Atomic Structure
Subject: Chemistry | Topic: Atomic Structure
For TLT, ECT, PST, JEST & All Competitive Exams
Everything around us, from water and air to our own bodies, is made of tiny particles called atoms. Understanding the structure of the atom is the foundation of Chemistry, because it explains how elements behave and how they combine to form compounds. This topic is a regular part of Chemistry and is frequently tested in ECT, BPS-15, PST, JEST, and other competitive exams.
What is an Atom?
An atom is the smallest particle of an element that still keeps the properties of that element. Atoms are made up of even smaller particles called subatomic particles: protons, neutrons, and electrons.
Structure of an Atom (Bohr Model)
Subatomic Particles
| Particle | Charge | Location | Discovered by |
|---|---|---|---|
| Proton | Positive (+) | Nucleus | Rutherford |
| Neutron | Neutral (no charge) | Nucleus | James Chadwick (1932) |
| Electron | Negative (-) | Shells around the nucleus | J. J. Thomson (1897) |
The nucleus is the small, dense, positively charged centre of the atom. It contains protons and neutrons, and almost all of the atom's mass is found here. Protons and neutrons have approximately equal mass, while the mass of an electron is negligible in comparison.
Atomic Number and Mass Number
Mass Number (A) = Number of protons + Number of neutrons
In a neutral atom, the number of electrons is equal to the number of protons, so the positive and negative charges balance each other.
Example: Sodium has atomic number 11 and mass number 23. So it has 11 protons, 11 electrons, and 23 - 11 = 12 neutrons.
Electronic Configuration (Shells)
Electrons move around the nucleus in fixed paths called shells or energy levels, named K, L, M, N and so on. The maximum number of electrons a shell can hold is given by the formula 2n², where n is the shell number.
| Shell | n | Maximum Electrons |
|---|---|---|
| K | 1 | 2 |
| L | 2 | 8 |
| M | 3 | 18 |
The electrons in the outermost shell are called valence electrons, and they decide how an atom reacts chemically.
- Carbon (6): 2, 4
- Oxygen (8): 2, 6
- Sodium (11): 2, 8, 1
- Chlorine (17): 2, 8, 7
Isotopes
Isotopes are atoms of the same element that have the same atomic number but different mass numbers, because they have different numbers of neutrons. Examples include Carbon-12 and Carbon-14, and the three isotopes of hydrogen: protium (no neutron), deuterium (one neutron), and tritium (two neutrons).
Scientists Behind the Atomic Theory
| Scientist | Contribution |
|---|---|
| John Dalton (1808) | Gave the first modern atomic theory |
| J. J. Thomson (1897) | Discovered the electron |
| Ernest Rutherford (1911) | Discovered the nucleus |
| Niels Bohr (1913) | Proposed electrons move in fixed shells |
| James Chadwick (1932) | Discovered the neutron |
Why Atomic Structure Matters
The arrangement of electrons, especially the valence electrons, explains why elements react the way they do, why they form bonds, and how the periodic table is organized. Understanding the atom is the first step toward understanding chemical bonding, reactions, and the properties of all matter.
Practice MCQs
1. Which particle has a positive charge?
a) Electron b) Neutron c) Proton d) None of these
2. Where are protons and neutrons found?
a) In the shells b) In the nucleus c) Outside the atom d) In the electron cloud only
3. Atomic number is equal to the number of:
a) Neutrons b) Protons c) Protons + Neutrons d) Shells
4. What is the maximum number of electrons in the L shell?
a) 2 b) 8 c) 18 d) 32
5. Who discovered the neutron?
a) J. J. Thomson b) Rutherford c) James Chadwick d) Niels Bohr
📚 Want Complete Preparation?
Join our full preparation course for TLT, ECT, PST, JEST and other competitive exams — daily MCQs, weekly mock tests, and chapter-wise notes covering the complete Sindh Textbook (Class 6–12).
💬 Message on WhatsApp#Chemistry #AtomicStructure #Atom #Isotopes #TLTExam #ECTExam #PSTExam #JESTExam #SindhTextbook #TeachingExamPakistan #MissMonoAcademy