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
8. Which compound has nitrogen and hydrogen?
– a) CO₂
– b) NaCl
– c) NH₃ ✅
– d) H₂O
9. The unit of atomic mass is:
– a) gram
– b) kg
– c) u ✅
– d) mg
10. What is the formula for calcium oxide?
– a) CaO₂
– b) Ca₂O
– c) CaO ✅
– d) Ca(OH)₂
11. Sodium chloride is made of:
– a) Na and O
– b) Na and Cl ✅
– c) Cl and H
– d) Na and N
12. What is the valency of calcium?
– a) 1
– b) 2 ✅
– c) 3
– d) 4
13. Which of these is a chemical formula?
– a) HCl ✅
– b) Water
– c) Hydrogen
– d) Atom
14. What is the atomic mass of oxygen?
– a) 8
– b) 10
– c) 16 ✅
– d) 18
15. What does a molecule consist of?
– a) Protons
– b) Two or more atoms ✅
– c) Electrons only
– d) Neutrons only
True or False
1. Dalton believed atoms can be divided. → False
2. Water is always made of hydrogen and oxygen in a 2:1 ratio by atoms. → True
3. The law of conservation of mass says mass can be created. → False
4. All atoms have the same mass. → False
5. Atoms are very large and visible. → False
6. A molecule of oxygen contains two atoms. → True
7. Sodium chloride is made of ions. → True
8. Chemical formulas represent compounds. → True
9. The atomic mass of hydrogen is 12 u. → False
10. Valency shows how an atom can combine. → True
11. CaCl₂ contains one calcium atom and two chlorine atoms. → True
12. An ion is always neutral. → False
13. The molecular mass of CO₂ is 44 u. → True
14. Polyatomic ions contain only one atom. → False
15. The formula of ammonium sulphate is (NH₄)₂SO₄. → True
