CBSE Class 11 Chemistry Chapter 11: The p-Block Elements NCERT Solutions

(i) Variation in oxidation states from Boron (B) to Thallium (Tl) (Group 13):

The general electronic configuration of Group 13 elements is $ns^2\; np^1$. Consequently, the most common oxidation state exhibited by these elements is expected to be +3, involving the loss of all three valence electrons. However, the stability of oxidation states varies down the group:

• Boron (B) and Aluminium (Al): These elements predominantly exhibit the +3 oxidation state. For Aluminium, the +3 state is highly stable.
• Gallium (Ga), Indium (In), and Thallium (Tl): These heavier elements exhibit both +1 and +3 oxidation states. As we move down the group from Ga to Tl, the +1 oxidation state becomes progressively more stable, while the +3 oxidation state becomes less stable.

This trend is attributed to the inert pair effect. The two electrons in the outermost s-orbital ($ns^2$) are increasingly attracted by the nucleus and become less available for bonding in heavier elements. This makes the +1 oxidation state (involving only the $np^1$ electron) more stable.

For example:

• Ga(+1) is unstable.
• In(+1) is fairly stable.
• Tl(+1) is very stable and Tl(+3) is a strong oxidizing agent.

The stability of the +3 oxidation state decreases significantly on moving down the group.

Summary Table for Group 13:

(ii) Variation in oxidation states from Carbon (C) to Lead (Pb) (Group 14):

The general electronic configuration of Group 14 elements is $ns^2\; np^2$. Thus, they are expected to show a +4 oxidation state by losing or sharing all four valence electrons. However, similar to Group 13, the stability of oxidation states changes down the group:

• Carbon (C) and Silicon (Si): These elements predominantly exhibit the +4 oxidation state.
• Germanium (Ge), Tin (Sn), and Lead (Pb): These heavier elements show both +2 and +4 oxidation states. As we move down the group, the +2 oxidation state becomes increasingly stable, while the +4 oxidation state becomes less stable.

This trend is again due to the inert pair effect. The $ns^2$ electrons become more reluctant to participate in bonding in heavier elements, favoring the +2 oxidation state (involving only the $np^2$ electrons).

Consequently:

• The stability of the +2 oxidation state increases down the group (Pb(+2) is very stable).
• The stability of the +4 oxidation state decreases down the group (Pb(+4) compounds are strong oxidizing agents).

Summary Table for Group 14:

Both Boron (B) and Thallium (Tl) belong to Group 13 of the periodic table. The general trend in Group 13 is that the +1 oxidation state becomes more stable as we move down the group, while the +3 oxidation state becomes less stable. This is due to the inert pair effect, where the $ns^2$ electrons become increasingly reluctant to participate in bonding.

Boron Trifluoride (BCl₃): Boron is the first element in Group 13. Its +3 oxidation state is its most stable oxidation state. Therefore, BCl₃ is a stable compound.

Thallium Trifluoride (TlCl₃): Thallium is at the bottom of Group 13. Due to the pronounced inert pair effect, the +1 oxidation state is significantly more stable for Thallium than the +3 oxidation state. Consequently, Tl(+3) compounds like TlCl₃ are highly oxidizing and tend to readily revert to the more stable Tl(+1) state. This makes TlCl₃ less stable compared to BCl₃.

Boron trifluoride (BF₃) behaves as a Lewis acid because the central boron atom is electron-deficient. Boron has the electronic configuration $1s^2\; 2s^2\; 2p^1$. In BF₃, boron forms three covalent bonds with three fluorine atoms, using its one 2p electron and two 2s electrons. This results in boron being surrounded by only six valence electrons (three bonding pairs), leaving its octet incomplete.

According to Lewis acid-base theory, a Lewis acid is a substance that can accept an electron pair. Since the boron atom in BF₃ has an incomplete octet and a vacant p-orbital, it readily accepts an electron pair from a Lewis base to form a coordinate covalent bond, thus completing its octet. This ability to accept an electron pair is characteristic of a Lewis acid.

The structure shows a vacant p-orbital on the boron atom, which is available for accepting electrons: