50 Very Short Answer Questions — Atoms and Molecules
These very short questions and answers are organised topic-wise and strictly follow the NCERT Class 9 syllabus for quick revision and board exam practice.
Topic: Laws of Chemical Combination
1
State the law of conservation of mass.
Mass of reactants = Mass of products in a chemical reaction.
2
What does the law of definite proportions state?
A compound always contains the same elements in the same fixed mass ratio.
3
Give a simple example that illustrates definite proportions.
Water always has hydrogen and oxygen in a mass ratio ≈1:8.
4
State the law of multiple proportions.
If two elements form more than one compound, masses of one element combining with a fixed mass of the other are in small whole-number ratios.
Topic: Dalton’s Atomic Theory
5
Who proposed the modern atomic theory?
John Dalton.
6
State one key postulate of Dalton’s theory.
Matter is made of tiny indivisible particles called atoms.
7
Does Dalton’s theory say atoms of an element are identical?
Dalton stated they are identical in mass and properties; modern science refines this due to isotopes.
8
According to Dalton, how are compounds formed?
By combination of atoms of different elements in simple whole-number ratios.
Topic: Atoms and Molecules — Basic Definitions
9
Define an atom in one line.
Smallest unit of an element that can take part in a chemical reaction.
10
Define a molecule.
Smallest neutral particle of a substance that shows its properties; composed of one or more atoms bonded together.
11
Give two examples of molecular elements.
O₂ and N₂.
12
Give two examples of molecular compounds.
H₂O and CO₂.
Topic: Chemical Formulae & Representation
13
What information does a chemical formula give?
Types of atoms present and their relative numbers in a molecule or formula unit.
14
Write formula for water.
H₂O.
15
Write formula for carbon dioxide.
CO₂.
16
What is a formula unit?
Smallest repeating unit of an ionic compound represented by its simplest whole-number ratio (e.g., NaCl).
Topic: Relative Atomic Mass (Ar) & Relative Molecular Mass (Mr)
17
What is relative atomic mass (Ar)?
Average mass of an atom of an element relative to 1/12 of carbon-12 atom.
18
How do you calculate relative molecular mass (Mr)?
Add the Ar values of all atoms present in the molecule.
19
Find Mr of H₂O (Ar: H=1, O=16).
Mr = 2×1 + 16 = 18.
20
Find Mr of CO₂ (Ar: C=12, O=16).
Mr = 12 + 2×16 = 44.
Topic: Counting Atoms in Formulae
21
How many H atoms are in 3H₂O?
3 × 2 = 6 hydrogen atoms.
22
How many O atoms are in 2CO₂?
2 × 2 = 4 oxygen atoms.
23
In Na₂SO₄, how many Na atoms?
2 sodium atoms.
24
In Ca(OH)₂, how many O atoms?
2 oxygen atoms (one in each OH group) ⇒ total 2.
Topic: Chemical Equations & Balancing
25
Why must chemical equations be balanced?
To obey the law of conservation of mass — equal number of each type of atom on both sides.
26
Balance: H₂ + O₂ → H₂O
Balanced equation: 2H₂ + O₂ → 2H₂O.
27
Balance: N₂ + H₂ → NH₃
Balanced: N₂ + 3H₂ → 2NH₃.
Topic: Classification & Examples
28
Is O₂ an element or compound?
Element (molecular form of oxygen).
29
Is NaCl an element or compound?
Compound (sodium chloride).
30
Is air a mixture or compound?
Mixture (homogeneous mixture of gases).
Topic: Bonding (Basic Distinction)
31
What type of bond is present in NaCl?
Ionic bond (between Na⁺ and Cl⁻ ions).
32
What type of bond is in H₂O?
Covalent bond (atoms share electrons).
Topic: Atomic Structure — Introductory
33
Name the three main subatomic particles.
Proton, neutron, electron.
34
Which subatomic particle has a positive charge?
Proton.
35
Which subatomic particle has negligible mass and negative charge?
Electron.
Topic: Isotopes — Simple Idea
36
What are isotopes?
Atoms of the same element with different masses (different number of neutrons).
37
Give an example of isotopes.
Carbon-12 and Carbon-14 are isotopes of carbon.
Topic: Formulae — Short Calculations
38
If Ar of Na = 23 and Cl = 35.5, what is Mr of NaCl?
Mr = 23 + 35.5 = 58.5 (approx.).
39
What is Mr of O₂ (Ar O = 16)?
Mr = 2×16 = 32.
Topic: Quick Facts & Misconceptions
40
Are atoms indivisible according to modern science?
No — atoms are made of subatomic particles and can be split in nuclear reactions.
41
Does a molecule always contain more than one atom?
No — some elements exist as single-atom species in certain contexts, but typically a molecule has two or more atoms; noble gases are monatomic.
42
Is the formula unit the same as a molecule for ionic compounds?
Yes — 'formula unit' is the term used because ionic compounds do not exist as discrete molecules.
Topic: Identify & Classify — Quick Questions
43
Classify: H₂ (element/compound/mixture)
Element (molecular form of hydrogen).
44
Classify: HCl (aq) — hydrogen chloride dissolved in water.
Solution (aqueous) — HCl is a compound; in water it forms an acidic solution.
45
Is diamond an element or a compound?
Diamond is an allotrope of carbon — composed of carbon atoms (elemental form).
Topic: Simple Applications
46
Why is Mr useful in chemistry?
Mr helps compare masses of molecules and is used in stoichiometric calculations.
47
How does knowing Ar help in chemistry?
Ar provides a basis for calculating molecular masses and comparing atomic masses.
48
State one use of balancing chemical equations in exams.
To verify conservation of mass and to determine mole ratios for calculations.
Topic: Final Quick Recall
49
Write formula of sulphuric acid.
H₂SO₄.
50
State one exam tip for this chapter.
Practice writing formulae, counting atoms, calculating Mr, and memorise key laws with examples.