Chemistry Class 9
Unit 5: Exercise Questions and Answers
Part 1: Bonding & Charge
State octet and duplet rules.
Octet Rule: Atoms react to achieve eight electrons in their outermost shell for stability.
Duplet Rule: The lightest atoms (H, Li, Be) react to achieve two electrons in their outermost shell, like He.
Explain formation of covalent bond between two nitrogen atoms.
Two nitrogen atoms N each need 3 more electrons. They achieve this by sharing three pairs of electrons, resulting in a strong triple covalent bond (N≡N). This sharing gives both N atoms a stable octet.
How does Al form cation?
Aluminum Al easily loses three electrons to achieve a stable octet (2, 8).
By losing 3 electrons, it forms the positively charged aluminum cation (Al3+).
Al → Al3+ + 3e-
How does O form anion?
Oxygen O readily gains two electrons to complete its stable octet (2, 8).
By gaining two electrons, it forms the negatively charged oxide anion (O2-).
O + 2e- → O2-
Draw electron cross and dot structure for H2O molecule.
The central oxygen O shares one electron pair with each of the two hydrogen H atoms, forming two single covalent bonds. It retains two non-bonded lone pairs.
= Oxygen (Dot) | = Hydrogen (Cross)
Part 2: Atomic Structure & Isotopes
Represent the formation of cations for metal atoms (Al, Sr, Ba).
Al (Aluminum): Al → Al3+ + 3e– (Loss of 3 electrons)
Sr (Strontium): Sr → Sr2+ + 2e– (Loss of 2 electrons)
Ba (Barium): Ba → Ba2+ + 2e– (Loss of 2 electrons)
Sulphur Isotopes & Calculations.
(a) Define isotope: Atoms of the same element with the same number of protons but different numbers of neutrons (different mass numbers).
(b) Define relative atomic mass: The weighted average mass of all naturally occurring isotopes of an element compared to 1/12th the mass of a C-12 atom.
(c) Calculate RAM: RAM = (0.950 × 32) + (0.0078 × 33) + (0.0422 × 34) ≈ 32.09.
(d) Table:
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| S-32 | 16 | 16 | 16 |
| S-34 | 16 | 18 | 16 |
(e) Periodic Table Placement: Group 16 (Chalcogens) and Period 3.
(f) Electrons to acquire stability: Sulphur will gain 2 electrons to achieve a stable octet (2, 8, 8).
(g) Atoms in 0.3 mole: 0.3 mol × (6.022 × 1023 atoms/mol) = 1.8066 × 1023 atoms.
Element with Atomic Number 9 (Fluorine).
(a) Protons and Neutrons: Protons = 9, Neutrons = 10 (19 – 9).
(b) Number of Electrons: 9 (for a neutral atom).
(c) Electronic Configuration: 2, 7.
(d) Ion Formation: The atom gains 1 electron to form a stable anion (F–: 2, 8).
F (2, 7) + 1e- → F- (2, 8)
(e) Group: Group 17 (Halogens).
(f) Period: Period 2.