Structure of the Atom | Science Grade 9 CBSE

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Inside the atom

An atom looks empty, but it is built from just three particles plus a few counting rules. Tap each term to see what it is, where it sits, and what it tells you about the element.

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The three big ideas

Proton — a positively charged particle in the nucleus; mass ≈ 1 u.
Neutron — a neutral (no charge) particle in the nucleus; mass ≈ 1 u.
Electron — a negatively charged particle that moves around the nucleus in shells; it has almost no mass (about 1/2000 of a proton).
The nucleus (protons + neutrons) holds nearly all the mass; the electrons take up nearly all the space.
A neutral atom has equal numbers of protons and electrons, so their charges cancel out.

Thomson's model: the atom is a sphere of positive charge with electrons embedded in it (like seeds in a watermelon). Overall, the atom is neutral.
Rutherford's model: from the alpha-particle scattering ("gold foil") experiment — a tiny, dense, positively charged nucleus at the centre, with electrons revolving around it; most of the atom is empty space.
Bohr's model: electrons revolve only in fixed shells (energy levels) named K, L, M, N… without losing energy.
The maximum number of electrons a shell can hold = 2n² → K = 2, L = 8, M = 18, N = 32. The outermost shell can hold at most 8 electrons.

Atomic number (Z) = the number of protons — it decides which element the atom is.
Mass number (A) = number of protons + neutrons.
Valence electrons = the electrons in the outermost shell; they decide how an atom bonds (its valency).
Isotopes = atoms of the same element (same Z) with different mass numbers, e.g. carbon-12 and carbon-14.
Isobars = atoms of different elements (different Z) with the same mass number, e.g. calcium-40 and argon-40.

Worked example. An atom has 11 protons and 12 neutrons. Find its atomic number and mass number.

Atomic number Z = number of protons = 11.

Mass number A = protons + neutrons = 11 + 12 = 23.

So this atom is sodium (Na), with the electron arrangement 2, 8, 1.

Common mistake: thinking the atomic number is the number of neutrons or electrons. The atomic number is the number of PROTONS — and that is what decides which element you have. (In a neutral atom the electron count happens to match, but Z is defined by the protons.)

Where you'll meet it

Atoms at work

Medicine & cancer treatment

Hospitals use radioactive isotopes every day. Cobalt-60 gives off gamma rays used to destroy cancer cells in radiotherapy, and other isotopes act as tracers that light up organs in medical scans.

Carbon-14 dating

Living things take in carbon-14 from the air; after they die it slowly decays. Measuring how much is left lets scientists estimate the age of ancient wood, bone and fossils.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.

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