GCSE Chemistry • Topic 1

Atomic Structure & The Periodic Table

Understanding atoms, elements, and how the periodic table is organised.

📌 What You’ll Learn

  • Structure of atoms (protons, neutrons, electrons)
  • Atomic number, mass number, and isotopes
  • Development of the atomic model (History)
  • Electronic structure and shell filling rules
  • How the Periodic Table is organised

🧠 How to Study

  • Memorise definitions before calculations
  • Always link atomic structure to periodic table position
  • Practice drawing electron shells neatly
  • Learn trends rather than isolated facts
1. Atoms, Elements & Compounds (Definitions)

Key Definitions

Everything is made of atoms. An element contains only one type of atom. A compound contains two or more elements chemically bonded together.

Particle Relative Charge Relative Mass
Proton +1 1
Neutron 0 1
Electron -1 Very small (≈ 0)

Q1: Define a compound.

A substance containing two or more elements chemically combined in fixed proportions.

Q2: Which subatomic particle has a relative mass of 1 but no charge?

The Neutron.

2. Structure of the Atom (Numbers & Isotopes)

Atomic & Mass Numbers

Atoms are neutral because the number of protons equals the number of electrons.

Atomic Number: Number of Protons (identifies the element).
Mass Number: Total number of Protons + Neutrons.

$$ \text{Neutrons} = \text{Mass Number} - \text{Atomic Number} $$

Isotopes

Isotopes are atoms of the same element (same protons) with different numbers of neutrons (different mass numbers).

Q1: An atom has an atomic number of 11 and a mass number of 23. Calculate the number of neutrons.

$$ 23 - 11 = 12 \, \text{neutrons} $$

Q2: Why do isotopes of an element have the same chemical properties?

Because they have the same electron configuration, especially in the outer shell.

3. History of the Atom (Changing Models)

Development of the Model

The atomic model changed as new experimental evidence was discovered.

[Image comparing the Plum Pudding Model and the Nuclear Model of the atom]
  • Dalton: Atoms are solid spheres.
  • Thomson: Discovered electrons; proposed the Plum Pudding Model (ball of positive charge with negative electrons embedded).
  • Rutherford: Alpha particle scattering experiment; discovered the nucleus (central, positive, concentrated mass).
  • Bohr: Suggested electrons orbit the nucleus in specific energy levels (shells).
  • Chadwick: Provided evidence for the neutron within the nucleus.

Q1: What did the alpha particle scattering experiment prove?

That the atom's mass is concentrated in a tiny, positively charged nucleus at the centre.

4. Electronic Structure (Shell Rules)

Shell Filling Rules

Electrons occupy the lowest available energy levels (innermost shells) first.

[Image showing the electronic shell structure of an atom with 2, 8, 8 configuration]

Rule: 1st shell (max 2), 2nd shell (max 8), 3rd shell (up to 8 electrons for GCSE).

Example: Sodium (Atomic No. 11) is 2, 8, 1.

⚠️ Exam Trap

Always draw electrons in pairs once the first shell is full; it makes them easier for examiners to count!

Q1: Write the electronic structure for Chlorine (Atomic Number 17).

2, 8, 7

Q2: How does the electronic structure relate to the Group number?

The number of electrons in the outer shell is the same as the element's Group number.

5. The Periodic Table (Organisation)

The Periodic Table is arranged in order of increasing atomic number, not mass. This ensures elements with similar properties fall into columns (Groups).

Feature Groups (Vertical) Periods (Horizontal)
What they represent Number of outer electrons Number of electron shells
Chemical Properties Very similar Gradual change

Metals vs Non-Metals

Metals: Found on the left/bottom. Form positive ions when they react.
Non-Metals: Found on the right/top. Form negative ions or covalent bonds.

Q1: Why are elements in the same group similar?

Because they have the same number of electrons in their outer shell (same electron configuration).

⚡ Quick Revision Checklist

Key Definitions:
  • Isotopes (Same protons, diff. neutrons)
  • Compound (Fixed proportions, bonded)
  • Atomic Number vs Mass Number
  • Plum Pudding vs Nuclear Model
Must-Know Facts:
  • Proton charge (+1), Electron charge (-1)
  • Shell rules (2, 8, 8)
  • Metals form positive ions
  • Noble Gases (Group 0) have full shells

🚫 Brutal Exam Trap Summary

1. Don't confuse mass number (total) with atomic number (protons).
2. Don't say electrons are in the nucleus; they are in shells/energy levels.
3. Don't forget that isotopes are the same element because they have the same protons.
4. Don't forget the Periodic Table is arranged by atomic number.

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