Atoms and Molecules — Important Questions

Latin

Step 1: Not all element symbols come from their English names. Step 2: Iron's symbol 'Fe' comes from its Latin name 'ferrum'. Step 3: Similarly, sodium (Na) comes from Latin 'natrium' and potassium (K) comes from Latin 'kalium'. Step 4: These symbols were adopted because Latin was the language of science in earlier times. Wrong options: Greek gave us the word 'atom' but not Fe; German and English are not the source of Fe's symbol.

Polyatomic (8 atoms)

Step 1: Atomicity is the number of atoms present in one molecule of an element. Step 2: Different non-metals have different atomicities. Step 3: Sulphur exists as S₈ — a ring of 8 sulphur atoms joined together — making it polyatomic. Step 4: Compare: Noble gases like Ar and He are monoatomic; O₂, H₂, N₂, Cl₂ are diatomic; phosphorus (P₄) is tetra-atomic. Wrong options: Sulphur is not monoatomic, diatomic, or tetra-atomic — it is a polyatomic molecule with 8 atoms.

Al₂O₃

Step 1: To write a chemical formula, we use the criss-cross method with valencies. Step 2: Aluminium has valency 3 (Al³⁺) and oxygen has valency 2 (O²⁻). Step 3: Criss-cross: the valency of Al (3) becomes the subscript of O, and the valency of O (2) becomes the subscript of Al. Step 4: This gives Al₂O₃, where the charges balance: 2 × (+3) = +6 and 3 × (−2) = −6. The compound is electrically neutral. Wrong options: AlO, Al₃O₂, and Al₂O do not balance the charges correctly.

18 u

Step 1: The molecular mass is the sum of atomic masses of all atoms in a molecule. Step 2: In H₂O, there are 2 hydrogen atoms and 1 oxygen atom. Step 3: Mass from H = 2 × 1 u = 2 u; Mass from O = 1 × 16 u = 16 u. Step 4: Total molecular mass = 2 + 16 = 18 u. Wrong options: 17 u would mean only one hydrogen; 16 u ignores the hydrogen atoms; 20 u is incorrect arithmetic.