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Structure of the Atom

An atom has a tiny, heavy nucleus of protons and neutrons, with electrons moving around it in shells (energy levels).

🎬 Step-by-step story

  1. Zoom in to the centre of the atom. This tiny, heavy ball is the nucleus.
  2. The red balls are protons. Each has a + charge. Sodium has 11 of them.
  3. The grey balls are neutrons. They have no charge, but they add mass.
  4. Now the electrons appear on shells: 2 on K, 8 on L, 1 on M. Sodium is 2, 8, 1.
  5. Your turn! Pick any element from the list and count its protons, neutrons and shells.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

The three particles inside an atom

Protons carry a positive charge (+1) and sit in the nucleus. Neutrons have no charge and also sit in the nucleus. Electrons carry a negative charge (−1), have almost no mass (about 1/1836 of a proton) and move around the nucleus.

An atom has equal numbers of protons and electrons, so overall it is neutral.

How the atomic model developed

Thomson (1897) found the electron and pictured the atom as a positive sphere with electrons stuck in it. Rutherford (1911) fired alpha particles at gold foil; a few bounced back, which showed that the positive charge and mass sit in a tiny nucleus. Bohr (1913) proposed that electrons move only in fixed shells with fixed energies. Chadwick (1932) discovered the neutron.

Atomic number and mass number

The atomic number (Z) is the number of protons; it decides which element the atom is. The mass number (A) is protons + neutrons. Atoms of the same element with different numbers of neutrons are isotopes, for example carbon-12 and carbon-14.

Electron shells (K, L, M, N)

A shell n can hold at most 2n² electrons: K holds 2, L holds 8, M holds 18. In the 2-8-8 scheme used in school, sodium (11 electrons) is 2, 8, 1. The electrons in the outer shell are the valence electrons, and they decide how the atom reacts. Rotate the 3D model and pick an element to see its shells.

Key formulas and definitions

Worked examples

1. Chlorine has Z = 17 and A = 35. How many protons, neutrons and electrons does it have?

Protons = 17, electrons = 17, neutrons = 35 − 17 = 18.

2. Write the electron distribution of oxygen (Z = 8).

K = 2, L = 6, so 2, 6. It has 6 valence electrons.

3. What is the valency of magnesium (Z = 12)?

Distribution 2, 8, 2. It loses its 2 outer electrons, so valency = 2.

Common mistakes

Practice quiz

1. Which particle has no charge?
2. Who discovered the nucleus?
3. Max electrons in the L shell:
4. An atom with 11 protons and 12 neutrons has mass number:
5. Isotopes of an element differ in the number of:

Frequently asked questions

What is the structure of an atom?

A small positive nucleus made of protons and neutrons, surrounded by negative electrons moving in shells.

What is the Bohr model?

Bohr's 1913 model in which electrons move in fixed circular shells with definite energies and do not lose energy while in a shell.

Which class has structure of the atom?

CBSE Class 9 Science, GCSE Chemistry/Physics (atomic structure), and US middle-school NGSS MS-PS1.

Where this is taught

CBSE (India)Class 9Structure of the Atom
England (KS3, GCSE, A-level)Year 9Atomic structure
USA (Common Core, NGSS, AP)Grade 8Structure of matter (MS-PS1)

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