Class 9 Science Atoms and Molecules
Fill in the Blanks
1. The smallest particle of an element is called an atom.
2. The scientist who introduced the atomic theory is John Dalton.
3. Atoms of the same element are ______ in mass and properties. → identical
4. The law which says mass is neither created nor destroyed is the Law of Conservation of Mass.
5. Water always contains hydrogen and oxygen in a 1:8 mass ratio.
6. The atomic symbol of sodium is Na.
7. The number of atoms in a molecule is called its atomicity.
8. A positively charged ion is called a cation.
9. A negatively charged ion is called an anion.
10. The molecular formula of water is H₂O.
11. The symbol for chlorine is Cl.
12. One atomic mass unit is equal to 1/12th the mass of a carbon-12 atom.
13. The valency of hydrogen is 1.
14. A group of atoms with a charge is called a polyatomic ion.
15. The formula of calcium chloride is CaCl₂.
Multiple Choice Questions
1. Who proposed the atomic theory?
– a) Einstein
– b) Newton
– c) Dalton ✅
– d) Bohr
2. What is the symbol of iron?
– a) I
– b) Fe
– c) Fe ✅
– d) Ir
3. What is the valency of oxygen?
– a) 1
– b) 2 ✅
– c) 3
– d) 4
4. Which gas has a diatomic molecule?
– a) Helium
– b) Argon
– c) Oxygen ✅
– d) Neon
5. Which one is a polyatomic ion?
– a) Na⁺
– b) Cl⁻
– c) H⁺
– d) SO₄²⁻ ✅
6. What is the molecular mass of H₂O?
– a) 15 u
– b) 18 u ✅
– c) 20 u
– d) 21 u
7. Who first used symbols for elements?
– a) Newton
– b) Dalton ✅
– c) Rutherford
– d) Bohr
