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IGCSE · Edexcel · Chemistry

Structure of the Atom

Learning Objectives

Describe the structure of an atom in terms of nucleus and electron shells
State the relative charges and masses of protons, neutrons, and electrons
Define atomic number and mass number

The Nucleus

Every atom contains a dense central nucleus composed of protons and neutrons, surrounded by electrons arranged in shells.

Atomic Number & Mass Number

Atomic number (Z) = number of protons; defines the element
Mass number (A) = protons + neutrons
In a neutral atom, proton count equals electron count

Electron Shells

Electrons occupy discrete energy levels (shells) around the nucleus. The first shell holds up to 2 electrons; subsequent shells hold up to 8…

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What determines the atomic number of an element?

The number of protons in the nucleus

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Step 3 · Questions

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State the charge and relative mass of an electron.

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Charge: −1. Relative mass: approximately 0 (negligible). Electrons occupy shells outside the nucleus.

Explain why atoms have no overall electric charge.

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Step 4 · Summary

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✓ Protons (+1, mass 1) and neutrons (0, mass 1) form the nucleus
✓ Electrons (−1, mass ≈ 0) occupy shells outside the nucleus
✓ Atomic number = proton count — defines the element
✓ Isotopes: same atomic number, different mass number

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