Pure substances can further be classified into elements and compounds. Particles of an element consist of only one type of atoms. These particles may exist as atoms or molecules.
-- NCERT Class 11 Chemistry, Ch. 1, p. 5Atom Molecule Element Compound
8 MCQs9-step worked example
Source: NCERT Some Basic Concepts of ChemistryPYQ coverage: NEET 2020, 2023Official key: NTA-verifiedLast updated: 8 Oct 2026
Try this first
Which of the following is an element whose constituent particles are molecules?
- A.Copper
- B.Hydrogen gas
- C.Water
- D.Ammonia
Tap to see the answer
Answer: B. Hydrogen, nitrogen and oxygen gases are elements whose particles are molecules of two combined atoms (NCERT Class 11 Chemistry, Chapter 1, page 6).
A is wrong: A is wrong because copper is an element whose particles are single atoms, not molecules (NCERT Class 11 Chemistry, Chapter 1, page 6).
C is wrong: C is wrong because water is a compound: its molecule joins hydrogen and oxygen atoms (NCERT Class 11 Chemistry, Chapter 1, page 6).
D is wrong: D is wrong because ammonia is a compound, listed by NCERT among the examples of compounds (NCERT Class 11 Chemistry, Chapter 1, page 6).
Atom Molecule Element Compound, explained for NEET
Is a molecule the same thing as a compound? No: O₂ is a molecule and it is an element. That one slip decides questions that look like simple classification.
Element. NCERT places elements and compounds under pure substances. The particles of an element consist of only one type of atom, and those particles may exist as atoms or as molecules (NCERT Class 11 Chemistry, Chapter 1, page 5). Sodium and copper contain atoms; hydrogen, nitrogen and oxygen gases consist of molecules in which two atoms combine (NCERT Class 11 Chemistry, Chapter 1, page 6).
Compound. When two or more atoms of different elements combine in a definite ratio, the molecule of a compound is obtained, and its constituents cannot be separated by physical methods, only by chemical methods (NCERT Class 11 Chemistry, Chapter 1, page 6). A water molecule has two hydrogen atoms and one oxygen atom; a carbon dioxide molecule has one carbon atom and two oxygen atoms (NCERT Class 11 Chemistry, Chapter 1, page 6).
Properties change. A compound behaves differently from its elements: hydrogen and oxygen are gases, but water is a liquid, and water puts out fires while oxygen supports combustion (NCERT Class 11 Chemistry, Chapter 1, page 6).
Two tests that never fail. Count the kinds of atom in one particle: one kind means an element (atom or molecule), two or more kinds in a fixed ratio means a compound. Then ask how it separates: physical methods point to a mixture, only chemical methods to a compound.
NEET link. The tagged question for this topic asked for protons, neutrons and electrons in a named atom (¹⁷⁵₇₁Lu). That needs the atomic number and the mass number from Chapter 2, which MCQ 6 practises.
Watch out: "Molecule" says how many atoms are bound together. "Compound" says how many kinds. Check both before you classify.
How do you solve a Atom Molecule Element Compound question? A worked example
- 1
Given
3 molecules of nitrogen gas (N₂) and 2 molecules of ammonia (NH₃).
- 2
Required
Classify each substance as an element or a compound, and count the total atoms and the atoms of each element.
- 3
Concept
An element has one type of atom in its particles; a compound has atoms of different elements in a definite ratio (NCERT Class 11 Chemistry, Chapter 1, pages 5 and 6).
- 4
Formula
Atoms in a sample = (molecules) × (atoms per molecule), counted separately for each kind of atom.
- 5
Substitution
N₂: 2 atoms of nitrogen per molecule. NH₃: 1 nitrogen + 3 hydrogen = 4 atoms per molecule.
- 6
Calculation
Atoms from N₂ = 3 × 2 = 6 (all nitrogen). Atoms from NH₃ = 2 × 4 = 8 (2 nitrogen + 6 hydrogen). Total atoms = 6 + 8 = 14. Nitrogen atoms = 6 + 2 = 8; hydrogen atoms = 6. The counts 3, 2 and the subscripts are exact counting numbers and do not affect any significant-figure count.
- 7
Final answer
N₂ is an element (molecules of one kind of atom); NH₃ is a compound. The sample has 14 atoms: 8 of nitrogen and 6 of hydrogen.
- 8
Common trap
Calling N₂ a compound because it is a molecule. A molecule says how many atoms are bound; a compound says how many kinds of atom.
