TNPSC Chemistry Atoms and Molecules Quiz 1 Important MCQ

Everything around us—from the air we breathe to the water we drink—is made of tiny particles called atoms and molecules. These fundamental concepts explain how matter is formed and why substances have different properties. For TNPSC aspirants, understanding atomic structure, molecules, chemical symbols, and related laws is essential because these concepts form the base of many Chemistry questions. Begin this quiz to evaluate your understanding and strengthen your preparation with exam-oriented questions.

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TNPSC Chemistry Atoms and Molecules Quiz 1 – Detailed Explanations

The journey of modern Chemistry began with the idea that matter is composed of extremely small particles. An atom is the smallest unit of an element that can participate in a chemical reaction, while a molecule is formed when two or more atoms combine through chemical bonds. Scientists such as John Dalton laid the foundation of atomic theory, which later helped explain chemical reactions, compounds, and the conservation of matter. In TNPSC examinations, candidates are expected to understand concepts like atomic number, mass number, valency, molecular formulae, and the laws governing the combination of elements. Mastering these basics makes advanced Chemistry topics much easier to learn.

1. Who proposed the Atomic Theory in 1808?

John Dalton proposed the Atomic Theory in 1808, laying the foundation for modern Chemistry. According to his theory, all matter is made up of tiny, indivisible particles called atoms. Dalton stated that atoms of the same element have similar properties and mass, while atoms of different elements differ in their characteristics. He also explained that atoms combine in simple whole-number ratios to form compounds and that chemical reactions involve only the rearrangement of atoms without creating or destroying them. Although later discoveries revealed the existence of subatomic particles such as electrons, protons, and neutrons, Dalton’s theory remains one of the most significant milestones in the history of Chemistry and continues to be an important topic for competitive examinations.

Key Points

John Dalton proposed the Atomic Theory in 1808.
Matter is made up of tiny particles called atoms.
Atoms of different elements have different properties.
Compounds are formed when atoms combine in simple whole-number ratios.
Dalton’s Atomic Theory is considered the foundation of modern Chemistry.

✅ Correct Answer: B. John Dalton

2. The smallest unit of an element that retains its chemical properties is called:

An atom is the smallest particle of an element that retains all the chemical properties of that element. It cannot be broken down into simpler substances by ordinary chemical reactions. Every element in the periodic table is made up of atoms, each having a unique number of protons known as the atomic number. Atoms consist of a central nucleus containing protons and neutrons, while electrons move around the nucleus in different energy levels. When atoms combine chemically, they form molecules and compounds. Understanding the structure and properties of atoms is essential because they form the basis of all chemical reactions and are fundamental to the study of Chemistry.

Key Points

An atom is the smallest unit of an element.
It retains the chemical properties of the element.
Atoms are made up of protons, neutrons, and electrons.
Every element is composed of only one type of atom.
Atoms combine chemically to form molecules and compounds.

✅ Correct Answer: A. Atom

3. A molecule is formed when:

A molecule is formed when two or more atoms are chemically bonded together. These atoms may belong to the same element, such as oxygen (O₂) and nitrogen (N₂), or different elements, such as water (H₂O) and carbon dioxide (CO₂). Molecules are the smallest units of a substance that can exist independently while retaining its chemical properties. Chemical bonds hold the atoms together, and the arrangement of these atoms determines the physical and chemical characteristics of the substance. Understanding molecules helps explain how different substances are formed and why they behave differently during chemical reactions.

Key Points

A molecule consists of two or more chemically bonded atoms.
Molecules can be formed from the same or different elements.
Molecules retain the chemical properties of a substance.
Chemical bonds hold atoms together in a molecule.
Examples include O₂, H₂O, CO₂, and NH₃.

✅ Correct Answer: A. Two or more atoms combine chemically

4. The chemical formula of water is:

Water is one of the most important compounds found on Earth and has the chemical formula H₂O. Each water molecule is made up of two hydrogen atoms and one oxygen atom, which are chemically bonded together. The subscript “2” indicates that two hydrogen atoms are present, while the absence of a subscript after oxygen means there is one oxygen atom. Water is essential for all living organisms and plays a vital role in biological processes, agriculture, industry, and the environment. Knowing common chemical formulae is important for understanding chemical compounds and reactions.

Key Points

Water has the chemical formula H₂O.
One molecule contains 2 hydrogen atoms and 1 oxygen atom.
Water is a compound, not an element.
Hydrogen and oxygen are joined by chemical bonds.
H₂O is one of the most commonly asked chemical formulae in competitive exams.

