Question

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.

• A. Both A and R are true and R is the correct explanation of A.
• B. Both A and R are true but R is NOT the correct explanation of A.
• C. A is true but R is false.
• D. A is false but R is true.

Answer 1

The Correct Option is A

To understand the given statements, let's analyze both the assertion and the reason: 

1. Assertion A: The energy of the 2s orbital of a hydrogen atom is greater than that of the 2s orbital of lithium.
   • The energy levels of orbitals within an atom depend on both the principal quantum number (\(n\)) and the effective nuclear charge experienced by the electrons.
   • In a hydrogen atom, only one electron is present, and it experiences the full nuclear charge, as there is no shielding effect from other electrons.
   • In a lithium atom, which has a higher atomic number, there are other electrons that cause repulsion, and the effective nuclear charge felt by the 2s electron is less than the actual nuclear charge. Thus, the electron is more tightly bound due to the higher nuclear charge, leading to lower energy of the orbital.
2. Reason R: Energies of the orbitals in the same subshell decrease with an increase in the atomic number.
   • The Reason R states a general principle in atomic structure: as the atomic number increases, more protons are present, increasing the nuclear charge and making the electrons in the same subshell more tightly bound.
   • This results in a decrease in the energy of the orbitals because the electrons are held closer to the nucleus due to the increased attraction.

Conclusion:

• Both Assertion A and Reason R are true. As we move from hydrogen to lithium, the increasing atomic number results in the 2s orbital of lithium having a lower energy compared to hydrogen, as explained by Reason R.
• Therefore, Reason R correctly explains why the energy of the 2s orbital in lithium is less than that in hydrogen.

Thus, the correct answer is: Both A and R are true and R is the correct explanation of A.

Answer 2

As the atomic number increases then the potential energy of electrons present in same shell becomes more and more negative. And therefore total energy also becomes more negative.

\(E_{total} = -13.6 \frac{z^2}{n^2}eV\)

∴ Energies of the orbitals in the same subshell decreases with increase in atomic number.

The correct answer is (A): Both A and R are true and R is the correct explanation of A