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Tamil Nadu (TNBSE)Class 7 Science★ 6-10 marks in the TN Class 7 Term 1 exam through atomic-particle, atomic-number and isotope questions

Atomic Structure

Samacheer Kalvi (TN State Board) Class 7 Science — Term 1 Chapter 4, Atomic Structure. Dalton's atomic theory, the subatomic particles (electron, proton, neutron), the nucleus and nucleons, atomic number (Z) and mass number, the electrically neutral atom, and isotopes, with the actual book-back questions, common mistakes and quick revision.

⏱ 14 min readEasy difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1State Dalton's atomic theory
2Name the subatomic particles and their charges
3Locate protons, neutrons and electrons
4Find atomic number and mass number
5Define isotopes

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Why this chapter matters

Atomic structure explains what every substance is built from and underlies all chemistry. The subatomic particles, atomic/mass number and isotopes are directly tested in the TN Class 7 Term 1 exam.

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A 5-minute refresher here will save you 30 minutes of confusion below.

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Chapter 3 — Matter Around Us

Atoms and elements.

Atomic Structure — Class 7 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 7 Science, Term 1 — Chapter 4. Inside the atom.

1. About this chapter

This chapter covers Dalton's atomic theory, the subatomic particles, the nucleus and nucleons, atomic number and mass number, and isotopes.

2. The atom and its particles

The atom is the basic unit of matter. It has three subatomic particles:

Particle	Charge	Location
Electron	negative (−1)	revolves around the nucleus
Proton	positive (+1)	in the nucleus
Neutron	no charge (0)	in the nucleus

The nucleus (centre) is surrounded by electrons. The protons and neutrons together are called nucleons.
An atom has equal numbers of protons and electrons, so it is electrically neutral.

3. Atomic number and mass number

Atomic number (Z) = the number of protons (= number of electrons in a neutral atom).
Mass number (A) = protons + neutrons (nucleons).

4. Isotopes

Isotopes are atoms of the same element (same atomic number) with different mass numbers — they have the same number of protons but different numbers of neutrons (e.g. isotopes of carbon and chlorine).

5. Worked examples

Example 1. What do nucleons consist of? Protons and neutrons.

Example 2. An atom has atomic number 11 and mass number 23. Find the neutrons. 23 − 11 = 12 neutrons.

Example 3. Why is an atom electrically neutral? It has equal numbers of protons (+) and electrons (−).

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

The basic unit of matter is the — (a) molecule / (b) atom. Ans: (b) atom.
The subatomic particle that revolves around the nucleus is the — (a) proton / (b) electron. Ans: (b) electron.
The positively charged particle is the — (a) proton / (b) electron. Ans: (a) proton.
The atomic number of an atom is the — (a) number of neutrons / (b) number of protons. Ans: (b) number of protons.
Nucleons comprise — (a) electrons and protons / (b) protons and neutrons. Ans: (b) protons and neutrons.

II. Fill in the blanks 6. The three subatomic particles are the proton, electron and neutron. 7. The atomic number is represented by the letter Z. 8. The nucleus is surrounded by electrons.

III. True or False 9. The basic unit of an element is the molecule. — False (it is the atom). 10. An atom is electrically neutral. — True.

7. Common mistakes

Mistake: Saying electrons are in the nucleus. Fix: Electrons revolve around the nucleus; protons and neutrons are inside it.
Mistake: Calling the molecule the basic unit of an element. Fix: The atom is the basic unit.
Mistake: Confusing atomic number and mass number. Fix: Atomic number = protons; mass number = protons + neutrons.

8. Quick revision

Term 1 · Ch 4 · atomic structure.
Atom = basic unit of matter; electron (−, around nucleus), proton (+, nucleus), neutron (0, nucleus).
Nucleons = protons + neutrons; atom is electrically neutral (equal protons and electrons).
Atomic number (Z) = protons; mass number = protons + neutrons; isotopes = same element, different mass number.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Subatomic particles

electron (−), proton (+), neutron (0)

Electrons in shells; nucleus has protons & neutrons.

Atomic number

Z = number of protons

= electrons in a neutral atom.

Mass number

A = protons + neutrons

Nucleons.

Isotopes

same atomic number, different mass number

Different neutrons.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Saying electrons are in the nucleus

✓ Electrons revolve around the nucleus; protons and neutrons are inside it.

WATCH OUT

✗ Confusing atomic number and mass number

✓ Atomic number = protons; mass number = protons + neutrons.

WATCH OUT

✗ Thinking isotopes are different elements

✓ Isotopes are the same element with different numbers of neutrons.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Choose the correct answer

Book-back MCQs on the atom, electron, proton, atomic number and nucleons.

Questions

Fill in the blanks

Subatomic particles, the letter Z and electrons around the nucleus.

Questions

True or False / Answer briefly

Basic unit, neutral atom, isotopes and mass number.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· MCQ

The subatomic particle that revolves around the nucleus is the ____.

▶ Show solution

electron.

Q2EASY· MCQ

The atomic number of an atom is the ____.

▶ Show solution

number of protons.

Q3EASY· MCQ

Nucleons comprise ____.

▶ Show solution

protons and neutrons.

Q4EASY· True/False

True or False: An atom is electrically neutral.

▶ Show solution

True (equal numbers of protons and electrons).

Q5MEDIUM· Neutrons

An atom has atomic number 11 and mass number 23. Find the number of neutrons.

▶ Show solution

23 − 11 = 12 neutrons.

Q6EASY· Answer briefly

Define isotopes with an example.

▶ Show solution

Atoms of the same element with the same atomic number but different mass numbers, e.g. the isotopes of carbon.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Term 1 Chapter 4 of Samacheer Kalvi Class 7 Science.
•Dalton: matter is made of indivisible atoms that combine in whole-number ratios.
•Electron (−) revolves in shells; proton (+) and neutron (0) are in the nucleus.
•The nucleus is positively charged.
•Atomic number = number of protons; mass number = protons + neutrons.
•Isotopes are atoms of the same element with different mass numbers.

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 6-10 marks across atomic structure

Question type	Marks each	Typical count	What it tests
Objective	1	3-4	Particles and locations
Numerical	2	1-2	Atomic/mass number
Short answer	2	1	Isotopes / Dalton

Prep strategy

Memorise the three particles and charges
Use Z = protons, A = protons + neutrons
Find neutrons as A − Z
Remember isotopes share the atomic number

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Medicine

Radioactive isotopes are used in scans and treatment.

Energy

Isotopes of uranium are used in nuclear power.

Dating

Carbon isotopes help date ancient remains.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

List the three particles with charges and locations
Use A − Z to find neutrons
Quote that isotopes share the atomic number
Recall Dalton's main points

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

An ion has 11 protons and 10 electrons; find its charge.
Compare two isotopes of chlorine by their particles.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

TN Class 7 Term 1 ExamHigh

NMMS / Foundation ScienceMedium

School unit testsHigh

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

The atomic number is the number of protons in an atom, while the mass number is the total of protons and neutrons in its nucleus.

Because they have the same number of protons and electrons (same atomic number); only the number of neutrons differs, which changes the mass but not the chemical behaviour.

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