Class 9 Science Chapter 9 MCQs – Atomic Foundations of Matter provides 50 important multiple-choice questions with answers to help students revise atoms, molecules, elements, compounds, atomic structure, valency, ions, chemical formulae, isotopes and related concepts. These MCQs are useful for Class 9 Science exam preparation, revision and practice.
1. What is the smallest unit of an element that retains the chemical properties of that element?
A. Molecule
B. Atom
C. Ion
D. Compound
Answer: B. Atom
2. Which scientist is credited with proposing the modern atomic theory?
A. J.J. Thomson
B. John Dalton
C. Ernest Rutherford
D. Niels Bohr
Answer: B. John Dalton
3. According to Dalton’s atomic theory, matter is made up of:
A. Cells
B. Atoms
C. Energy waves
D. Tissues
Answer: B. Atoms
4. Which of the following represents an element?
A. H₂O
B. CO₂
C. O₂
D. NaCl
Answer: C. O₂
5. Two or more atoms chemically bonded together form a:
A. Molecule
B. Proton
C. Electron
D. Nucleus
Answer: A. Molecule
6. Which of the following is a molecule of an element?
A. H₂
B. H₂O
C. CO₂
D. NH₃
Answer: A. H₂
7. Which of the following is a molecule of a compound?
A. O₂
B. N₂
C. H₂O
D. Cl₂
Answer: C. H₂O
8. Which of the following is a compound?
A. Oxygen
B. Hydrogen
C. Sodium chloride
D. Nitrogen
Answer: C. Sodium chloride
9. The chemical symbol for sodium is:
A. So
B. S
C. Na
D. Sd
Answer: C. Na
10. The chemical symbol for potassium is:
A. P
B. Po
C. K
D. Pt
Answer: C. K
11. The chemical symbol for iron is:
A. Ir
B. Fe
C. I
D. In
Answer: B. Fe
12. The chemical symbol for copper is:
A. Co
B. Cu
C. Cp
D. C
Answer: B. Cu
13. Which of the following symbols is correctly written?
A. na
B. NA
C. Na
D. nA
Answer: C. Na
14. The atomicity of oxygen gas (O₂) is:
A. 1
B. 2
C. 3
D. 4
Answer: B. 2
15. The atomicity of ozone (O₃) is:
A. 1
B. 2
C. 3
D. 4
Answer: C. 3
16. What is the atomicity of phosphorus molecule P₄?
A. 2
B. 3
C. 4
D. 5
Answer: C. 4
17. What is the atomicity of sulfur molecule S₈?
A. 4
B. 6
C. 7
D. 8
Answer: D. 8
18. The relative atomic mass of an atom is expressed by comparing it with:
A. Hydrogen atom only
B. Oxygen atom only
C. 1/12 of the mass of a carbon-12 atom
D. Mass of an electron
Answer: C. 1/12 of the mass of a carbon-12 atom
19. The atomic mass of oxygen is approximately:
A. 8 u
B. 12 u
C. 16 u
D. 32 u
Answer: C. 16 u
20. The atomic mass of carbon is approximately:
A. 6 u
B. 12 u
C. 16 u
D. 24 u
Answer: B. 12 u
21. The molecular mass of water (H₂O) is:
A. 16 u
B. 17 u
C. 18 u
D. 20 u
Answer: C. 18 u
22. The molecular mass of carbon dioxide (CO₂) is:
A. 28 u
B. 32 u
C. 40 u
D. 44 u
Answer: D. 44 u
23. The molecular mass of oxygen (O₂) is:
A. 8 u
B. 16 u
C. 32 u
D. 64 u
Answer: C. 32 u
24. The formula of calcium chloride is:
A. CaCl
B. Ca₂Cl
C. CaCl₂
D. Ca₂Cl₂
Answer: C. CaCl₂
25. The formula of magnesium chloride is:
A. MgCl
B. MgCl₂
C. Mg₂Cl
D. Mg₂Cl₂
Answer: B. MgCl₂
26. The formula of aluminium oxide is:
A. AlO
B. Al₂O
C. AlO₂
D. Al₂O₃
Answer: D. Al₂O₃
