Structure of the Atom

Why This Chapter Matters

Scientific models change when evidence demands a better explanation. The history of the atom is a perfect example. Dalton's indivisible atom became inadequate after electrons were discovered. Thomson proposed a new model, but Rutherford's scattering experiment contradicted it. Bohr then introduced a shell model that better explained atomic stability. The chapter is therefore not just a list of diagrams; it is a story of observation, model, contradiction and refinement.

Bridge from Earlier Learning

• Dalton atomic theory
• Chemical symbols
• Atomic and molecular mass
• Electric charge

These ideas are reused and deepened in this chapter.

1. Discovery of Subatomic Particles

2. Thomson Model

J.J. Thomson proposed a positively charged sphere with electrons embedded in it. The model explained electrical neutrality because positive and negative charges could balance. But it spread positive charge throughout the atom, and Rutherford's later evidence showed that this could not be correct.

3. Rutherford Alpha-Scattering Experiment

Fast alpha particles were directed at very thin gold foil. Most passed straight through, showing that most of the atom is empty space. Some were deflected, showing concentrated positive charge. Very few turned through large angles, indicating a tiny dense nucleus containing most of the mass and positive charge.

4. From Rutherford to Bohr

A classical orbiting electron should radiate energy and spiral into the nucleus, so Rutherford's model could not explain atomic stability. Bohr proposed allowed shells or energy levels around the nucleus. At Class 9 level these are labelled K, L, M, N and so on. The shell model is simplified compared with modern quantum mechanics but useful for electronic configuration and valency.

5. Electronic Configuration

In the school-level shell model, shell capacity is related to 2n², with additional Bohr-Bury rules for outer shells. K holds up to 2 electrons and L up to 8. For the first 18 elements, the outermost shell is considered stable with up to 8 electrons. Sodium, Z=11, has 2,8,1; chlorine, Z=17, has 2,8,7.

6. Valency from Outer Electrons

Atoms tend toward stable outer-shell arrangements. An atom with one outer electron may lose one; an atom with seven may gain one. Valency is the combining capacity represented by the number of electrons lost, gained or shared to reach a stable configuration in this simple model. This connects atomic structure directly to chemical formulae.

7. Atomic Number and Mass Number

Atomic number Z is the number of protons. In a neutral atom it also equals the number of electrons. Mass number A is total protons plus neutrons. Therefore neutron number is A-Z. Element identity depends on Z, not A.

8. Isotopes and Isobars

Isotopes are atoms of the same element with the same proton number but different neutron numbers, so they have the same Z but different A. Isobars are atoms of different elements with the same A but different Z. Isotopes often have similar chemical behaviour because neutral atoms have the same electron arrangement.

Compare the Ideas

Visualise the Concept

Atomic models changed because new experiments revealed features older models could not explain.

Formula / Relationship Sheet

Worked Examples & Scientific Reasoning

Observe Like a Scientist

Exam Thinking Strategy

Common Mistakes

Quick Revision

Focused Practice

Final Chapter Summary