Question

Which of the following statements concerning effective nuclear charge are correct?

I: The most outer electrons in O2− and F− experience the same effective nuclear charge

II: For carbon, the 2s electrons experience a greater effective nuclear charge than the 2p electrons.

III: In lithium, all three electrons experience an identical effective nuclear charge.

Answer is II. only , please explain

Answer 1

There are two terms that we need to understand:

Shielding effect: It is basically the inter-electronic repulsion occurring between the inner electrons in the core of an atom with that of the outer electrons present in the valence shell. So as the outer valence electrons are shielded by the inner electrons hence the outer valence electrons do not feel the more attractive force of the nucleus and hence the outer electrons are loosely bound and can be easily ionized.

Orbital penetration effect: It is basically a effect that just explains how close the electrons can come with nucleus and more close the electrons are with the nucleus so more attractive force of nucleus they feel and are more closely bound with the nucleus. The penetration effect of electrons would depend on the orbital in which electrons are residing.

The orbitals having more electron density near the nucleus would have greater penetration effect as compared to orbital having lesser electron density near the nucleus.

The penetration effect of orbital having same value of principal quantum number (n) follows this order:

s>p>d>f

The penetration effect for electrons having different values of n is:

1s>2s>2p>3s>3p>4s>3d>4p etc.

Effective nuclear charge: On account of shielding effect the outer valence electrons are not able to feel the complete charge present on the nucleus and they feel less nuclear charge so this is known as the effective nuclear charge that is the charge actually felt by the outer valence electrons.

The 2^nd statement is correct

Since the penetration power of a 2s orbital is more than the 2p orbital so the electron present in 2s orbital is more attracted to the nucleus than the electron present in 2p. So the 2s orbital feels more charge of nucleus as compared to the 2p orbital hence the effective nuclear charge felt by the 2s electrons would be more as compared to the electrons present in 2p orbitals.

The 2s electron feels more nuclear charge as compared to the 2p electron and hence the effective nuclear charge of 2s electron would be more than the 2p electron

The 3^rd statement is incorrect as all the three electrons in Lithium would not feel the same effective nuclear charge.

Li=1s²,2s¹

The only electron present in 2s orbital would feel less effective nuclear charge as compared to the electrons present in 1s orbital because the 1s electron have more penetration power than the 2s electron.

The 1^st statement is incorrect as the effective nuclear charge felt by O^2- and F^- would not be same

O^2-=1s²,2s²,2p⁶

F^-=1s²,2s²,2p⁶,

The effective nuclear charge felt by F- would be higher than felt by O^2- because the number of protons in F^- is higher than the number of protons in O^2- so the overall attractive force of nucleus would be more for F^- and so the effective nuclear charge felt by F^- would also be greater than O^2-