Molecules

A molecule is defined as:
a) A single atom
b) A charged particle
c) A group of atoms chemically bonded
d) A neutral proton
✅ c) A group of atoms chemically bonded
Explanation: Molecules are formed when two or more atoms bond chemically.
Which of the following is a diatomic molecule?
a) H₂O
b) CO₂
c) O₂
d) NH₃
✅ c) O₂
Explanation: O₂ has two oxygen atoms, hence diatomic.
The smallest unit of a compound that retains its chemical identity is:
a) Atom
b) Ion
c) Molecule
d) Proton
✅ c) Molecule
Explanation: Molecules represent the smallest identifiable unit of a compound.
Which of the following is a polyatomic molecule?
a) H₂
b) Cl₂
c) H₂O
d) N₂
✅ c) H₂O
Explanation: Water (H₂O) has three atoms → polyatomic.
A molecule of methane contains:
a) One atom
b) Two atoms
c) Three atoms
d) Five atoms
✅ d) Five atoms
Explanation: CH₄ = 1 carbon + 4 hydrogen = 5 atoms.
Which of the following is not a molecule?
a) O₂
b) Na⁺
c) HCl
d) NH₃
✅ b) Na⁺
Explanation: Na⁺ is an ion, not a molecule.
Molecules with the same atoms but different arrangements are:
a) Isotopes
b) Isomers
c) Isobars
d) Ions
✅ b) Isomers
Explanation: Isomers have same molecular formula but different structures.
Which of the following is a triatomic molecule?
a) O₂
b) N₂
c) CO₂
d) HCl
✅ c) CO₂
Explanation: Carbon dioxide contains three atoms: 1C + 2O.
Water is:
a) Monoatomic
b) Diatomic
c) Triatomic
d) Polyatomic
✅ c) Triatomic
Explanation: Water (H₂O) has three atoms.
A molecule of nitrogen (N₂) is held together by:
a) Single bond
b) Double bond
c) Triple bond
d) No bond
✅ c) Triple bond
Explanation: N₂ is connected by a strong triple covalent bond.

NCERT-Based MCQs

Which of the following is not a compound molecule?
a) NH₃
b) H₂
c) CH₄
d) H₂O
✅ b) H₂
Explanation: H₂ is an element, not a compound.
A molecule with two atoms of the same kind is called:
a) Polyatomic
b) Heteronuclear
c) Diatomic
d) Monatomic
✅ c) Diatomic
Explanation: Diatomic molecules like H₂, O₂ have two identical atoms.
The molecular formula of ammonia is:
a) NH
b) NH₂
c) NH₃
d) N₂H
✅ c) NH₃
Explanation: Ammonia has one nitrogen and three hydrogen atoms.
Which molecule has only single bonds?
a) CO₂
b) O₂
c) H₂O
d) N₂
✅ c) H₂O
Explanation: Water contains only single covalent bonds.
Which type of bond is found in molecules?
a) Ionic
b) Covalent
c) Metallic
d) Hydrogen
✅ b) Covalent
Explanation: Most molecules are formed via covalent bonding.
Which of the following is a monoatomic molecule?
a) He
b) H₂
c) Cl₂
d) N₂
✅ a) He
Explanation: Helium exists as a single atom.
Ozone (O₃) is:
a) Monoatomic
b) Diatomic
c) Triatomic
d) Tetraatomic
✅ c) Triatomic
Explanation: Ozone consists of three oxygen atoms.
Which molecule is responsible for greenhouse effect?
a) O₂
b) CO₂
c) H₂
d) Cl₂
✅ b) CO₂
Explanation: Carbon dioxide traps heat and contributes to global warming.
Which of these molecules exists in solid, liquid and gas states?
a) CH₄
b) NH₃
c) H₂O
d) CO₂
✅ c) H₂O
Explanation: Water is found in all three physical states on Earth.
The chemical formula of glucose is:
a) C₆H₁₂O₆
b) C₆H₁₂
c) C₆O₆
d) CH₄
✅ a) C₆H₁₂O₆
Explanation: Glucose contains carbon, hydrogen and oxygen atoms.

