Level 1: Remembering (Knowledge-Based Questions)
1. Define the following:
(a) Valence electrons(b) Valency
Answer:
(a) Valence electrons are the electrons present in the outermost shell of an atom.(b) Valency is the combining capacity of an atom, i.e., the number of electrons it can lose, gain, or share.
Explanation:
Atoms tend to achieve stability by completing their outermost shell (duplet or octet). Valence electrons participate in bonding, and valency indicates how many electrons are involved in this process.
2. Name one example each of:
(a) A molecule having a single covalent bond(b) A molecule having a double covalent bond
(c) A molecule having a triple covalent bond
Answer:
(a) Hydrogen (H₂)(b) Oxygen (O₂)
(c) Nitrogen (N₂)
Explanation:
Covalent bonds are formed by sharing electrons:
Double bond → two pairs shared
Triple bond → three pairs shared
3. What is a lone pair of electrons?
Name one molecule in which the oxygen atom has two lone pairs of electrons.
Answer:
A lone pair is a pair of valence electrons not involved in bonding.
Example: Water (H₂O)
Explanation:
In water, oxygen has six valence electrons. It forms two bonds with hydrogen and retains four electrons as two lone pairs.
4. Give one example each of:
(a) A compound with a polar covalent bond(b) A compound with a non-polar covalent bond
Answer:
(a) Hydrogen chloride (HCl) or Water (H₂O)(b) Methane (CH₄) or Chlorine (Cl₂)
Explanation:
Polar covalent bonds form when electrons are shared unequally due to electronegativity difference.Non-polar covalent bonds form when electrons are shared equally or nearly equally.
5. Name one nitrogen-containing compound that:
(a) Contains only covalent bonds(b) Contains covalent bonds and a lone pair of electrons
(c) Contains both covalent and ionic bonds
Answer:
(a) Nitrogen (N₂)(b) Ammonia (NH₃)
(c) Ammonium chloride (NH₄Cl)
Explanation:
N₂ has a triple covalent bond.NH₃ has three covalent bonds and one lone pair on nitrogen.
NH₄Cl contains covalent bonds within NH₄⁺ and an ionic bond between NH₄⁺ and Cl⁻.
Level 2: Understanding-based Questions
1. Why do covalent compounds have low melting points?
Answer:
Covalent compounds have low melting points because the forces between their molecules are weak.
Explanation:
Covalent compounds consist of separate molecules. While the bonds within each molecule are strong, the intermolecular forces are weak, so only a small amount of heat is needed to separate the molecules.
2. State the type of bond present in each of the following:
(a) Water(b) Nitrogen
(c) Magnesium oxide
(d) Calcium chloride
Answer:
(a) Water – Polar covalent bond(b) Nitrogen – Triple covalent bond
(c) Magnesium oxide – Ionic (electrovalent) bond
(d) Calcium chloride – Ionic (electrovalent) bond
Explanation:
Water and nitrogen involve sharing of electrons (covalent bonding).Water is polar due to unequal sharing of electrons.
Magnesium oxide and calcium chloride are formed by transfer of electrons from metals to non-metals, resulting in ionic bonds.
3. What type of bonding is formed between atoms that do not easily lose or gain electrons?
Answer:
Covalent bonding.
Explanation:
Such atoms (usually non-metals) achieve stability by sharing electrons instead of transferring them, leading to the formation of covalent bonds.
4. Why is electrolysis not possible in covalent compounds in the liquid state?
Answer:
Because covalent compounds do not contain free ions.
Explanation:
Electrolysis requires free-moving ions to conduct electricity. Covalent compounds consist of neutral molecules and do not dissociate into ions in the liquid state.
5. Where are the elements that form covalent compounds located in the periodic table?
Answer:
On the right side of the periodic table (non-metals).
Explanation:
Non-metals tend to share electrons due to high electronegativity, leading to the formation of covalent bonds instead of ionic bonds.
