For an element with atomic number 35, which one of the following will be the correct number of electrons in its valence shell based on Bohr's model of an atom?
- (a)1
- (b)3
- (c)5
- (d)7
Correct — D, 7. Element 35 is bromine. Filling the Bohr shells using the 2n² rule gives K = 2, L = 8, M = 18, N = 7 (and 2 + 8 + 18 + 7 = 35). The outermost shell (N) holds 7 electrons — the valence electrons — which is why bromine is a halogen in Group 17, needing just one more electron to complete its octet.
- (a)1 — One valence electron would suit a Group-1 alkali metal, not bromine; the 2,8,18,7 filling of element 35 leaves 7 in the outer shell.
- (b)3 — Three valence electrons belong to a Group-13 element; it does not match the Bohr filling of atomic number 35.
- (c)5 — Five valence electrons would be a Group-15 element; bromine's outermost shell holds 7, not 5.
In Bohr's model, electrons occupy shells K, L, M, N... that can hold at most 2n² electrons (2, 8, 18, 32...), filled from the innermost outward. The number of electrons.
The task is simply to distribute 35 electrons: 2, then 8, then 18 (total 28), leaving 7 for the fourth shell. Seven valence electrons is the signature of a halogen.
- Shell capacities follow 2n²: K = 2, L = 8, M = 18, N = 32 (maximum).
- Bromine (Z = 35) has the configuration 2, 8, 18, 7.
- Seven valence electrons place bromine in Group 17 (the halogens).
- Bromine gains one electron to form the bromide ion, Br⁻.
2 + 8 + 18 + 7 = 35 → bromine, a halogen with 7 valence electrons.
- Filling shells as 2, 8, 8... instead of 2, 8, 18... for higher elements.
- Confusing valence electrons (7 for bromine) with valency (which is 1 for bromine).
Given the atomic number, fill the Bohr shells and read off the number of electrons in the outermost shell.
No directly related past PYQ was found.
- practice — not a real PYQ
An element has atomic number 17. The number of electrons in its valence shell is
- (a)2
- (b)5
- (c)7
- (d)8
Answer(c) 7 — chlorine has the configuration 2, 8, 7, so it is a halogen with 7 valence electrons.
- practice — not a real PYQ
An element with electronic configuration 2, 8, 1 belongs to which group of the periodic table?
- (a)Group 1
- (b)Group 8
- (c)Group 17
- (d)Group 18
Answer(a) Group 1 — one valence electron marks an alkali metal (sodium).