2. Give short answer.
(i) State octet and duplet rules.
Answer:
Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with eight electrons in their outermost shell (valence shell), similar to the noble gases (except helium). Duplet Rule: Some atoms, particularly hydrogen and helium, achieve stability with two electrons in their outermost shell (the 1s orbital). Hydrogen tends to gain, lose, or share one electron to achieve a duplet.
(ii) Explain formation of covalent bond between two nitrogen atoms.
Answer: Nitrogen (N) has the electronic configuration 1s², 2s², 2p³. It has 5 valence electrons and needs 3 more electrons to complete its octet. In the formation of a nitrogen molecule (N₂), two nitrogen atoms share three pairs of electrons (a triple bond). Each nitrogen atom contributes three electrons to the shared pairs, allowing both atoms to achieve a stable octet. The triple bond (N[source symbol unavailable]N) is very strong, making nitrogen gas very stable and unreactive.
(iii) How does Al form cation?
Answer: Aluminium (Al) has the electronic configuration 1s², 2s², 2p⁶, 3s², 3p¹. It has 3 valence electrons. To achieve a stable noble gas configuration (like Neon, 1s², 2s², 2p⁶), it loses these 3 electrons. This loss of electrons results in the formation of an Al³⁺ cation.
Equation: Al [source symbol unavailable] Al³⁺ + 3e⁻
(iv) How does O form anion?
Answer: Oxygen (O) has the electronic configuration 1s², 2s², 2p⁴. It has 6 valence electrons and needs 2 more electrons to complete its octet (like Neon). It gains 2 electrons to form an O²⁻ anion.
Equation: O + 2e⁻ [source symbol unavailable] O²⁻
(v) Draw electron cross and dot structure for H₂O molecule.
Answer: (Diagram Description)
Central Atom: Oxygen (O) with 6 valence electrons (dots). Surrounding Atoms: Two Hydrogen (H) atoms, each with 1 valence electron (cross). Bonding: Oxygen shares one electron pair with each hydrogen atom (single covalent bonds). Oxygen also has two lone pairs of electrons. Structure: H - O - H, with two lone pairs on oxygen.
3. Describe the importance of noble gas electronic configuration.
Answer: The noble gas electronic configuration (ns² np⁶, except He which is 1s²) is important because:
Stability: Atoms with a complete valence shell are chemically inert and stable. They do not readily gain, lose, or share electrons. Octet Rule: Other atoms tend to gain, lose, or share electrons to achieve this stable configuration. Predicting Bonding: The type of bond formed (ionic, covalent, metallic) depends on how atoms achieve noble gas configuration. Periodic Trends: Elements near noble gases show trends in reactivity and ionization energy. It is the driving force behind chemical bonding.
4. Explain how elements attain stability?
Answer: Elements attain stability by achieving a stable electronic configuration, usually the noble gas configuration (a complete octet or duplet).
Ionic Bonding: Metals lose electrons to form cations, and non-metals gain electrons to form anions, achieving noble gas configurations. Covalent Bonding: Atoms share electrons to complete their valence shells. Metallic Bonding: Metal atoms share a sea of delocalized electrons. The tendency to achieve a stable configuration (octet or duplet) is the fundamental driving force for chemical bonding.
5. Describe the ways in which bonds may be formed.
Answer: Bonds can be formed in several ways:
Ionic Bond: Formed by the complete transfer of electrons from a metal to a non-metal (e.g., NaCl). Covalent Bond: Formed by the sharing of electron pairs between non-metal atoms (e.g., H₂O, CH₄). Coordinate Covalent Bond: Formed when one atom provides both electrons for sharing (e.g., NH₄⁺). Metallic Bond: Formed by the attraction between metal cations and a sea of delocalized electrons (e.g., Na, Cu). Hydrogen Bond: A weak attraction between a hydrogen atom bonded to an electronegative atom and another electronegative atom (e.g., in water).
