Multiple Choice Questions

ATOMS AND MOLECULES

Updated for CBSE 2027
88+ Practice Questions
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Total 88 questions available

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Q. 1
What are the building blocks of matter?
A
Atoms
B
Compound
C
Molecules
D
Element
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Q. 2
Cl₂ stands for:
A
2 ions of chlorine
B
2 molecules of chlorine
C
3 molecules of chlorine
D
2 atoms of chlorine
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Q. 3
What kind of particles do ionic compounds consist of?
A
Ions
B
Atoms
C
Elements
D
Molecules
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Q. 4
How are the elements in a chemical formula typically arranged?
A
Randomly
B
Metals first, then non-metals
C
Non-metals first
D
Alphabetically
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Q. 5
What is the difference between molecular mass and formula unit mass?
A
None
B
Molecular mass is for covalent compounds; formula unit mass is for ionic compounds
C
Molecular mass is always higher
D
They are the same
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Q. 6
Which of the following is the correct IUPAC symbol for the element Cobalt?
A
Co
B
Cb
C
C
D
none of these
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Q. 7
Which atom is the smallest atom of all?
A
Hydrogen
B
Lithium
C
Helium
D
Carbon
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Q. 8
What is an atom?
A
Smallest particle of a compound
B
Smallest particle of a mixture
C
Largest particle of an element
D
Smallest particle of an element
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Q. 9
How do atoms usually exist in nature?
A
In the free state
B
In the form of molecules
C
In the form of ions
D
In the form of molecules and Ions
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Q. 10
Who gave the Law of constant proportions?
A
Dalton
B
Cavendish
C
Proust
D
Lavoisier
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Q. 11
1 u or 1 amu means
A
Mass of O-16 atom
B
1/12th mass of C-12 atoms
C
Mass of C-12 atom
D
Mass of Hydrogen molecule
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Q. 12
In the formula Mg(OH)₂, what does the "2" outside the brackets indicate?
A
2 H atoms
B
2 OH ions
C
2 O atoms
D
2 Mg atoms
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Q. 13
Which of the following is the correct pair of elements and its symbol?
A
Potassium – Pt
B
Sodium – So
C
Sulphur – S
D
Silver – Si
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Q. 14
What does the term "atomicity" refer to?
A
Mass of an atom
B
Number of atoms in a molecule
C
Size of an atom
D
Number of protons in an atom
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Q. 15
The balancing of chemical equations is based upon:
A
Law of combining volumes
B
Law of definite proportion
C
Law of conservation of mass
D
Law of multiple proportions
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Q. 16
Law of conservation of mass can be derived from which postulate of Dalton’s atomic theory?
A
Matter is made of tiny particles.
B
Atoms of a given element have the same mass.
C
Atom can neither be created nor destroyed.
D
The relative number and kind of atoms are constant in a given compound.
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Q. 17
What is the formula for sodium chloride?
A
NaCl₃
B
NaCl₂
C
Na₂Cl
D
NaCl
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Q. 18
One atomic mass unit is a mass unit equal to exactly one twelfth of the mass of ___________.
A
Carbon-1
B
Nitrogen -14
C
Silicon-14
D
Carbon-12
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Q. 19
Calculate the formula unit mass of ZnCl2?
A
123 u
B
136.3 u
C
111 u
D
124 u
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Q. 20
The atomic mass of sodium is 23. The number of moles in 46g of sodium is ________.
A
2
B
0
C
4
D
½
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Q. 21
Molecules of compounds are made up of the same type of atoms.
A
True
B
False
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Q. 22
Which of the following can be used to see atoms?
A
Microscope
B
Optical microscope
C
Most powerful microscope
D
Scanning tunneling microscope
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Q. 23
9 grams of water decompose to give:
A
4g oxygen and 16g hydrogen
B
2g hydrogen and 1g oxygen
C
8g oxygen and 1g hydrogen
D
none
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Q. 24
What does the Law of Constant Proportions tell us about chemical compounds?
A
They have elements that vary in mass randomly
B
They contain different elements in different proportions
C
They change their elements when prepared differently
D
They always have the same elements in the same proportion by mass
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Q. 25
How are the names of elements typically derived?
A
Randomly
B
From places or names
C
From numbers
D
From colors
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Q. 26
Which is the smallest particle of an element that can take part in a chemical reaction?
A
Compound
B
Mixture
C
Molecule
D
Atom
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Q. 27
Which of the following represents the correct relation between Avogadro's number (N₀), number of particles (N), and moles (n)?
A
n = N / N₀
B
n = N₀ / N
C
n = N N₀
D
All are correct
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Q. 28
Which type of reaction verifies the law of conservation of mass?
A
Displacement reaction
B
Decomposition reaction
C
Double decomposition reaction
D
Precipitation reaction
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Q. 29
Atoms of a given element are different in mass and chemical properties.
A
False
B
True
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Q. 30
Which of the following statements is not true about an atom?
