Part 1: Particles, ions, isotopes & the atomic model

1. Developing the model of the atom

Models of the atom changed as new experiments produced evidence the old model could not explain.

1. Solid sphere (early 1800s). Atoms were pictured as tiny, indivisible solid spheres, with each element having its own type of atom.
2. Discovery of the electron → "plum pudding" (1897–1904). J.J. Thomson showed atoms contain small negative electrons, so atoms are not indivisible. The model became a sphere of positive charge with electrons embedded in it.
3. Nuclear model (1909–1911). Geiger and Marsden fired alpha particles at thin gold foil and Rutherford interpreted the results: a tiny, dense, positive nucleus surrounded by mostly empty space.
4. Bohr model (1913). Electrons occupy fixed energy levels (shells) at set distances from the nucleus, and can move between them only by absorbing or emitting specific amounts of energy.
5. Neutron discovered (1932). Chadwick identified the neutral neutron in the nucleus. This explained why nuclei are heavier than their protons alone, and why isotopes of one element have different masses.
6. Modern model. Electrons occupy orbitals, which are regions of space where there is a high probability of finding an electron. They are not fixed planet-like paths.

Alpha-particle scattering: observation → inference

Note that the particles passing straight through is evidence for empty space. It is the rare large deflections that show the nucleus is small, dense and contains most of the mass.

2. Fundamental particles

An atom has a tiny, dense nucleus containing protons and neutrons (hydrogen-1 is the exception — its nucleus is a single proton with no neutrons). Electrons occupy orbitals around the nucleus. Almost all of the atom's mass is in the nucleus.

These are relative values — comparisons with each other, not masses in kg or charges in coulombs.

A neutral atom has equal numbers of protons and electrons, so the +1 and −1 charges cancel and the overall charge is zero.

3. Atomic number, mass number and notation

Nuclide notation places the mass number A at the upper left, the atomic number Z at the lower left, the element symbol in the middle and any charge at the upper right:

2713Al3+

• Atomic number, Z = number of protons in the nucleus. It identifies the element.
• Mass number, A = total number of protons + neutrons in the nucleus.
• Neutrons = A − Z
• Electrons: neutral atom = Z; positive ion with charge n+ = Z − n; negative ion with charge n− = Z + n.

Positive ions form by losing electrons; negative ions form by gaining electrons. Ordinary ion formation does not change the number of protons or neutrons.

4. Worked examples: atoms and ions

Check the charge: charge = protons − electrons.

• Al³⁺: 13 − 10 = +3 ✓
• S²⁻: 16 − 18 = −2 ✓

In both ions the proton and neutron numbers are unchanged from the neutral atom — only the electron count changes.

5. Isotopes

Example: neon-20 and neon-22 (Z = 10)

Contrast: ions of different isotopes

Same protons, different neutrons — but the charges differ, so the electron numbers differ too. You cannot assume equal electrons for ions.

6. Why isotopes have the same chemical properties

The same number of protons tells you the atoms are the same element, but on its own it does not explain chemical behaviour — chemistry depends on electrons. At A-level, treat isotopes' chemical properties as the same. Physical properties that depend on mass (such as density) can differ.

7. Command words and mark-scheme logic

These are Finesse model explanations, not official AQA mark allocations.

8. Common errors

• Writing "same protons" instead of "same number of protons".
• Confusing mass number with relative atomic mass (A_r).
• Changing the number of protons when an ion forms.
• Adding electrons to make a positive ion (positive ions lose electrons).
• Saying all ions of isotopes have equal electron numbers.
• Giving "same number of protons" alone to explain the same chemical properties.

9. Finesse practice

Original Finesse practice questions — not copied AQA past-paper questions.

10. Sources

We have not found evidence that AQA always requires electrons in an isotope definition, so this lesson does not state that as a rule.