Given below are two statements. One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Energy of $$2s$$ orbital of hydrogen atom is greater than that of $$2s$$ 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

We need to evaluate the assertion and reason about orbital energies of hydrogen and lithium.

The assertion (A) states that the energy of the $$2s$$ orbital of hydrogen atom is greater than that of the $$2s$$ orbital of lithium. The reason (R) given is that energies of the orbitals in the same subshell decrease with increase in the atomic number.

For hydrogen ($$Z = 1$$), the energy of the $$2s$$ orbital is:

$$E = -\frac{13.6}{n^2} = -\frac{13.6}{4} = -3.4 \text{ eV}$$

For lithium ($$Z = 3$$), the effective nuclear charge experienced by the $$2s$$ electron is greater than 1 due to incomplete shielding. The $$2s$$ electron in lithium is more tightly bound (has more negative energy) than the $$2s$$ electron in hydrogen. Since the energy of the $$2s$$ orbital in hydrogen is less negative (higher/greater) than that in lithium, the assertion is true.

As atomic number increases, the nuclear charge increases. For the same subshell, the increased nuclear charge (even after accounting for shielding) pulls the electrons closer to the nucleus, making the energy more negative (lower). So the energy of orbitals in the same subshell decreases (becomes more negative) with increasing atomic number, and the reason is true.

The increased nuclear attraction in lithium directly explains why the $$2s$$ energy in hydrogen (lower $$Z$$) is higher (less negative) than in lithium (higher $$Z$$), so the reason correctly explains the assertion. The correct answer is Option A: Both A and R are true and R is the correct explanation of A.