Q4. Which of the following are isoelectronic, that is, have the same number of electrons: Na+, K+, Mg2+, Ca2+, S2−, Ar (NCERT Exercise 2.22, page 70)?
Na+ (11 − 1) and Mg2+ (12 − 2) have 10 electrons each. K+ (19 − 1), Ca2+ (20 − 2), S2− (16 + 2) and Ar have 18 each.
Q5. (i) Write the configurations of H−, Na+, O2− and F−. (ii) Give the atomic numbers of the elements whose outermost electrons are 3s1, 2p3 and 3p5. (iii) Name the atoms [He] 2s1, [Ne] 3s2 3p3 and [Ar] 4s2 3d1 (NCERT Exercise 2.23, page 70).
(i) H− 1s2; Na+, O2− and F− all have 10 electrons: 1s2 2s2 2p6. (ii) 3s1: 2 + 8 + 1 = 11 (Na); 2p3: 2 + 2 + 3 = 7 (N); 3p5: 2 + 8 + 2 + 5 = 17 (Cl). (iii) 3 electrons, Li; 15, P; 21, Sc.
Q6. (a) What is the lowest value of n that allows g orbitals? (b) An electron is in one of the 3d orbitals. Give its possible n, l and ml (NCERT Exercises 2.24 and 2.25, page 70).
(a) A g orbital has l = 4, and l ≤ n − 1, so n is at least 5. (b) n = 3, l = 2, ml = −2, −1, 0, +1 or +2.
Q7. (i) For n = 3, what values can l and ml take? (ii) List l and ml for the 3d orbitals. (iii) Which of 1p, 2s, 2p and 3f are possible (NCERT Exercise 2.28, page 70)?
(i) l = 0 (ml = 0); l = 1 (ml = −1, 0, +1); l = 2 (ml = −2 to +2). (ii) l = 2; ml = −2, −1, 0, +1, +2. (iii) 2s and 2p are possible; 1p (l = 1 needs n ≥ 2) and 3f (l = 3 needs n ≥ 4) are not.
Q8. How many electrons in an atom can have (a) n = 4, ms = −½; (b) n = 3, l = 0 (NCERT Exercise 2.31, page 71)?
(a) Shell 4 has 4 = 16 orbitals, each with one electron of spin −½: 16. (b) n = 3, l = 0 is the single 3s orbital: 2.
Q9. Bromine has 6 electrons in 2p, 6 in 3p and 5 in 4p. Which of these feel the lowest effective nuclear charge (NCERT Exercise 2.63, page 73)?
The 4p electrons: they are farthest from the nucleus and shielded by all the inner shells.
Q10. In each pair, which orbital’s electron feels the larger effective nuclear charge: (i) 2s and 3s; (ii) 4d and 4f; (iii) 3d and 3p (NCERT Exercise 2.64, page 73)?
(i) 2s, being closer to the nucleus. (ii) 4d: in the same shell, Zeff falls as l rises. (iii) 3p, for the same reason.
Q11. The unpaired electrons of Al and Si are in 3p orbitals. Which feel the larger effective nuclear charge (NCERT Exercise 2.65, page 73)?
Silicon’s: its nuclear charge is larger (Z = 14 against 13) while the inner shells shielding the 3p electrons are the same.
Q12. (a) How many subshells does n = 4 have? (b) How many electrons in the n = 4 shell have ms = −½ (NCERT Exercise 2.67, page 73)?
(a) Four: 4s, 4p, 4d and 4f. (b) 16, half of the shell’s 32 electrons (the same as Q8(a), which NCERT also asks as Exercise 2.31).
Q13. An atom contains 29 electrons and 35 neutrons. Find its number of protons and its electronic configuration (NCERT Exercise 2.26, page 70).
A neutral atom has as many protons as electrons: 29, so it is copper. Configuration: 1s2 2s2 2p6 3s2 3p6 3d10 4s1, the exceptional filled-3d arrangement.
Q14. (a) How many electrons in an atom can have n = 3 and ml = 0? (b) Which rule does each of these break: beryllium written as 1s2 2s1 2p1 in the ground state; nitrogen as 1s2 2s2 2px2 2py1; two ↑ arrows in one box?
(a) 3s, 3p and 3d each have one orbital with ml = 0, holding 2 electrons each: 6. (b) The aufbau principle (2s must fill before 2p); Hund’s rule (the 2p electrons must occupy separate orbitals before pairing); the Pauli principle (two electrons in one orbital must have opposite spins).