Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

The stability of dihalides of \(\mathrm{Si}\), Ge, \(\mathrm{Sn}\) and \(\mathrm{Pb}\) increases steadily in the sequence (a) \(\mathrm{SiX}_{2}<\mathrm{GeX}_{2}<\mathrm{PbX}_{2}<\mathrm{Sn} \mathrm{X}_{2}\) (b) \(\mathrm{SiX}_{2}<\mathrm{GeX}_{2}<\mathrm{SnX}_{2}<\mathrm{PbX}_{2}\) (c) \(\mathrm{PbX}_{2}<\mathrm{SnX}_{2}<\mathrm{GeX}_{2}<\mathrm{SiX}_{2}\) (d) \(\mathrm{GeX}_{2}<\mathrm{SiX}_{2}<\mathrm{SnX}_{2}<\mathrm{PbX}_{2}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{SiX}_{2}<\mathrm{GeX}_{2}<\mathrm{SnX}_{2}<\mathrm{PbX}_{2}\) is correct.

Step 1: Identify the Group Trend

The elements Si, Ge, Sn, and Pb belong to the same group (14) of the periodic table. As we move down the group, the inert pair effect becomes more pronounced. This effect is due to poor shielding by d- and f-electrons, causing the electrons in the s-orbital to be less available for bonding, making the +2 oxidation state more stable.

Step 2: Analyze Stability in +2 Oxidation State

Due to the inert pair effect, as we move down the group from Si to Pb, the stability of the +2 oxidation state increases. This means that for Pb, the +2 oxidation state is quite stable compared to the +4 oxidation state, which is opposite for Si.

Step 3: Determine Correct Sequence Based on Stability

Applying our understanding from Step 2, the correct sequence of increasing stability for dihalides (MX2) should reflect the increasing stability of the +2 oxidation state. Thus, PbX2 should be the most stable, followed by SnX2, then GeX2, with SiX2 being the least stable.

Key Concepts

Stability of Oxidation States

The concept of oxidation states pertains to the charge of an atom within a molecule, describing how many electrons are gained or lost from an atom compared to its elemental state. In the context of group 14 elements, such as Silicon (Si), Germanium (Ge), Tin (Sn), and Lead (Pb), oxidation states of +2 and +4 are commonly observed.

The stability of these oxidation states shifts as you move down the group, primarily due to the inert pair effect. This effect influences the availability of the s-electrons for bonding. For heavier elements like Sn and Pb, the +2 oxidation state becomes more stable compared to the +4 state. This is because the s-electrons (which tend to remain non-bonding due to poor shielding by d- and f-electrons) make the +2 state energetically favorable. Thus, dihalides, or compounds where two halogen atoms are bonded to a central atom, display varying stability based on this effect.

Group 14 Elements

Group 14 of the periodic table includes Carbon (C), Silicon (Si), Germanium (Ge), Tin (Sn), and Lead (Pb). These elements exhibit a diverse range of chemical behaviors.

- Carbon is a non-metal. - Silicon and Germanium are metalloids, demonstrating properties of both metals and non-metals. - Tin and Lead are metals.

As you progress down the group, several patterns emerge:

• The atomic size increases, making extra-nuclear electrons less tightly held.
• Ionization energy decreases—it's easier for the atoms to lose electrons.
• Electronegativity decreases, which affects how these atoms form bonds with other elements.
  

This progression results in different stability of oxidation states. In tin and lead, the higher atomic number and larger electron cloud significantly stabilize the +2 oxidation state due to the inert pair effect.

Periodic Trends

Periodic trends refer to the predictable changes in properties of elements as you move across periods or down groups in the periodic table. In group 14, a few key trends can be observed:

• Atomic Size: As you move down the group, atomic size increases. This results from additional electron shells being occupied.
• Metallic Nature: Transitioning from non-metallic Carbon to metallic Lead demonstrates how elements become more metallic down a group.
• Reactive Nature: As atomic size increases, elements' ability to gain or lose electrons changes, affecting their reactivity.
• Oxidation States: Higher oxidation states are more stable for lighter elements like Si and Ge, whereas lower oxidation states become prevalent for heavier elements like Sn and Pb due to the inert pair effect.
  

Understanding these trends helps predict chemical reactions and the type of compounds formed by these elements.

Dihalides

Dihalides are chemical compounds featuring a central atom bound to two halogen atoms. These compounds are particularly important when discussing the chemistry of group 14 elements.

The stability of dihalides in group 14 shows a clear trend:

• Silicon Dihalides ( SiX₂): Least stable due to a preference for the +4 oxidation state.
• Germanium Dihalides ( GeX₂): More stable than silicon dihalides, yet less so compared to tin and lead dihalides.
• Tin Dihalides ( SnX₂): Quite stable, owing to the balance between +2 and +4 oxidation states.
• Lead Dihalides ( PbX₂): Most stable dihalides among the group 14 elements; the +2 oxidation state is particularly stable due to the inert pair effect.

The stabilization trend showcases the periodic trend of increasing preference for lower oxidation states as you move from Si to Pb, influenced by the inert pair effect and other periodic trends.