✅ Correct Answer: B. H₂O

5. Which of the following is a molecule of an element?

A molecule of an element contains two or more atoms of the same element chemically bonded together. Oxygen gas (O₂) consists of two oxygen atoms joined by a covalent bond, making it a molecule of an element. Similarly, hydrogen (H₂), nitrogen (N₂), and chlorine (Cl₂) are also molecules of elements. In contrast, substances such as water (H₂O) and carbon dioxide (CO₂) contain atoms of different elements and are therefore classified as compounds. Understanding this distinction helps in identifying whether a substance is an element or a compound.

Key Points

A molecule of an element contains only one type of atom.
O₂ is made up of two oxygen atoms.
H₂, N₂, and Cl₂ are also molecules of elements.
Compounds contain different kinds of atoms.
O₂ is essential for respiration in living organisms.

✅ Correct Answer: C. O₂

6. Avogadro’s number is:

Avogadro’s number is a fundamental constant in Chemistry that represents the number of particles present in one mole of any substance. Its value is 6.022 × 10²³, and these particles may be atoms, molecules, or ions depending on the substance. Named after the Italian scientist Amedeo Avogadro, this number helps chemists measure and compare quantities of matter accurately. The mole concept connects the microscopic world of atoms and molecules with measurable laboratory quantities such as mass, making it one of the most important concepts in Chemistry.

Key Points

Avogadro’s number is 6.022 × 10²³.
One mole contains 6.022 × 10²³ particles.
Particles may be atoms, molecules, or ions.
It forms the basis of the mole concept.
Named after Amedeo Avogadro.

✅ Correct Answer: A. 6.022 × 10²³

7. The SI unit of amount of substance is:

The mole (mol) is the SI unit used to measure the amount of substance. One mole contains exactly 6.022 × 10²³ elementary entities, such as atoms, molecules, or ions. Instead of counting individual particles, which is practically impossible due to their extremely small size, scientists use the mole as a convenient unit for measuring chemical substances. The mole plays an important role in calculating chemical reactions, determining molecular masses, and preparing solutions in laboratories. It is one of the seven fundamental SI units used in science.

Key Points

The SI unit of amount of substance is the mole (mol).
One mole contains 6.022 × 10²³ particles.
The mole is one of the seven SI base units.
It is widely used in chemical calculations.
The mole helps relate the mass of a substance to the number of particles.

✅ Correct Answer: B. Mole

8. Which particle carries a negative charge?

An electron is a negatively charged subatomic particle found outside the nucleus of an atom. Electrons move around the nucleus in different energy levels or shells and play a major role in chemical bonding and chemical reactions. The movement and arrangement of electrons determine the chemical properties of an element. Unlike protons and neutrons, electrons have a very small mass but are essential for the formation of ions and compounds. The discovery of the electron by J.J. Thomson marked an important milestone in understanding the internal structure of atoms.

Key Points

Electrons carry a negative (-) charge.
They move around the nucleus in energy levels.
Electrons participate in chemical bonding.
They were discovered by J.J. Thomson.
The arrangement of electrons determines an element’s chemical properties.

✅ Correct Answer: C. Electron

9. Which subatomic particle has no charge?

A neutron is a neutral subatomic particle found inside the nucleus of an atom. Unlike protons and electrons, neutrons do not carry any electric charge. They contribute significantly to the mass of an atom and help maintain the stability of the nucleus by reducing the repulsive force between positively charged protons. The number of neutrons may vary among atoms of the same element, leading to the formation of isotopes. Neutrons were discovered by James Chadwick in 1932, which greatly improved our understanding of atomic structure.

Key Points

Neutrons have no electric charge.
They are located in the nucleus of an atom.
Neutrons contribute to the mass of an atom.
Different numbers of neutrons produce isotopes.
Neutrons were discovered by James Chadwick.

✅ Correct Answer: C. Neutron

10. The center of an atom is called:

The nucleus is the central region of an atom that contains protons and neutrons. Although it occupies only a very small portion of the atom’s volume, it contains almost all of the atom’s mass. Electrons move around the nucleus in different energy levels. The nucleus was discovered through Ernest Rutherford’s gold foil experiment, which showed that most of an atom is empty space with a dense, positively charged center. Understanding the nucleus is important because it determines the identity and stability of an element.

Key Points

The nucleus is the center of an atom.
It contains protons and neutrons.
Nearly all the mass of an atom is concentrated in the nucleus.
Electrons revolve around the nucleus.
The nucleus was discovered by Ernest Rutherford.

