Structure of the Atom: Class 9 Study Guide
Atoms have a small nucleus and electrons around it. Their arrangement decides how elements behave.
An atom contains three kinds of subatomic particles: protons (positive charge) and neutrons (no charge) in the central nucleus, and electrons (negative charge) outside it. J. J. Thomson discovered the electron, Ernest Rutherford showed that the atom has a tiny dense nucleus from his gold-foil experiment, and James Chadwick discovered the neutron.
Niels Bohr proposed that electrons move in fixed shells or orbits without losing energy.
Numbers that describe an atom
The atomic number (Z) is the number of protons. The mass number (A) is protons plus neutrons. A neutral atom has equal numbers of protons and electrons. Sodium has Z = 11 and A = 23, so it has 11 protons, 12 neutrons and 11 electrons.
| Atom | Protons | Neutrons | Electrons |
|---|---|---|---|
| Carbon-12 | 6 | 6 | 6 |
| Sodium-23 | 11 | 12 | 11 |
| Chlorine-35 | 17 | 18 | 17 |
Filling the shells
Electrons fill shells from the inside. The first shell (K) holds up to 2 electrons, the second (L) up to 8 and the third (M) up to 8 in the first 20 elements. Sodium has the configuration 2, 8, 1. Chlorine has 2, 8, 7. The number of electrons in the outermost shell decides the valency: sodium has 1 electron to lose, so valency 1, and chlorine has 7 electrons and needs 1 more, so valency 1.
Equal to the atomic number.
Then L with 8.
Up to 8.
From the outer shell.
Isotopes and isobars
Isotopes are atoms of the same element with different mass numbers, such as hydrogen (protium), deuterium and tritium, which have 0, 1 and 2 neutrons. Isobars are atoms of different elements with the same mass number, such as argon-40 and calcium-40.
Worked examples and common mistakes
Example: Oxygen has Z = 8 and A = 16, so 8 neutrons and the configuration 2, 6. Example: Neon, 2, 8, has a full outer shell and is unreactive. Common mistakes: mixing up the atomic number and the mass number, forgetting that neutrons have no charge, and putting more than 8 electrons in the second shell.
Check your understanding: model answers
1. 14 − 6 = 8 neutrons. 2. 2, 8, 2. 3. Atoms of one element with the same number of protons but different numbers of neutrons.
✨ Key Takeaways
- Protons and neutrons are in the nucleus, and electrons are in shells.
- Atomic number = protons.
- Mass number = protons + neutrons.
- Outer-shell electrons give the valency.
💬 Questions for Discussion & Reflection
- How many neutrons does carbon-14 have (Z = 6)?
- Write the electron configuration of magnesium (Z = 12).
- What are isotopes?
Print a science observation log →