore$ Number of electrons present in the atom of the element
= 2 + 2 + 6 + 1 = 11
$\therefore$ Atomic number of the element = 11

(b) 2$p^3$
Completing the electron configuration of the element as
$1s^{2}2s^{2}2p^3$.
$\therefore$ Number of electrons present in the atom of the element
= 2 + 2 + 3 = 7
$\therefore$ Atomic number of the element = 7

(c) 3$p^5$
Completing the electron configuration of the element as
$1s^{2}2s^{2}2p^5$.
$\therefore$ Number of electrons present in the atom of the element
= 2 + 2 + 5 = 9
$\therefore$ Atomic number of the element = 9

(iii).

(a) [He] 2$s^1$
The electronic configuration of the element is [He] $2s^1=1s^{2}2s^1.$
$\therefore$ Atomic number of the element = 3
Hence, the element with the electronic configuration [He] 2$s^1$ is lithium (Li).

(b) [Ne] 3$s^{2}3p^3$
The electronic configuration of the element is [Ne] $3s^{2}3p^3=1s^{2}2s^{2}2p^{6}3s^{2}3p^3.$
$\therefore$ Atomic number of the element = 15
Hence, the element with the electronic configuration [Ne] 3$s^2$ 3$p^3$ is phosphorus (P).

(c) [Ar] 4$s^{2}3d^1$
The electronic configuration of the element is [Ar] $4s^{2}3d^1=1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^1$.
$\therefore$ Atomic number of the element = 21 Hence, the element with the electronic configuration [Ar] $4s^{2}3d^1$ is scandium (Sc).