JEE Main 2025 · 29 January · evening · Single correct · +4 / −1
First ionisation enthalpy values of first four group 15 elements are given below. Choose the correct value for the element that is a main component of apatite family :
- (a) 834 kJ mol⁻¹
- (b) 947 kJ mol⁻¹
- (c) 1402 kJ mol⁻¹
- (d) 1012 kJ mol⁻¹
Answer and solution
Answer (d) 1012 kJ mol⁻¹
Solution
Chemistry ma’am · 3 steps
Apatite is a calcium phosphate, so find phosphorus, the second step down the ionisation ladder of group 15.
- Identify the element. The apatite family is calcium phosphate minerals, Ca₅(PO₄)₃(F,Cl,OH). The group 15 element in them is phosphorus.
- Trend down group. Going down group 15 (N, P, As, Sb), the atom gets bigger and the outer electrons sit farther from the nucleus. So the first ionisation enthalpy falls at each step.
Element Period First ionisation enthalpy N 2 1402 kJ mol⁻¹ P 3 1012 kJ mol⁻¹ As 4 947 kJ mol⁻¹ Sb 5 834 kJ mol⁻¹ - So. Arrange the four values in decreasing order and match them to N, P, As, Sb. Phosphorus gets the second highest value, 1012 kJ mol⁻¹, which is option (d).
The idea to keep. First connect the mineral to its element, then use the down-the-group trend: ionisation enthalpy falls as atomic size grows, so rank the values and assign them in order.
Answer from the NTA final answer key. Solution by PrepWiser.