JEE Main 2022 · 26 June · evening · Single correct · +4 / −1
s-block element which cannot be qualitatively confirmed by the flame test is :
- (a) Li
- (b) Na
- (c) Rb
- (d) Be
Answer and solution
Answer (d) Be
Solution
Chemistry ma’am · 3 steps
A flame can only show colour if it can excite an electron, and Be holds its electrons too tightly.
- Why flames colour. The heat of the flame lifts a valence electron to a higher energy level. When the electron falls back, it gives out light. If that light is in the visible range, you see a colour.
- Why Be fails. Beryllium is very small and has a high ionisation enthalpy, so its electrons are held very strongly. The flame does not supply enough energy to excite them, so no visible colour appears. Magnesium behaves the same way for the same reason.
Element Flame colour Confirmed by flame test? Li crimson red yes Na golden yellow yes Rb red-violet yes Be no colour no - So. All the alkali metals colour the flame, but Be does not. The answer is (d) Be.
The idea to keep. Every alkali metal colours the flame. Among group 2, Be and Mg do not, because they are small and their electrons are held too tightly for flame energy to excite them.
Answer from the NTA final answer key. Solution by PrepWiser.