Part 1: Multiple-Choice Questions (MCQs)
1. When molten copper and molten zinc are mixed together, they give rise to a new substance called brass. Predict what type of bond is formed between copper and zinc.
Explanation:
Brass is an alloy, a homogeneous mixture of metals. The bonding in alloys is metallic, characterized by a ‘sea of delocalized electrons’ surrounding the metal cations (Cu²⁺ and Zn²⁺ in this case).
2. Which element is capable of forming all three types of bonds; covalent, coordinate covalent, and ionic?
Explanation:
Covalent bond: e.g., in H₂O and O₂.Coordinate covalent bond: e.g., in H₃O⁺ (O donates a lone pair to H⁺).
Ionic bond: e.g., in metal oxides like CaO (O gains electrons to form O²⁻).
3. Why is H₂O a liquid while H₂S is a gas?
Explanation:
The strong forces are hydrogen bonds. H₂O molecules form extensive hydrogen bonding due to the high electronegativity of O and small size of H, requiring more energy to separate them. H₂S molecules experience much weaker dipole-dipole forces.
4. Which of the following bonds is expected to be the weakest?
Explanation:
The O-O single bond (e.g., in peroxides like H₂O₂) is exceptionally weak due to strong lone-pair repulsions on the two small, highly electronegative oxygen atoms. While F-F is also weak, the O-O bond is generally weaker.
5. Which form of carbon is used as a lubricant?
Explanation:
Graphite has a layered structure where atoms within a layer are held together by strong covalent bonds, but the layers are held together by weak London dispersion forces. These layers can slide over each other easily, making it an excellent solid lubricant.
6. Keeping in view the intermolecular forces of attraction, indicate which compound has the highest boiling point.
Explanation:
H₂O has the strongest intermolecular forces. It can form two hydrogen bonds per molecule, leading to a massive network. This gives water an anomalously high boiling point compared to other Group 16 hydrides. HF and NH₃ also have hydrogen bonding, but it is stronger and more extensive in H₂O.
7. Which metal has the lowest melting point?
Explanation:
Moving down Group 1 (alkali metals), the atomic size increases. The strength of the metallic bond decreases because the single valence electron is farther from the nucleus and less effectively holds the cations together. Thus, Rb has the weakest metallic bonds and the lowest melting point.
8. Which ionic compound has the highest melting point?
Explanation:
For ions of similar charge, the melting point increases as the size of the ions decreases (lattice energy increases). Lithium chloride (LiCl) is an exception to this trend. While lithium is the smallest alkali metal, LiCl has a lower melting point than NaCl. This is because the small size and high charge density of the lithium ion (Li⁺) cause it to polarize the chloride ion (Cl⁻), leading to a significant covalent character in the LiCl bond.
9. Which compound contains both covalent and ionic bonds?
Explanation:
NH₄Cl contains:
- Covalent bonds: Within the ammonium ion (NH₄⁺), N and H atoms are bonded covalently.
- Ionic bond: The NH₄⁺ ion and the Cl⁻ ion are held together by an ionic bond.
10. Which among the following has a double covalent bond?
Explanation:
Ethylene has the formula H₂C=CH₂, which contains a carbon-carbon double bond. Ethane has only single bonds, and acetylene has a carbon-carbon triple bond.
Part 2: Short Answer Questions
1. What type of elements lose their outer electron easily and what type of elements gain electron easily?
Answer: Metals (especially alkali and alkaline earth metals) lose their outer electrons easily to form cations. Non-metals (especially halogens and oxygen) gain electrons easily to form anions.
2. Why do lower molecular mass covalent compounds exist as gases or low boiling liquids?
Answer: Lower molecular mass covalent compounds have weak intermolecular forces (like London dispersion forces) between their molecules. Very little energy is required to overcome these forces, allowing them to vaporize easily into gases or boil at low temperatures.
3. Give one example of an element which exists as a crystalline solid and it has covalent bonds between its atoms.
Answer: Diamond (a form of carbon) or Silicon (Si). Both form giant covalent network structures held together by covalent bonds, making them very hard crystalline solids.
4. Which property of metals makes them malleable and ductile?
Answer: The ability of layers of positive metal ions to slide over one another without breaking the metallic bond. This is due to the non-directional nature of the “sea of delocalized electrons,” which continues to hold the lattice together even when layers are displaced.
5. Is coordinate covalent bond a strong bond?
Answer: Yes, once formed, a coordinate covalent bond is identical in strength and properties to a regular covalent bond. The difference is only in the method of formation.