Level 3: Application-Based Questions (Using Concepts)
1. Oxygen (O₂) and Nitrogen (N₂) are covalent molecules.
Draw their electron dot structures and compare their bonding.
Answer:
Oxygen (O₂): Double covalent bond (O=O) – two pairs of electrons sharedNitrogen (N₂): Triple covalent bond (N≡N) – three pairs of electrons shared
Explanation:
Oxygen has 6 valence electrons and needs 2 more to complete its octet, so it shares two pairs. Nitrogen has 5 valence electrons and needs 3 more, so it shares three pairs. Hence, nitrogen forms a triple bond while oxygen forms a double bond—this shows how valency determines bond multiplicity.
2. An element A has atomic number 7.
Determine the number of electrons in its outer shell that do not participate in bonding when it forms a molecule.
Answer:
Two electrons
Explanation:
Atomic number 7 corresponds to nitrogen (2,5). In N₂, each nitrogen shares three electrons to form a triple bond. The remaining two electrons form a lone pair and do not participate in bonding.
3. Elements A, B, and C have atomic numbers 12, 14, and 16 respectively.
(a) Predict the type of compound formed between A and C and represent its Lewis structure.
(b) Write the formulae of the compounds formed when A, B, and C react with hydrogen.
Answer:
(a) Compound: Magnesium sulphide (MgS), ionic bond → Mg²⁺ [:S:]²⁻
(b) Hydrides:
- A (Mg): MgH₂
- B (Si): SiH₄
- C (S): H₂S
Explanation:
A (Mg) is a metal with 2 valence electrons and transfers them to C (S), a non-metal with 6 valence electrons, forming an ionic compound. With hydrogen:
- Metals like Mg form ionic hydrides (MgH₂)
- Non-metals like Si and S form covalent compounds (SiH₄, H₂S) by sharing electrons
4. Based on the given electronic configurations, classify the elements as electropositive or electronegative:
Mg(2,8,2), Al(2,8,3), N(2,5), C(2,4), Li(2,1), O(2,6), Br(2,8,18,7), Ne(2,8)
Answer:
- Electropositive: Li, Mg, Al
- Electronegative: N, O, C, Br
Explanation:
Electropositive elements (metals) have fewer valence electrons and tend to lose them. Electronegative elements (non-metals) have more valence electrons and tend to gain or share electrons. Neon is inert due to a complete octet.
5. Using your answer from the previous question, predict the ionic charges formed by the electronegative elements.
Answer:
- Nitrogen (N): −3
- Oxygen (O): −2
- Bromine (Br): −1
Explanation:
These elements gain electrons to complete their octet:
- N gains 3 → N³⁻
- O gains 2 → O²⁻
-
Br gains 1 → Br⁻
Carbon generally shares electrons instead of forming ions.
Level 4: Analysis-Based Questions (Break, Compare, Examine)
1. Compare the bonding and properties of sodium chloride (NaCl) and methane (CH₄). Explain why their melting points differ significantly.
Answer:
NaCl has ionic bonding, while CH₄ has covalent bonding. NaCl has a high melting point, whereas CH₄ has a low melting point.
Explanation:
NaCl forms a giant ionic lattice with strong electrostatic forces between ions, requiring a large amount of energy to break. CH₄ consists of discrete molecules with weak intermolecular forces, so less energy is needed to melt it.
2. Analyse why water (H₂O) is a polar molecule while carbon dioxide (CO₂) is non-polar, even though both contain polar bonds.
Answer:
H₂O is polar, while CO₂ is non-polar.
Explanation:
In H₂O, the bent shape causes unequal distribution of charge, resulting in a net dipole moment. In CO₂, the linear shape causes dipoles to cancel out, making the molecule non-polar despite having polar bonds.
3. A student says, “All compounds formed between non-metals are non-polar.” Examine this statement and justify whether it is correct.
Answer:
The statement is incorrect.
Explanation:
Compounds formed between non-metals are covalent, but they can be either polar or non-polar depending on the electronegativity difference and molecular shape. For example, HCl is polar, while Cl₂ is non-polar.
