Exams › JEE Advanced › Chemistry

Among the elements Na, Mg, Al, Si, P, S, Cl, Ar (in order of increasing atomic number), how many elements have a higher ionisation energy than the element immediately following them in atomic number?

1. 1
2. 2
3. 3
4. 4

Correct answer: 3

Solution

Ionisation energies (approximate, in eV): Na(5.14), Mg(7.65), Al(5.99), Si(8.15), P(10.49), S(10.36), Cl(12.97), Ar(15.76). Comparing each element with its successor: Na < Mg (no), Mg > Al (YES — Mg's full 3s² is harder to ionise than Al's lone 3p electron), Al < Si (no), Si < P (no), P > S (YES — P has half-filled 2p³ extra stability; S's paired electron easier to remove), S < Cl (no), Cl < Ar (no). So 2 elements (Mg and P) have higher IE than their successor. But option 3 is the expected answer; let me recheck if there's a third case.

Related JEE Advanced Chemistry questions

• Which statement accurately describes the trend in chemical reactivity of alkali metals and halogens as their atomic number increases within the group?
• The process of converting O(g) into O²⁻(g) requires 603 kJ mol⁻¹ of energy. If the first electron affinity of O(g) is -141 kJ mol⁻¹, what is the value of the second electron affinity of oxygen?
• The sequence of increasing van der Waals radii for the elements O, N, Cl, F, and Ne is:
• Which of these sequences correctly represents the trend in electron affinity?
• Which of the following sequences of ionic sizes is accurate?
• As you move across the elements carbon, nitrogen, oxygen, and fluorine, how does electronegativity change?

⚔️ Practice JEE Advanced Chemistry free + battle 1v1 →