Ncert Books Class 11 Chemistry Chapter 11 Free Download

From Group 13, you can expect 3-mark questions that test deep conceptual understanding. Some frequently asked types include:

• Explaining why Boron exhibits anomalous behaviour compared to other elements in its group.
• Describing the unique bridged structure of diborane (B₂H₆), including its 'banana bonds'.
• Reasoning why Boron trifluoride (BF₃) acts as a Lewis acid.
• Explaining the trend in stability of +1 and +3 oxidation states down the group due to the inert pair effect. For example, 'Why is Thallium (I) more stable than Thallium (III)?'

The inert pair effect is a high-yield topic for exams. It is usually tested through reasoning-based questions. The inert pair effect is the reluctance of the outermost ns² electrons to participate in bonding, which increases down a group. For exams, you should be prepared for questions like:

• Why does Ga have a lower atomic radius than Al?
• Explain why PbCl₄ is a strong oxidising agent and less stable than PbCl₂.
• Compare the stability of +2 and +4 oxidation states for elements of Group 14 (C, Si, Ge, Sn, Pb).
• Why does Indium (In) show +1 and +3 oxidation states, but +1 is more common for Thallium (Tl)?

This is a classic important question based on structure and bonding. The difference arises from the hybridisation and arrangement of carbon atoms:

• In diamond, each carbon atom is sp³ hybridised and bonded to four other carbon atoms in a rigid tetrahedral lattice. All four valence electrons are used in forming strong covalent bonds, so there are no free electrons to conduct electricity.
• In graphite, each carbon atom is sp² hybridised and bonded to three other carbon atoms, forming hexagonal layers. The fourth valence electron of each carbon atom is delocalised in the layers, forming a cloud of free electrons. These mobile electrons are responsible for graphite's electrical conductivity.

For 5-mark questions from Group 14, expect comprehensive questions that combine multiple concepts. Likely questions include:

• (a) Comparing the properties (hardness, conductivity, structure) of diamond and graphite. (b) Explaining why carbon shows catenation to a much greater extent than silicon.
• (a) What are silicones? (b) How are they prepared? (c) List three important properties and two uses.
• Explaining the trends in atomic radius, ionisation enthalpy, and electronegativity as you move down Group 14.
• (a) Why is CO₂ a gas while SiO₂ is a solid? (b) Describe the structures of both.

To score well on this chapter, a systematic approach is needed:

• Focus on Trends: Don't just memorise properties. Understand the 'why' behind trends like ionisation enthalpy, atomic size, and oxidation states.
• Master the Exceptions: Pay special attention to the anomalous behaviour of Boron and Carbon, and exceptions like the atomic radius of Gallium. These are frequently asked.
• Practice Structures: Be thorough with the structures of key compounds like diborane, diamond, graphite, and SiO₂.
• Use Summary Notes: Create a table for important compounds (Borax, Boric acid, Silicones) with their formula, preparation, properties, and uses for quick revision.
• Solve Reasoning Questions: This chapter is full of 'Give Reason' questions. Practice them extensively to build conceptual clarity.

This difference is a crucial concept tested in exams and relates to the ability to form multiple bonds.

• Carbon, due to its small size and high electronegativity, forms strong pπ-pπ multiple bonds with oxygen. This results in discrete, linear CO₂ molecules. The only forces between these molecules are weak van der Waals forces, which require little energy to overcome, making CO₂ a gas.
• Silicon is larger and cannot form stable pπ-pπ bonds. Instead, it forms four strong Si-O single bonds with neighbouring oxygen atoms. This creates a giant three-dimensional covalent network structure (SiO₄ tetrahedra), which is very strong and requires a large amount of energy to break, making SiO₂ a high-melting solid.

Understanding the anomalous properties of Boron is key. The important differences from Aluminium are:

• Nature of Oxides: Boron forms acidic oxides (e.g., B₂O₃), while Aluminium forms amphoteric oxides (Al₂O₃), meaning it reacts with both acids and bases.
• Halides: Boron halides (like BCl₃) are monomeric and act as Lewis acids, whereas Aluminium halides (like AlCl₃) exist as dimers (Al₂Cl₆) in the vapour phase to complete their octet.
• Reaction with Alkalies: Boron does not react with non-oxidising acids and alkalies, but Aluminium dissolves in alkalies to form aluminates, liberating hydrogen gas.