6. Write down dot and cross formula of HNO₃.
Answer:
Dot and cross diagram for HNO₃ would appear here
(Image representation of HNO₃ structure with electron dots and crosses)
(Image representation of HNO₃ structure with electron dots and crosses)
Part 3: Constructed Response Questions
1. Why HF is a liquid while HCl is a gas?
Answer: HF molecules form very strong intermolecular hydrogen bonds due to the high electronegativity of fluorine and the small size of the H and F atoms. HCl molecules, on the other hand, only experience weaker dipole-dipole interactions, so it is a gas.
2. Why covalent compounds are generally not soluble in water?
Answer: Most covalent compounds are non-polar or have low polarity. Water is a highly polar solvent. Since “like dissolves like,” the non-polar covalent molecules cannot effectively interact with or break the strong hydrogen-bonded network of water molecules, making them insoluble or poorly soluble.
3. How do metals conduct heat?
Answer: When heat is applied to one part of a metal, the delocalized electrons gain kinetic energy and move faster. These energetic electrons quickly transfer the thermal energy by colliding with other electrons and metal ions throughout the structure, conducting heat efficiently.
4. How many oxides does nitrogen form. Write down the formulae of oxides?
Answer: Nitrogen forms several oxides. The common ones are:
- Nitrous oxide: N₂O
- Nitric oxide: NO
- Dinitrogen trioxide: N₂O₃
- Nitrogen dioxide: NO₂
- Dinitrogen tetroxide: N₂O₄ (dimer of NO₂)
- Dinitrogen pentoxide: N₂O₅
5. What will happen if NaBr is treated with AgNO₃ in water?
Answer: A pale yellow precipitate of silver bromide (AgBr) will form. This is a characteristic precipitation reaction used to test for bromide ions (Br⁻).
Chemical Equation: NaBr(aq) + AgNO₃(aq) → AgBr(s)↓ (pale yellow) + NaNO₃(aq)
Chemical Equation: NaBr(aq) + AgNO₃(aq) → AgBr(s)↓ (pale yellow) + NaNO₃(aq)
6. Why does iodine exist as a solid while Cl₂ exist as a gas?
Answer: Iodine (I₂) has a much larger molecular size than chlorine (Cl₂). Larger molecules have stronger London dispersion forces (instantaneous dipole-induced dipole forces) between them. These forces are strong enough in iodine to hold the molecules in a solid-state at room temperature, whereas they are much weaker in smaller chlorine molecules, allowing it to be a gas.
Part 4: Descriptive Questions (Key Points)
1. Explain the formation of an ionic bond and a covalent bond.
Ionic Bond: Formed by the complete transfer of electrons from a metal atom to a non-metal atom. The metal becomes a cation (+) and the non-metal becomes an anion (-). The electrostatic attraction between these oppositely charged ions forms the ionic bond (e.g., NaCl).
Covalent Bond: Formed by the mutual sharing of electrons between two non-metal atoms. Each atom contributes one electron to the shared pair to achieve a stable noble gas configuration (e.g., H₂, CH₄).
Covalent Bond: Formed by the mutual sharing of electrons between two non-metal atoms. Each atom contributes one electron to the shared pair to achieve a stable noble gas configuration (e.g., H₂, CH₄).
2. How do ions arrange themselves to form NaCl crystal?
Answer: In a NaCl crystal, each Na⁺ ion is surrounded by six Cl⁻ ions, and each Cl⁻ ion is surrounded by six Na⁺ ions. This regular, repeating, three-dimensional pattern forms a face-centered cubic crystal lattice. This arrangement maximizes the attractive forces between oppositely charged ions and minimizes the repulsive forces between like charges.
3. Explain the properties of metals keeping in view the nature of metallic bond.
Properties due to Metallic Bond (Sea of Electrons):
- Malleability/Ductility: Layers of ions can slide over each other without breaking the bond because the electron sea readjusts.
- Electrical Conductivity: Delocalized electrons are free to move and carry electric current.
- Thermal Conductivity: Mobile electrons transfer kinetic energy (heat) rapidly.
- Luster: Electrons absorb and re-emit light energy.
4. Compare the properties of ionic and covalent compounds.
| Property | Ionic Compounds | Covalent Compounds |
|---|---|---|
| State at RT | Usually crystalline solids | Gases, liquids, or soft solids |
| Melting/Boiling Point | High | Low |
| Solubility in Water | Usually high | Usually low |
| Electrical Conductivity | Conduct when molten/dissolved | Do not conduct |
| Bond Formation | Electron transfer | Electron sharing |
5. How will you explain the electrical conductivity of graphite crystals?
Answer: In graphite, each carbon atom is covalently bonded to three others in a hexagonal layered structure. The fourth electron of each carbon atom is delocalized and free to move throughout the entire layer. These mobile delocalized electrons can carry an electric current parallel to the layers, making graphite a good conductor of electricity.