- 9
Similar NEET-style question
How many atoms are present in 2 molecules of CO₂ and 3 molecules of O₂ together, and which of the two is the element? (Answer: CO₂ has 1 + 2 = 3 atoms, so 2 × 3 = 6; O₂ has 2 atoms, so 3 × 2 = 6; total = 6 + 6 = 12 atoms. O₂ is the element; CO₂ is the compound.)
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Can you answer these Atom Molecule Element Compound MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which of the following is an element whose constituent particles are molecules?
Show answer and why every option is right or wrong
Answer: B. Hydrogen, nitrogen and oxygen gases are elements whose particles are molecules of two combined atoms (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why A is wrong: A is wrong because copper is an element whose particles are single atoms, not molecules (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why C is wrong: C is wrong because water is a compound: its molecule joins hydrogen and oxygen atoms (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why D is wrong: D is wrong because ammonia is a compound, listed by NCERT among the examples of compounds (NCERT Class 11 Chemistry, Chapter 1, page 6).
MCQ 2Easy RecallPractice
A compound is best described as:
Show answer and why every option is right or wrong
Answer: D. NCERT describes a compound as atoms of different elements combined in a definite ratio, separable into simpler substances only by chemical methods (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why A is wrong: A is wrong because atoms of the same kind joined make a molecule of an element, and filtration is a physical method that does not break a substance into its elements (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why B is wrong: B is wrong because a mixture has components in any ratio and they separate by physical methods; a compound has a definite ratio (NCERT Class 11 Chemistry, Chapter 1, page 5).
Why C is wrong: C is wrong because that is the description of an element, not a compound (NCERT Class 11 Chemistry, Chapter 1, page 5).
MCQ 3Easy RecallPractice
One molecule of carbon dioxide contains:
Show answer and why every option is right or wrong
Answer: A. A molecule of carbon dioxide contains two oxygen atoms combined with one carbon atom (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why B is wrong: B is wrong because it reverses the counts; it is oxygen that appears twice (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why C is wrong: C is wrong because one carbon with one oxygen is a different compound (carbon monoxide), not carbon dioxide (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why D is wrong: D is wrong because it doubles the carbon; the molecule has one carbon atom (NCERT Class 11 Chemistry, Chapter 1, page 6).
MCQ 4Direct ApplicationPractice
Which set lists only compounds?
Show answer and why every option is right or wrong
Answer: C. Water, ammonia and carbon dioxide are all named by NCERT as compounds (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why A is wrong: A is wrong because oxygen and nitrogen gases are elements: their molecules contain two atoms of one kind (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why B is wrong: B is wrong because air is a homogeneous mixture, not a compound (NCERT Class 11 Chemistry, Chapter 1, page 5).
Why D is wrong: D is wrong because sodium and copper are elements made of single atoms (NCERT Class 11 Chemistry, Chapter 1, page 6).
MCQ 5Direct ApplicationPractice
Hydrogen burns with a pop, oxygen supports combustion, and water is used as a fire extinguisher. What does this show?
Show answer and why every option is right or wrong
Answer: B. NCERT uses exactly this example: the properties of a compound are different from those of its constituent elements (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why A is wrong: A is wrong because water is a liquid that extinguishes fire; no averaging of a gas that pops and a gas that supports combustion gives that (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why C is wrong: C is wrong because a mixture has variable composition and separates physically; water has a fixed ratio and separates only chemically (NCERT Class 11 Chemistry, Chapter 1, page 5).
Why D is wrong: D is wrong because the elements do not keep their separate properties once combined in a compound (NCERT Class 11 Chemistry, Chapter 1, page 6).
MCQ 6Direct ApplicationPractice
The numbers of protons, neutrons and electrons in ¹⁷⁵₇₁Lu, respectively, are:
Show answer and why every option is right or wrong
Answer: A. The atomic number (71) is the number of protons and also of electrons; neutrons = mass number − protons = 175 − 71 = 104 (NCERT Class 11 Chemistry, Chapter 2, page 35).
Why B is wrong: B is wrong because 175 is the mass number (protons plus neutrons), not the number of protons (NCERT Class 11 Chemistry, Chapter 2, page 35).
Why C is wrong: C is wrong because it swaps protons and neutrons; the atomic number 71 is the proton count (NCERT Class 11 Chemistry, Chapter 2, page 35).
Why D is wrong: D is wrong because it makes the electron count 104; an atom is neutral, so electrons equal protons, 71 (NCERT Class 11 Chemistry, Chapter 2, page 35).
MCQ 7CalculationPractice
Consider H₂, H₂O, CO₂, N₂ and NH₃. How many are elements and how many are compounds?