4. Analyze the bonding in ammonium chloride (NH₄Cl) and explain how it contains both ionic and covalent bonds.
Answer:
NH₄Cl contains both ionic and covalent bonds.
Explanation:
Within the ammonium ion (NH₄⁺), nitrogen and hydrogen are bonded covalently (including a coordinate bond). The ammonium ion and chloride ion (Cl⁻) are held together by ionic bonding, making it a compound with mixed bonding.
5. Examine why magnesium oxide (MgO) conducts electricity in the molten state but not in the solid state.
Answer:
MgO conducts electricity in the molten state but not in the solid state.
Explanation:
In solid MgO, ions are fixed in a rigid lattice and cannot move. In the molten state, ions become free to move and carry an electric charge, allowing conduction.
Level 5: Evaluation-Based Questions (Justify, Critique, Defend)
1. Evaluate the factors responsible for the formation of an ionic bond—low ionization energy, high electron affinity, and high lattice energy.
Which factor plays the most significant role in ensuring stability? Justify your answer.
Answer:
High lattice energy plays the most significant role in ensuring stability.
Explanation:
Although low ionization energy and high electron affinity contribute to ion formation, the overall stability depends on the large amount of energy released when oppositely charged ions form a crystal lattice. This lattice energy outweighs the energy required for ionization, making the compound stable.
2. Justify why ionic solids are described as hard yet brittle.
Answer:
Ionic solids are hard due to strong electrostatic forces, but brittle because they break when layers shift.
Explanation:
The strong attraction between oppositely charged ions makes ionic solids hard. However, when a force is applied, layers shift and bring like charges together, causing repulsion and leading to the crystal breaking.
3. A student claims that covalent and ionic compounds should have similar melting points since both can be crystalline.
Critically evaluate this statement.
Answer:
The statement is incorrect.
Explanation:
Ionic compounds have strong electrostatic forces in a lattice, requiring high energy to break. Covalent compounds consist of molecules held by weak intermolecular forces, so they melt at lower temperatures despite sometimes being crystalline.
4. Assess why atoms form bonds instead of remaining as individual atoms.
Answer:
Atoms form bonds to achieve a lower-energy, more stable state.
Explanation:
Isolated atoms often have incomplete valence shells, making them unstable. By bonding, they achieve a complete octet or duplet, lowering their energy and increasing stability.
5. Evaluate why water (H₂O) and ammonia (NH₃) are classified as polar covalent compounds.
Answer:
They are polar due to unequal sharing of electrons.
Explanation:
In both molecules, electrons are drawn toward more electronegative atoms (O in H₂O, N in NH₃), creating partial charges. This unequal distribution results in polarity.
Level 6: Creation-Based Questions (Design, Formulate, Construct)
1. A chemical manufacturer needs to design a liquid compound (Compound P) with the following properties:
- Low melting and boiling points
- Good electrical conductivity in aqueous solution
Design the type of bonding required in Compound P and explain the nature of the electron pair involved.
Answer:
Compound P should contain polar covalent bonds.
Explanation:
To achieve low melting and boiling points, the compound must be covalent, as covalent molecules have weak intermolecular forces. However, for electrical conductivity in aqueous solution, the compound must ionize in water. Polar covalent compounds fulfill both conditions—they have unequal sharing of electrons, creating partial charges that allow them to form ions in solution and conduct electricity.
2. You are asked to create a quick-learning guide to help beginners distinguish between electrovalent and covalent bonds.
Formulate two clear rules—one for each bond type—based on electron origin and electron movement.
Answer:
- Electrovalent (Ionic) Bond Rule: Formed by complete transfer of electrons from a metal to a non-metal.
- Covalent Bond Rule: Formed by mutual sharing of electron pairs between non-metal atoms.
Explanation:
In ionic bonding, electrons originate from a metal atom and are transferred to a non-metal, forming oppositely charged ions. In covalent bonding, both atoms contribute electrons and share them to achieve stable electronic configurations.
Take Your Preparation to the Next Level
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