A
Atoms are the basic units from which molecules and ions are formed.
B
Atoms are not able to exist independently.
C
Atoms are always neutral in nature.
D
Atoms aggregate in large numbers to form matter.
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Q. 31
Which element's atomic mass was originally used as a standard before carbon-12?
A
Hydrogen
B
Oxygen
C
Nitrogen
D
None
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Q. 32
What is the molecular mass of nitric acid (HNO₃)?
A
70 u
B
64 u
C
63 u
D
58 u
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Q. 33
Which of the following contains the maximum number of molecules?
A
19 CO₂
B
1 g CH₄
C
1 g N₂
D
1 g H₂
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Q. 34
A student heats 25g of reactant ‘A’ with 50g of reactant ‘B’... What law is confirmed?
A
Law of conservation of mass
B
Law of constant proportion
C
Law of multiple proportions
D
Law of conservation of mass and Law of constant proportion
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Q. 35
Molecules can exist in free state because:
A
They are very stable
B
They are electrically charged
C
They are very reactive
D
They are bigger in size
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Q. 36
How many atoms of oxygen are present in 300 grams of CaCO₃?
A
54.207 × 10²³
B
22.2 × 10²³
C
6.207 × 10²³
D
12.207 × 10²³
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Q. 37
Study of internal structure of Earth is known as
A
nuclear physics
B
geophysics
C
atomic physics
D
mechanics
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Q. 38
3.42 g of sucrose are dissolved in 18 g of water. The number of oxygen atoms in the solution is:
A
6.68 × 10²³
B
6.09 × 10²²
C
6.022 × 10²¹
D
6.022 × 10²³
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Q. 39
How many atoms are in a molecule of phosphorus (P₄)?
A
4
B
1
C
8
D
2
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Q. 40
The reaction used to verify the law of conservation of mass is included in which category?
A
Precipitation reaction
B
Double displacement reaction
C
Neutralization reaction
D
Both Precipitation reaction and Double displacement reaction
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Q. 41
Which statement is not in accordance with Dalton’s atomic theory?
A
All matter is made of tiny particles.
B
Atoms can be divided into subatomic particles.
C
Atoms of given elements are identical in mass and properties.
D
Atoms combine in small whole number ratios.
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Q. 42
What is a molecule?
A
A single atom
B
A group of atoms
C
A compound
D
A charged ion
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Q. 43
An element X is divalent and another element Y is tetravalent. The compound formed will be:
A
XY₂
B
XY₄
C
XY
D
X₂Y
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Q. 44
Which element has an atomic mass closest to 12 u?
A
Nitrogen
B
Hydrogen
C
Carbon
D
Oxygen
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Q. 45
How is the molecular mass of a substance expressed?
A
In grams
B
In liters
C
In atomic mass units
D
In moles
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Q. 46
What is the formula mass unit of ZnO?
A
18 u
B
88 u
C
81 u
D
188 u
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Q. 47
What do the subscripts in a chemical formula represent?
A
Number of ions
B
Number of molecules
C
Number of atoms
D
Number of elements
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Q. 48
When CaCO3 is heated, it forms CaO and CO2. What best describes the mass of products?
A
The mass of each product is equal to CaCO3.
B
The sum of the products’ masses equals the mass of CaCO3.
C
The sum of the products’ masses is less than the mass of CaCO3.
D
The difference in products’ masses is equal to the mass of CaCO3.
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Q. 49
Which constituent of air is monoatomic?
A
Water vapour
B
Argon
C
Oxygen
D
Nitrogen
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Q. 50
What is the atomic mass of nitrogen?
A
14 a.m.u.
B
7 a.m.u.
C
8 a.m.u.
D
16 a.m.u.
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Q. 51
Which of the following is a polyatomic ion?
A
Cl⁻
B
H₂
C
OH⁻
D
Na⁺
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Q. 52
Which statement is true about the law of conservation of mass?
A
Elements are always present in a constant proportion.
B
If two elements form more than one compound...
C
The rate of reaction is proportional to active mass.
D
In a chemical reaction, matter is conserved.
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Q. 53
What is a major drawback of Dalton’s atomic theory?
A
Atoms of an element have exactly the same mass.
B
All of these
C
Atoms of different elements have same masses.
D
Atoms are indivisible.
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Q. 54
A change in the physical state can be brought about:
A
only when energy is given to the system
B
Without any energy change
C
only when energy is taken out from the system
D
When energy is either given to or taken out
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Q. 55
What is the atomic mass unit (amu) based on?
A
Oxygen
B
Carbon-12
C
Nitrogen
D
Hydrogen
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Q. 56
Valency of Calcium element is:
A
5
B
4
C
2
D
3
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Q. 57
Which of the following is a triatomic molecule?
A
O₃
B
Cl₂
C
H₂
D
N₂
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Q. 58
The molecular formula of potassium nitrate is ________.
A
KNO
B
KNO₂
C
KNO3
D
KON
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Q. 59
What type of molecule is helium (He)?
