## 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 ? (a) [He]2s1 (b) [Ne]3s2 3p3 (c) [Ar] 4s2 3d1 .

### (iii) Which atoms are indicated by the following configurations ? (a) [He]2s1 (b) [Ne]3s2 3p3 (c) [Ar] 4s2 3d1 .

(i) (a) 1H = 1s1 . A negative charge means gain of electron.
∴ electronic configuration of H– = 1s2
(b) 11Na = 1s22s22p63s1 . A positive charge means the shortage of an electron.
∴ electronic configuration of Na= 1s22s22p6
(c) 8O = 1s22s22p4
∴ electronic configuration of O2- = 1s22s22p6

(d) 9F = 1s22s22p5

∴ electronic configuration of F = 1s22s22p6

(ii) (a) 3s1
Completing the electron configuration of the element as 1s22s22p63s1
∴ 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 1s22s22p3
∴ 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 1s22s22p63s23p5
∴ Number of electrons present in the atom of the element = 2+2+6+2+5 = 17
∴ Atomic number of the element = 9

(iii)  (a) [He]2s1
electronic configuration = 1s22s1
∴ Atomic number of the element = 2+1 = 3
Hence, the element with the electronic configuration [He]2s1 is lithium (Li).
(b) [Ne]3s23p3
electronic configuration = 1s22s22p63s23p3
∴ Atomic number of the element = 2+2+6+2+3 = 15
Hence, the element with the electronic configuration  [Ne]3s23p3 is phosphorus (P).
(c) [Ar] 4s23d1
electronic configuration = 1s22s22p63s23p64s23d1
∴ Atomic number of the element = 2+2+6+2+6+2+1 = 21
Hence, the element with the electronic configuration [Ar] 4s23d1 is scandium (Sc).