Show answer and why every option is right or wrong
Answer: D. H₂ and N₂ contain one kind of atom each, so they are elements (2); H₂O, CO₂ and NH₃ each contain atoms of different elements in a fixed ratio, so they are compounds (3) (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why A is wrong: A is wrong because it calls every molecule an element; a molecule with atoms of different kinds is a compound (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why B is wrong: B is wrong because it counts one multi-atom compound molecule as an element; CO₂, for example, joins carbon and oxygen (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why C is wrong: C is wrong because it forgets that N₂ is also a molecule of one kind of atom, like hydrogen and oxygen gases (NCERT Class 11 Chemistry, Chapter 1, page 6).
MCQ 8CalculationPractice
Which of the following statements are correct? (i) Every molecule is a compound. (ii) The constituents of a compound can be separated only by chemical methods. (iii) Oxygen gas consists of molecules. (iv) A molecule of an element contains atoms of one type only.
Show answer and why every option is right or wrong
Answer: C. (ii) is stated for compounds, (iii) holds because oxygen gas has two-atom molecules, and (iv) holds because an element has one type of atom; (i) is false because O₂ is a molecule of an element (NCERT Class 11 Chemistry, Chapter 1, pages 5 and 6).
Why A is wrong: A is wrong because it keeps (i); O₂ is a molecule yet it is an element, not a compound (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why B is wrong: B is wrong because it keeps (i) and drops the true statements (ii) and (iii) (NCERT Class 11 Chemistry, Chapter 1, page 6).
Why D is wrong: D is wrong because it drops (iii); oxygen gas does consist of molecules of two combined atoms (NCERT Class 11 Chemistry, Chapter 1, page 6).
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What to remember before solving Atom Molecule Element Compound questions
5 NCERT lines
Key Fact
Elements: sodium and copper consist of atoms; hydrogen, nitrogen, oxygen of two-atom molecules
Sodium, copper, silver, hydrogen, oxygen, etc., are some examples of elements. Their all atoms are of one type. However, the atoms of different elements are different in nature. Some elements, such as sodium or copper, contain atoms as their constituent particles, whereas, in some others, the constituent particles are molecules which are formed by two or more atoms. For example, hydrogen, nitrogen and oxygen gases consist of molecules, in which two atoms combine to give their respective molecules.
-- NCERT Class 11 Chemistry, Ch. 1, p. 6When two or more atoms of different elements combine together in a definite ratio, the molecule of a compound is obtained. Moreover, the constituents of a compound cannot be separated into simpler substances by physical methods. They can be separated by chemical methods.
-- NCERT Class 11 Chemistry, Ch. 1, p. 6Note that a water molecule comprises two hydrogen atoms and one oxygen atom. Similarly, a molecule of carbon dioxide contains two oxygen atoms combined with one carbon atom.
-- NCERT Class 11 Chemistry, Ch. 1, p. 6Also, the properties of a compound are different from those of its constituent elements. For example, hydrogen and oxygen are gases, whereas, the compound formed by their combination i.e., water is a liquid. It is interesting to note that hydrogen burns with a pop sound and oxygen is a supporter of combustion, but water is used as a fire extinguisher.
-- NCERT Class 11 Chemistry, Ch. 1, p. 6Atom Molecule Element Compound: NEET previous year questions (PYQs) with answers
2 questions from NEET 2020, 2023, answers verified against NTA official keys
NEET 2023
1C, D and E only
2A and E only
3B, C and E only
4A, B and C only
NTA Answer: Option 3(final)
Why the other options are wrong
- Option 1: D is false: protons and neutrons present in the nucleus are collectively known as nucleons. B is also missing.
- Option 2: A is false: atoms consist of three fundamental particles, electrons, protons and neutrons. B and C are missing.
- Option 4: A is false: atoms are made of three fundamental particles, not two. E is missing.
NEET 2020
The number of protons, neutrons and electrons in ¹⁷⁵₇₁Lu, respectively, are :
1175, 104 and 71
271, 104 and 71
3104, 71 and 71
471, 71 and 104
NTA Answer: Option 2(final)
Why the other options are wrong
- Option 1: 175 is the mass number, not the proton count. Protons = Z = 71; neutrons = 175 − 71 = 104; electrons = 71.
- Option 3: Protons and neutrons are swapped. The number of protons equals the atomic number, 71, and neutrons = A − Z = 104.
- Option 4: Neutrons and electrons are swapped. A neutral atom has electrons equal to protons (71), and neutrons = 175 − 71 = 104.
All 12 past-paper questions from Some Basic Concepts of Chemistry →
More in Some Basic Concepts of Chemistry: 7 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.
Sources
NCERT refs: Class 11 Chemistry Chapter 1, p.5 | Class 11 Chemistry Chapter 1, p.6
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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