Building Blocks: Atomic Structure — AQA Combined Science: Synergy
This topic covers how the model of the atom developed and the structure of atoms in terms of subatomic particles, isotopes and electron arrangement.
Scientific models of the atom
Our model of the atom changed as new evidence appeared:
- Atoms were first thought to be tiny solid spheres that could not be divided.
- The discovery of the electron led to the plum pudding model (a ball of positive charge with electrons embedded in it).
- The alpha particle scattering experiment — most particles passed straight through a thin gold foil, but a few were deflected. This showed the atom is mostly empty space with a small, dense, positively charged nucleus — the nuclear model.
- Niels Bohr proposed electrons orbit at specific distances (energy levels).
- Later experiments identified the proton, and James Chadwick found evidence for the neutron.
This shows how scientific models are revised in the light of new experimental evidence.
The size of atoms
Atoms are extremely small, with a radius of about 1 × 10⁻¹⁰ m. The nucleus is around 1/10 000 of the size of the atom but contains almost all of its mass.
Sub-atomic particles
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | very small |
- Atomic (proton) number = number of protons — this identifies the element.
- Mass number = protons + neutrons.
- Atoms are neutral because the number of protons equals the number of electrons.
Isotopes
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. The relative atomic mass is the average mass of the isotopes, weighted by their abundance.
Electrons in atoms
Electrons occupy shells (energy levels), filling from the inside out: 2, 8, 8 for the first 20 elements. The electronic structure links directly to an element's position in the periodic table. Electrons can move to a higher energy level by absorbing electromagnetic radiation and fall back by emitting it.
Exam tips
- Be able to describe the alpha scattering experiment and what each observation proved.
- Work out protons, neutrons and electrons from the atomic and mass numbers.
- Define isotopes precisely (same protons, different neutrons).
- Write electronic structures (e.g. sodium = 2,8,1) and link them to the periodic table.