✅ Correct Answer: D. Nucleus

11. Atoms of the same element having different mass numbers are called:

Isotopes are atoms of the same element that have the same atomic number but different mass numbers because they contain different numbers of neutrons. Since the number of protons remains unchanged, isotopes have similar chemical properties but may differ in physical properties such as mass and stability. Some isotopes occur naturally, while others are artificially produced for scientific and medical purposes. For example, Carbon-12 and Carbon-14 are isotopes of carbon. Understanding isotopes is important in Chemistry, medicine, archaeology, and nuclear science.

Key Points

Isotopes have the same atomic number.
They have different mass numbers.
The difference is due to the number of neutrons.
Isotopes show similar chemical properties.
Examples include Carbon-12 and Carbon-14.

✅ Correct Answer: B. Isotopes

12. The number of protons in an atom is known as:

The atomic number is the number of protons present in the nucleus of an atom. It is represented by the symbol Z and uniquely identifies an element in the periodic table. Every atom of a particular element has the same atomic number. For example, hydrogen has one proton, so its atomic number is 1, while oxygen has eight protons, giving it an atomic number of 8. In a neutral atom, the number of electrons is equal to the atomic number. The atomic number is one of the most fundamental properties used in Chemistry.

Key Points

Atomic number equals the number of protons.
It is represented by the symbol Z.
It determines the identity of an element.
Neutral atoms have equal numbers of protons and electrons.
Elements are arranged in the periodic table according to their atomic numbers.

✅ Correct Answer: C. Atomic Number

13. The mass number of an atom is equal to:

The mass number of an atom is the total number of protons and neutrons present in its nucleus. It is represented by the symbol A and helps distinguish different isotopes of the same element. Since electrons have negligible mass, they are not included in the calculation of the mass number. For example, a carbon atom with 6 protons and 6 neutrons has a mass number of 12. Knowing the mass number is useful for calculating the number of neutrons using the formula: Mass Number = Protons + Neutrons.

Key Points

Mass number = Protons + Neutrons.
It is represented by the symbol A.
Electrons are not included in the calculation.
Mass number helps identify isotopes.
Number of neutrons = Mass Number − Atomic Number.

✅ Correct Answer: B. Protons + Neutrons

14. Which of the following is a compound?

A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio. The properties of a compound are different from those of the elements that make it. Water (H₂O) is a compound because it is formed from hydrogen and oxygen atoms chemically bonded together. Unlike mixtures, compounds cannot be separated by simple physical methods. They can only be broken down into their constituent elements through chemical reactions. Compounds are essential in everyday life and include substances such as water, carbon dioxide, and sodium chloride.

Key Points

Compounds contain two or more different elements.
Elements are combined in a fixed proportion.
H₂O is a compound made of hydrogen and oxygen.
Compounds have properties different from their constituent elements.
They can be separated only by chemical methods.

✅ Correct Answer: C. H₂O

15. The combining capacity of an atom is called:

Valency is the combining capacity of an atom, indicating how many electrons an atom can lose, gain, or share to achieve a stable electronic configuration. It helps determine how atoms combine to form molecules and compounds. For example, hydrogen has a valency of 1, oxygen has a valency of 2, and nitrogen has a valency of 3. Knowing the valency of elements is essential for writing correct chemical formulae and understanding chemical bonding. Valency is one of the fundamental concepts in Chemistry and is frequently tested in competitive examinations.

Key Points

Valency is the combining capacity of an atom.
It depends on the outermost electrons.
Valency helps in writing chemical formulae.
Elements combine by losing, gaining, or sharing electrons.
Common valencies include H = 1, O = 2, and N = 3.

✅ Correct Answer: D. Valency

16. Which gas is represented by the formula CO₂?

Carbon dioxide (CO₂) is a compound consisting of one carbon atom and two oxygen atoms. It is a colorless and odorless gas naturally present in the Earth’s atmosphere. Carbon dioxide is released during respiration, combustion of fuels, and various industrial processes. Plants absorb CO₂ during photosynthesis to produce food and release oxygen, making it essential for maintaining the balance of atmospheric gases. Because of its role in the greenhouse effect and climate change, carbon dioxide is one of the most important gases studied in environmental science and Chemistry.

Key Points

CO₂ stands for carbon dioxide.
It contains one carbon and two oxygen atoms.
Plants use CO₂ during photosynthesis.
It is produced by respiration and combustion.
CO₂ is an important greenhouse gas.

✅ Correct Answer: B. Carbon dioxide

17. The formula of ammonia is:

Ammonia is a chemical compound with the formula NH₃, consisting of one nitrogen atom and three hydrogen atoms. It is a colorless gas with a strong, pungent smell and is highly soluble in water. Ammonia is widely used in the manufacture of fertilizers, cleaning agents, explosives, and many industrial chemicals. In agriculture, it is an important source of nitrogen, which is essential for plant growth. Understanding common chemical formulae such as NH₃, H₂O, CO₂, and NaCl is important for solving Chemistry questions in competitive examinations.

