Given below are two statements. One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Energy of 2 s orbital of hydrogen atom is greater than that of 2 s orbital of lithium.
Reason R: Energies of the orbitals in the same subshell decrease with increase in the atomic number.
In the light of the above statements, choose the correct answer from the options given below.

Solution

Chemistry ma’am · 4 steps

A bigger nucleus pulls the same 2s orbital in tighter, so its energy drops as Z goes up.

1. Hydrogen 2s. For hydrogen, Eₙ = -13.6/n² eV, so the 2s orbital sits at -3.4 eV.
2. Lithium 2s. Lithium has 3 protons. Even after the two 1s electrons shield some of that charge, the 2s electron feels more nuclear pull than in hydrogen. Removing it takes 5.39 eV, so lithium's 2s sits near -5.39 eV, lower than -3.4 eV.
3. Check A and R. A is true: -3.4 eV is higher than -5.39 eV. R is the general rule behind it: a larger atomic number means a stronger effective nuclear charge, which lowers the energy of the same subshell.
4. So. Both statements are true, and R directly explains A. The answer is (a).

The idea to keep. The same orbital is not at the same energy in every atom. More protons give a stronger pull, so 1s, 2s and 2p all sink lower as atomic number rises.

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