27. The formula of sodium oxide is:
A. NaO
B. Na₂O
C. NaO₂
D. Na₂O₂
Answer: B. Na₂O
28. The formula of calcium oxide is:
A. CaO
B. Ca₂O
C. CaO₂
D. Ca₂O₃
Answer: A. CaO
29. The valency of sodium is:
A. 1
B. 2
C. 3
D. 4
Answer: A. 1
30. The valency of magnesium is:
A. 1
B. 2
C. 3
D. 4
Answer: B. 2
31. The valency of aluminium is:
A. 1
B. 2
C. 3
D. 4
Answer: C. 3
32. The valency of oxygen is generally:
A. 1
B. 2
C. 3
D. 4
Answer: B. 2
33. The valency of chlorine in sodium chloride is:
A. 1
B. 2
C. 3
D. 4
Answer: A. 1
34. An atom becomes a positive ion when it:
A. Gains electrons
B. Loses electrons
C. Gains neutrons
D. Loses protons
Answer: B. Loses electrons
35. An atom becomes a negative ion when it:
A. Loses electrons
B. Gains electrons
C. Loses neutrons
D. Gains protons
Answer: B. Gains electrons
36. A positively charged ion is called a:
A. Anion
B. Cation
C. Molecule
D. Neutron
Answer: B. Cation
37. A negatively charged ion is called an:
A. Cation
B. Proton
C. Anion
D. Atom
Answer: C. Anion
38. Which particle has a negative charge?
A. Proton
B. Neutron
C. Electron
D. Nucleus
Answer: C. Electron
39. Which particle has no electrical charge?
A. Proton
B. Electron
C. Neutron
D. Ion
Answer: C. Neutron
40. Which particle has a positive charge?
A. Electron
B. Proton
C. Neutron
D. Atom
Answer: B. Proton
41. The central part of an atom is called the:
A. Shell
B. Orbit
C. Nucleus
D. Electron cloud
Answer: C. Nucleus
42. The nucleus of an atom contains:
A. Only electrons
B. Protons and neutrons
C. Only neutrons
D. Protons and electrons
Answer: B. Protons and neutrons
43. The atomic number of an element represents the number of:
A. Neutrons
B. Electrons + neutrons
C. Protons
D. Protons + neutrons
Answer: C. Protons
44. In a neutral atom, the number of protons is equal to the number of:
A. Neutrons
B. Electrons
C. Nucleons
D. Shells
Answer: B. Electrons
45. The mass number of an atom is equal to:
A. Number of protons only
B. Number of electrons + protons
C. Number of protons + neutrons
D. Number of neutrons − protons
Answer: C. Number of protons + neutrons
46. An atom has 11 protons and 12 neutrons. Its mass number is:
A. 11
B. 12
C. 23
D. 24
Answer: C. 23
47. An atom has atomic number 17 and mass number 35. How many neutrons does it contain?
A. 17
B. 18
C. 35
D. 52
Answer: B. 18
48. Isotopes are atoms of the same element having:
A. Same mass number but different atomic numbers
B. Same atomic number but different mass numbers
C. Different atomic numbers and mass numbers
D. Same number of neutrons only
Answer: B. Same atomic number but different mass numbers
49. Which pair represents isotopes?
A. ¹²C and ¹⁴C
B. ¹²C and ¹²N
C. ²³Na and ²⁴Mg
D. ¹H and ⁴He
Answer: A. ¹²C and ¹⁴C
50. Which statement about an atom is correct?
A. An atom always has a net positive charge
B. An atom always has a net negative charge
C. A neutral atom has equal numbers of protons and electrons
D. An atom contains only protons and electrons
Answer: C. A neutral atom has equal numbers of protons and electrons