Key Points

Ammonia has the chemical formula NH₃.
It contains one nitrogen and three hydrogen atoms.
It is widely used in the fertilizer industry.
Ammonia is a colorless gas with a pungent smell.
NH₃ is one of the most important chemical formulae to remember.

✅ Correct Answer: A. NH₃

18. Which element has atomic number 1?

Hydrogen is the first element in the periodic table and has an atomic number of 1, meaning each hydrogen atom contains one proton in its nucleus. It is the lightest and most abundant element in the universe, making up a large portion of stars, including the Sun. In its natural state, hydrogen usually exists as a diatomic molecule (H₂). It is widely used in industries, fuel cells, and the production of ammonia. The atomic number is a unique property that distinguishes one element from another and determines its position in the periodic table.

Key Points

Hydrogen has an atomic number of 1.
It contains one proton in its nucleus.
It is the lightest element in the periodic table.
Hydrogen commonly exists as H₂.
Atomic number determines an element’s identity.

✅ Correct Answer: C. Hydrogen

19. Which particles determine the chemical properties of an atom?

The electrons, especially those present in the outermost shell (called valence electrons), determine the chemical properties of an atom. These electrons participate in chemical bonding and reactions by being shared, gained, or lost. The arrangement of electrons influences an element’s valency, reactivity, and the type of compounds it can form. Elements with similar valence electron configurations often show similar chemical properties, which is why they are grouped together in the periodic table. Understanding the role of electrons is essential for learning chemical bonding and reactions.

Key Points

Valence electrons determine chemical properties.
Electrons participate in chemical bonding.
They can be shared, gained, or lost during reactions.
Electron arrangement determines an element’s reactivity.
Elements in the same group have similar valence electrons.

✅ Correct Answer: C. Electrons

20. Matter is made up of tiny particles called:

According to Dalton’s Atomic Theory, all matter is made up of extremely tiny particles called atoms. These atoms are the fundamental building blocks of all elements and compounds. Every substance, whether solid, liquid, or gas, consists of atoms arranged in different ways. Atoms combine to form molecules, and molecules combine to form the various materials found in nature. Although atoms themselves contain smaller particles such as protons, neutrons, and electrons, they remain the basic units responsible for the chemical properties of elements. This concept forms the foundation of modern Chemistry.

Key Points

Matter is made up of tiny particles called atoms.
Atoms are the basic building blocks of matter.
Atoms combine to form molecules and compounds.
The concept was explained by John Dalton’s Atomic Theory.
All solids, liquids, and gases are composed of atoms.

✅ Correct Answer: A. Atoms

TNPSC Exam Tips

Revise John Dalton’s Atomic Theory and its basic postulates.
Understand the difference between an atom, molecule, element, and compound.
Memorize common chemical formulae such as H₂O, CO₂, NH₃, O₂, N₂, and NaCl.
Learn the definitions of atomic number, mass number, and valency.
Remember the charges of protons (+), electrons (−), and neutrons (0).
Study the concept of isotopes and how they differ from atoms with different atomic numbers.
Remember Avogadro’s number (6.022 × 10²³) and the SI unit mole (mol).
Practice writing simple chemical formulae using valency.
Revise important scientists such as John Dalton, J.J. Thomson, Ernest Rutherford, James Chadwick, and Niels Bohr along with their contributions.
Solve previous TNPSC General Science questions to improve accuracy and speed.

TNPSC Chemistry Atoms and Molecules Quiz 1 – Detailed Explanations

1. Who proposed the Atomic Theory in 1808?

2. The smallest unit of an element that retains its chemical properties is called:

3. A molecule is formed when:

4. The chemical formula of water is:

5. Which of the following is a molecule of an element?

6. Avogadro’s number is:

7. The SI unit of amount of substance is:

8. Which particle carries a negative charge?

9. Which subatomic particle has no charge?

10. The center of an atom is called:

11. Atoms of the same element having different mass numbers are called:

12. The number of protons in an atom is known as:

13. The mass number of an atom is equal to:

14. Which of the following is a compound?

15. The combining capacity of an atom is called:

16. Which gas is represented by the formula CO₂?

17. The formula of ammonia is:

18. Which element has atomic number 1?

19. Which particles determine the chemical properties of an atom?

20. Matter is made up of tiny particles called:

TNPSC Exam Tips

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