Identify the elements X and Y using the ionisation energy values given below :

Solution

Chemistry ma’am · 4 steps

Look at where the big jump in ionisation energy comes, because it tells you the number of valence electrons.

1. Read the jump. Electrons are removed one by one. Energy rises steadily while you take electrons from the outer shell. It jumps sharply once you start breaking into a full inner shell.

   Element	1st IE	2nd IE	2nd IE ÷ 1st IE	What it means
   X	495	4563	about 9	huge jump after one electron, so 1 valence electron
   Y	731	1450	about 2	no big jump yet, so at least 2 valence electrons

2. X is sodium. A low 1st IE with one outer electron fits an alkali metal. After Na loses its 3s electron, Na⁺ has the neon core 1s² 2s² 2p⁶. The next electron must come from that closed inner shell, which sits much closer to the nucleus, so the 2nd IE is huge.
3. Y is magnesium. Mg ([Ne]3s²) still has one 3s electron left after the first removal. That electron leaves a positive ion, so the energy roughly doubles, but no closed shell is broken.
4. So. X = Na and Y = Mg, which is option (c). Fluorine is ruled out for both, because its 1st IE is about 1681 kJ mol⁻¹, far above both 495 and 731.

The idea to keep. Count the ionisation energies up to the first giant jump. That count is the number of valence electrons, and it tells you the group of the element straight away.

Answer from the NTA final answer key. Solution by PrepWiser.