Write the electronic configurations of the following ions: (a) H- (b) Na+ (c) O2-(d) F- (ii) What are the atomic numbers of elements whose outermost electrons are represented by (a) 3s1 (b) 2p3 and (c) 3p5? (iii) Which atoms are indicated by the following configurations?

Question 2.23:  (i) Write the electronic configurations of the following ions: (a) H^- (b) Na⁺ (c) O^2-(d) F^-
(ii) What are the atomic numbers of elements whose outermost electrons are represented by (a) 3s¹ (b) 2p³ and (c) 3p⁵? (iii) Which atoms are indicated by the following configurations?
(a) [He] 2s¹ (b) [Ne] 3s² 3p³ (c) [Ar] 4s² 3d¹.
Answer
(i) (a) H^- ion
The electronic configuration of H atom is 1s¹.
A negative charge on the species indicates the gain of an electron by it.
∴ Electronic configuration of H^- = 1s²
(b) Na⁺ ion

The electronic configuration of Na atom is 1s² 2s² 2p⁶ 3s¹.
A positive charge on the species indicates the loss of an electron by it.
∴ Electronic configuration of Na⁺ = 1s² 2s² 2p ⁶ 3s⁰ or 1s² 2s² 2p⁶
(c) O^2- ion
The electronic configuration of 0 atom is 1s² 2s² 2p⁴.
A dinegative charge on the species indicates that two electrons are gained by it.
∴ Electronic configuration of O^2- ion = 1s² 2s ² p⁶
(d) F^- ion
The electronic configuration of F atom is 1s² 2s² 2p⁵.
A negative charge on the species indicates the gain of an electron by it.
∴ Electron configuration of F^- ion = 1s² 2s² 2p ⁶
(ii) (a) 3s¹
Completing the electron configuration of the element as 1s² 2s² 2p⁶ 3s¹.
∴ Number of electrons present in the atom of the element = 2 + 2 + 6 + 1 = 11
∴ Atomic number of the element = 11
(b) 2p³
Completing the electron configuration of the element as 1s² 2s² 2p³.
∴ Number of electrons present in the atom of the element = 2 + 2 + 3 = 7
∴ Atomic number of the element = 7
(c) 3p⁵
Completing the electron configuration of the element as 1s² 2s² 2p⁵.
∴ Number of electrons present in the atom of the element = 2 + 2 + 5 = 9
∴ Atomic number of the element = 9
(iii) (a) [He] 2s¹
The electronic configuration of the element is [He] 2s¹ = 1s² 2s¹.
∴ Atomic number of the element = 3
Hence, the element with the electronic configuration [He] 2s¹ is lithium (Li).
(b) [Ne] 3s² 3p³
The electronic configuration of the element is [Ne] 3s² 3p³= 1s² 2s² 2p⁶ 3s² 3p ³.

∴ Atomic number of the element = 15
Hence, the element with the electronic configuration [Ne] 3s² 3p³ is phosphorus (P).
(c) [Ar] 4s² 3d¹
The electronic configuration of the element is [Ar] 4s² 3d¹= 1s² 2s² 2p⁶ 3s² 3p ⁶ 4s² 3d¹.
∴ Atomic number of the element = 21
Hence, the element with the electronic configuration [Ar] 4s² 3d¹ is scandium (Sc).