A
Monoatomic
B
Diatomic
C
Polyatomic
D
Triatomic
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Q. 60
Which statement about atoms is correct?
A
They cannot bond
B
They are visible
C
They always form molecules
D
They can exist independently
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Q. 61
Molecular mass is defined as the:
A
Mass of one molecule compared to one atom of C-12
B
Mass of one atom compared to the mass of one atom of hydrogen
C
Mass of one atom compared to one molecule
D
None of the above
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Q. 62
The atomic masses and chemical properties of Sulphur and Phosphorus are different. This can be explained on the basis of:
A
Law of constant proportions
B
Dalton’s Atomic theory
C
Law of conservation of mass
D
Laws of Chemical combination
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Q. 63
What is the valency of an element?
A
Its color
B
Its atomic mass
C
Its ability to combine
D
Its charge
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Q. 64
Who proposed the Law of Constant Proportions?
A
Antoine Lavoisier
B
Joseph Proust
C
John Dalton
D
Dmitri Mendeleev
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Q. 65
According to Law of conservation of mass, mass of reactants will be equal to the mass of:
A
Apparatus used for reaction
B
Gases evolved
C
Catalysts
D
Products
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Q. 66
What is the formula unit mass of CaCl₂?
A
150 u
B
71 u
C
100 u
D
111 u
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Q. 67
The number of atoms present in one molecule of an element is called as:
A
Atomicity
B
Molecule
C
Empirical formula
D
Compound
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Q. 68
What does a cation represent?
A
Neutral ion
B
Negatively charged ion
C
Positively charged ion
D
A group of atoms
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Q. 69
Molecules of compounds are made up of different types of atoms in ______ proportions.
A
definite
B
specific
C
unlimited
D
random
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Q. 70
A sample of NH3 molecule contains 82.35% Nitrogen and 17.65% Hydrogen by mass. This data supports:
A
Law of Definite Proportions
B
Law of Conservation of Mass
C
Avogadro’s Law
D
Law of Multiple Proportions
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Q. 71
What is the molecular mass of water (H₂O)?
A
20 u
B
16 u
C
12 u
D
18 u
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Q. 72
How are formulas for compounds with polyatomic ions written?
A
With numbers
B
With brackets
C
With no brackets
D
With letters
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Q. 73
Which of the following is a diatomic molecule?
A
H₂O
B
N₂
C
O₃
D
CO₂
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Q. 74
Maharishi Kanad suggested that matter could be divided until reaching an indivisible unit called "_____".
A
Parmanus
B
Protons
C
Atoms
D
Molecules
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Q. 75
Which of the following represents a correct chemical formula?
A
NaS
B
CaCl
C
NaSO₄
D
BiPO₄
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Q. 76
What is a negatively charged ion called?
A
Cation
B
Anion
C
Proton
D
Neutron
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Q. 77
5 grams of compound A reacts with 10 grams of compound B... What should be the mass of C?
A
50 g
B
15 g
C
10 g
D
5 g
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Q. 78
NaCl molecule is made of which of the following ions?
A
Na cation and Cl anion
B
Cl cation and Na anion
C
Both Na and Cl cation
D
Both Na and Cl anion
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Q. 79
Which one of the following is the modern symbol of Gold?
A
Gl
B
Go
C
Si
D
Au
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Q. 80
Which is correct for the ‘Law of Conservation of Mass’?
A
Mass can be created or destroyed.
B
The rate of reaction is directly proportional to active mass.
C
Mass can neither be created nor destroyed.
D
Elements are always present in a constant proportion.
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Q. 81
What does the Law of Conservation of Mass state?
A
Mass can be created
B
Mass changes in reactions
C
Mass remains constant
D
Mass can be destroyed
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Q. 82
Who stated the Law of conservation of mass?
A
Thomson
B
Dalton
C
Cavendish
D
Lavoisier
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Q. 83
What should be the formula of hydrogen sulphide?
A
H2S2
B
H2S
C
HS
D
HS2
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Q. 84
Which of the following is a characteristic of a molecule?
A
Cannot exist alone
B
Cannot form bonds
C
Is visible
D
Has fixed mass
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Q. 85
An atom has a mass number of 23 and atomic number 11. The number of protons are ________.
A
11
B
23
C
12
D
44
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Q. 86
Which is not a postulate of Dalton’s atomic theory?
A
The number of atoms in a compound is fixed.
B
Atoms cannot be divided.
C
Atoms can neither be created nor destroyed.
D
Atoms of the same elements cannot combine in more than one ratio.
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Q. 87
If all reactants in a chemical reaction are completely used, which statement accurately describes the relationship?
A
Products must have different physical states than reactants.
B
Total mass of reactants = total mass of products.
C
Density of reactants must equal density of products.
D
Reactants must contain more complex molecules than products.
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Q. 88
Chemical formula of a compound is A2B3. The valency of A is
A
5
B
6
C
3
D
2
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