6. Describe the formation of covalent bond between two non-metallic elements.
Answer: A covalent bond is formed when two non-metallic atoms share one or more pairs of electrons to achieve a stable noble gas configuration. For example, in a chlorine molecule (Cl₂), each chlorine atom has 7 valence electrons and needs 1 more electron to complete its octet. The two chlorine atoms share one pair of electrons (a single covalent bond), so each atom effectively has 8 valence electrons. The shared pair is attracted to both nuclei, holding the atoms together.
7. Explain with examples simple, double and triple covalent bond.
Answer:
Single Covalent Bond: Formed by sharing one pair of electrons. Example: H₂ (H-H), Cl₂ (Cl-Cl).
Double Covalent Bond: Formed by sharing two pairs of electrons. Example: O₂ (O=O), CO₂ (O=C=O). Triple Covalent Bond: Formed by sharing three pairs of electrons. Example: N₂ (N[source symbol unavailable]N), C₂H₂ (H-C[source symbol unavailable]C-H).
8. Find the number of valence electrons in the following atoms using the periodic table:
(a) Boron
Answer: Group IIIA [source symbol unavailable] 3 valence electrons
(b) Neon
Answer: Group VIIIA [source symbol unavailable] 8 valence electrons
(c) Rubidium
Answer: Group IA [source symbol unavailable] 1 valence electron
(d) Barium
Answer: Group IIA [source symbol unavailable] 2 valence electrons (e) Arsenic
Answer: Group VA [source symbol unavailable] 5 valence electrons
9. Represent the formation of cations for the following metal atoms using electron dot structures.
(a) Al
Answer: Al (3 valence electrons) [source symbol unavailable] Al³⁺ + 3e⁻
(b) Sr
Answer: Sr (2 valence electrons) [source symbol unavailable] Sr²⁺ + 2e⁻
(c) Ba
Answer: Ba (2 valence electrons) [source symbol unavailable] Ba²⁺ + 2e⁻
10. A sample of sulphur from a volcano was analysed to give the following composition of isotopes (At no of S = 16)
| Isotope | Abundance (%) |
| S - 32 | 95.0 |
| S - 33 | 0.76 |
| S - 34 | 4.22 |
(a) Define the term Isotope
Answer: Isotopes are atoms of the same element that have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers).
(b) Define the term relative atomic mass
Answer: The relative atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, compared to 1/12th the mass of a carbon-12 atom.
(c) Calculate the relative atomic mass of sulphur
Answer:
Relative Atomic Mass = (95.0 × 32) + (0.76 × 33) + 4.22 × 34100
= (3040) + (25.08) + 143.48100
= 3208.56100 = 32.0856 amu
(d) Complete the following table. | Isotope | Protons | Neutrons | Electrons | |---|---|---|---| | S - 32 | 16 | 16 | 16 | | S - 34 | 16 | 18 | 16 | (e) Where will you place S in the periodic table.
Answer: Period 3, Group VIA (16). (f) How many electrons S will loss or gain to acquire stable configuration.
Answer: S will gain 2 electrons to form S²⁻ and achieve a stable noble gas configuration (Argon). (g) How many atoms of S are there in 0.3 mole of sulphur.
Answer: Number of atoms = 0.3 × 6.022 × 10²³ = 1.8066 × 10²³ atoms.
11. An atom of an element has atomic number 9 and mass number 19.
(a) State the number of protons and neutrons in the nucleus of this atom.
Answer: Protons = 9, Neutrons = 19 - 9 = 10.
(b) State the number of electrons in this atom.
Answer: Electrons = 9 (in a neutral atom).
(c) Show with electron cross-dot diagrams, the formation of ions by this atom.
Answer: (Diagram Description) The atom is Fluorine (F), with 7 valence electrons. It gains 1 electron (from a metal like Na) to form F⁻, achieving a stable octet (like Neon).
(d) Write electronic configuration of this element.
Answer: 1s², 2s², 2p⁵ (e) Point out its group in the periodic table.
Answer: Group VIIA (17) (f) Point out its period in the periodic table.
Answer: Period 2
