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
Answer
Why
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.
Why the others are wrong
- (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.
Concept
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 in the outermost occupied shell is the count of valence electrons, and it fixes the element's group in the periodic table.
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.
Key facts
- 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.
Study next
Common traps
- 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.
Related PYQs
For Aluminium (Al) (atomic number: 13, mass number: 27), which one among the following statements is NOT correct?
- (a) Number of electrons present in Al is 13
- (b) Number of protons present in Al is 13
- (c) Number of neutrons present in Al is 14
- (d) The valency of Al is 2
Answer(d) The valency of Al is 2
The same atomic-structure skill — reading electrons, protons, neutrons and valency from an atomic number, here for aluminium instead of bromine.
Practice
- 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).