6. Why are metals usually hard and heavy?
Answer: Metals are hard because of the strong electrostatic forces of attraction between the closely packed positive metal ions and the sea of delocalized electrons. They are heavy (have high density) because their atoms/ions are packed very closely together in the crystal lattice.
Part 5: Investigative Questions
1. The formula of AlCl₃ in vapour phase is Al₂Cl₆ which means it exists as a dimer. Explain the bonding between its two molecules?
Answer: Aluminium chloride (AlCl₃) is electron-deficient. A chlorine atom on one AlCl₃ molecule donates a lone pair of electrons into the empty orbital of the aluminium atom in the other molecule, forming a coordinate covalent bond. This creates a bridge-like structure completing the octet for both aluminium atoms.
2. Explain the structure of sand (SiO₂).
Answer: Sand is primarily silicon dioxide (SiO₂). It has a giant covalent structure (network solid). Each silicon atom is covalently bonded to four oxygen atoms in a tetrahedral arrangement. Each oxygen atom is bonded to two silicon atoms. This creates a very hard, rigid, and high-melting-point three-dimensional network with a formula ratio of SiO₂.
Structure diagram of SiO₂ would appear here
(Tetrahedral arrangement of silicon and oxygen atoms)
(Tetrahedral arrangement of silicon and oxygen atoms)
Part 6: Miscellaneous Questions
1. What types of elements form ionic bonds?
Answer: Ionic bonds are typically formed between metals (which lose electrons to form cations) and non-metals (which gain electrons to form anions).
2. What are the conditions for an ionic bond to be formed?
Answer: The conditions are:
- A large difference in electronegativity (usually >1.7) between the two atoms.
- A low ionization energy for the metal atom (to easily lose electrons).
- A high electron affinity for the non-metal atom (to readily gain electrons).
3. What type of elements form covalent bond?
Answer: Covalent bonds are typically formed between non-metal atoms.
4. How covalent bond is different from an ionic bond?
Answer:
Covalent Bond: Formed by sharing of electrons between atoms. Results in the formation of molecules.
Ionic Bond: Formed by complete transfer of electrons from one atom to another. Results in the formation of ions held in a lattice.
Ionic Bond: Formed by complete transfer of electrons from one atom to another. Results in the formation of ions held in a lattice.
5. Draw electron dot and cross structure of the following compounds. SiH₄, PCl₃, SO₂
SiH₄ (Silane):
PCl₃ (Phosphorus Trichloride): Phosphorus is central, with 3 single bonds to Cl atoms and one lone pair.
SO₂ (Sulfur Dioxide):
Electron dot and cross structure of SiH₄ would appear here
(Silicon in center with four hydrogen atoms)
(Silicon in center with four hydrogen atoms)
PCl₃ (Phosphorus Trichloride): Phosphorus is central, with 3 single bonds to Cl atoms and one lone pair.
Electron dot and cross structure of PCl₃ would appear here
SO₂ (Sulfur Dioxide):
Electron dot and cross structure of SO₂ would appear here
6. Draw the pictures of coordinate covalent bond formed between:
(a) BF₃ and AlCl₃
(b) CH₃OCH₃ and H⁺
(a) BF₃ and AlCl₃
(b) CH₃OCH₃ and H⁺
(a) BF₃ and AlCl₃: Both are Lewis acids (electron acceptors). They do not typically form a coordinate bond with each other. A more common example is BF₃ forming a coordinate bond with a Lewis base like NH₃: F₃B ← NH₃.
(b) CH₃OCH₃ and H⁺: The oxygen atom in dimethyl ether (CH₃OCH₃) has two lone pairs. It donates one lone pair to a H⁺ ion, forming a coordinate covalent bond and creating an oxonium ion: CH₃-O(CH₃) → H⁺ or [CH₃OCH₃H]⁺
(b) CH₃OCH₃ and H⁺: The oxygen atom in dimethyl ether (CH₃OCH₃) has two lone pairs. It donates one lone pair to a H⁺ ion, forming a coordinate covalent bond and creating an oxonium ion: CH₃-O(CH₃) → H⁺ or [CH₃OCH₃H]⁺
7. Which compound is not able to form a coordinate covalent bond?
Answer: Generally, compounds with no lone pairs or no empty orbitals cannot form coordinate bonds, e.g., Alkanes like CH₄.
8. What type of atoms form metallic bond?
Answer: Metal atoms form metallic bonds with each other.
9. Give a comparison of metallic bond with an ionic bond.
| Feature | Metallic Bond | Ionic Bond |
|---|---|---|
| Formed between | Atoms of the same/different metals | Metal and non-metal atoms |
| Nature of Bond | Attraction between cations & electron sea | Electrostatic attraction between ions |
| Electron Role | Delocalized | Transferred |
| Resulting Structure | Lattice of cations in electron sea | Crystal lattice of alternating ions |