Molecules – MCQs 21 to 50 with Explanations

The molecule of hydrogen chloride (HCl) is composed of:
a) Two hydrogen atoms
b) Two chlorine atoms
c) One hydrogen and one chlorine atom
d) One hydrogen and two chlorine atoms
✅ c) One hydrogen and one chlorine atom
Explanation: HCl is a diatomic molecule made of hydrogen and chlorine.
Which of these molecules contains polar covalent bonds?
a) O₂
b) CO₂
c) H₂O
d) N₂
✅ c) H₂O
Explanation: Water has unequal electron sharing, making it polar.
A molecule of oxygen has:
a) One atom
b) Two atoms
c) Three atoms
d) Four atoms
✅ b) Two atoms
Explanation: O₂ is a diatomic molecule.
The number of atoms in a molecule of sulfur (S₈) is:
a) 1
b) 2
c) 4
d) 8
✅ d) 8
Explanation: S₈ is a polyatomic molecule with 8 sulfur atoms.
Which of the following molecules is linear in shape?
a) H₂O
b) CH₄
c) CO₂
d) NH₃
✅ c) CO₂
Explanation: CO₂ is linear due to double bonds on either side of carbon.
What is the molar mass of a water molecule?
a) 16 g/mol
b) 17 g/mol
c) 18 g/mol
d) 20 g/mol
✅ c) 18 g/mol
Explanation: H₂O = 2(1) + 16 = 18 g/mol.
Which molecule is responsible for acid rain formation?
a) O₂
b) SO₂
c) N₂
d) CO
✅ b) SO₂
Explanation: Sulfur dioxide forms sulfuric acid in rainwater.
What kind of molecule is methane (CH₄)?
a) Linear
b) Angular
c) Tetrahedral
d) Octahedral
✅ c) Tetrahedral
Explanation: Methane has a tetrahedral geometry due to 4 bonds.
In H₂O, oxygen forms how many covalent bonds?
a) One
b) Two
c) Three
d) Four
✅ b) Two
Explanation: Oxygen bonds with two hydrogen atoms using two electrons.
Which molecule is used as a refrigerant gas?
a) CH₄
b) CCl₂F₂
c) NH₃
d) CO₂
✅ b) CCl₂F₂
Explanation: This is Freon-12, a common refrigerant.
Which molecule is responsible for depletion of ozone layer?
a) H₂O
b) CO₂
c) CFCs
d) N₂
✅ c) CFCs
Explanation: Chlorofluorocarbons break ozone into oxygen molecules.
The common salt molecule is composed of:
a) Na and Cl atoms
b) Na and O atoms
c) Cl and H atoms
d) H and O atoms
✅ a) Na and Cl atoms
Explanation: NaCl is formed by ionic bonding between Na⁺ and Cl⁻.
Which molecule is known as laughing gas?
a) CO₂
b) N₂
c) N₂O
d) CH₄
✅ c) N₂O
Explanation: Nitrous oxide is called laughing gas.
Which of these is not a molecule?
a) He
b) H₂
c) CO₂
d) HCl
✅ a) He
Explanation: Helium exists as monatomic, not molecular.
The ozone molecule has:
a) 2 atoms
b) 3 atoms
c) 4 atoms
d) 1 atom
✅ b) 3 atoms
Explanation: O₃ is a triatomic molecule of oxygen.
Which molecule is used in fire extinguishers?
a) CH₄
b) CO₂
c) O₂
d) NH₃
✅ b) CO₂
Explanation: Carbon dioxide displaces oxygen, suffocating fire.
What is Avogadro’s number of molecules?
a) 6.02 × 10²²
b) 6.02 × 10²³
c) 6.02 × 10²⁴
d) 6.02 × 10²⁵
✅ b) 6.02 × 10²³
Explanation: It is the number of molecules in one mole.
A molecule of glucose contains how many atoms?
a) 6
b) 12
c) 24
d) 24
✅ d) 24
Explanation: C₆H₁₂O₆ = 6 + 12 + 6 = 24 atoms.
Which of the following is a molecule but not a compound?
a) H₂O
b) CO₂
c) O₂
d) NH₃
✅ c) O₂
Explanation: O₂ is made of the same element, so not a compound.
Which molecule is used for respiration in cells?
a) O₂
b) CO₂
c) H₂O
d) CH₄
✅ a) O₂
Explanation: Oxygen is essential for aerobic respiration.
Molecules held together by ionic bonds are generally:
a) Soft
b) Volatile
c) Soluble in water
d) Non-polar
✅ c) Soluble in water
Explanation: Ionic molecules dissolve easily in polar solvents like water.
Covalent molecules usually have:
a) High melting points
b) Good conductivity
c) Low boiling points
d) Metallic properties
✅ c) Low boiling points
Explanation: Covalent bonds have weak intermolecular forces.
Which molecule is essential for plant photosynthesis?
a) CO₂
b) O₂
c) CH₄
d) H₂O
✅ a) CO₂
Explanation: Plants use carbon dioxide to make food.
Molecules of hydrogen are formed by:
a) Ionic bonding
b) Polar bonding
c) Metallic bonding
d) Covalent bonding
✅ d) Covalent bonding
Explanation: H₂ has a non-polar covalent bond.
Which molecule is essential for DNA structure?
a) O₂
b) H₂O
c) CH₄
d) Phosphate group
✅ d) Phosphate group
Explanation: DNA molecules are linked by phosphate-sugar backbones.
Which of the following molecules helps regulate blood pH?
a) CO₂
b) O₂
c) H₂
d) NaCl
✅ a) CO₂
Explanation: Carbon dioxide forms carbonic acid, regulating blood pH.
Which gas exists as diatomic molecule?
a) Argon
b) Nitrogen
c) Helium
d) Neon
✅ b) Nitrogen
Explanation: N₂ is a diatomic molecule.
Water molecules are held together by:
a) Ionic bonds
b) Hydrogen bonds
c) Metallic bonds
d) Coordinate bonds
✅ b) Hydrogen bonds
Explanation: Hydrogen bonding gives water its unique properties.
A molecule of carbon dioxide is made of:
a) One C and one O
b) One C and two O
c) Two C and two O
d) One C and three O
✅ b) One C and two O
Explanation: CO₂ = 1 carbon + 2 oxygen atoms.
Which of these has molecules in all 3 states at Earth’s temperature?
a) Oxygen
b) Nitrogen
c) Water
d) Carbon
✅ c) Water
Explanation: Water exists as ice (solid), liquid, and vapor (gas).