Electronic StructureAQA GCSE Chemistry: Revision notes
Section 1
How has the model of the atom changed over time?
Our idea of the atom has developed through several stages, each replacing the last as new evidence emerged:
- Indivisible spheres — atoms were thought to be solid, uncuttable balls
- Plum pudding model — after the discovery of the electron, atoms were pictured as a ball of positive charge with negative electrons embedded in it
- Nuclear model — the alpha particle scattering experiment showed most of an atom's mass and all its positive charge is concentrated in a tiny central nucleus
- Bohr's model — electrons were shown to orbit the nucleus at specific, fixed distances (energy levels/shells)
- Later work identified protons and neutrons as the particles making up the nucleus
The alpha particle scattering experiment is the key piece of evidence for the nuclear model — most particles passed straight through the gold foil, but a few bounced back, showing a dense, small, positive nucleus.
Section 2
What are the charges and masses of subatomic particles?
An atom is made up of three types of subatomic particle, found either in the nucleus or surrounding it:
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | Very small |
Almost all of an atom's mass is concentrated in the nucleus, since electrons have a negligible mass in comparison.
Section 3
How is an atom's structure described using atomic number and mass number?
In a neutral atom, the number of electrons equals the number of protons — this keeps the atom electrically neutral overall.
The atomic number (proton number) is the number of protons in the nucleus, and it identifies the element.
The mass number is the total number of protons and neutrons.
- Number of protons = atomic number
- Number of neutrons = mass number − atomic number
- Number of electrons = number of protons (for a neutral atom)
Atoms of the same element can have different numbers of neutrons — these are called isotopes.
Chlorine-35 has 17 protons and 18 neutrons (35 − 17 = 18). Chlorine-37 also has 17 protons but 20 neutrons — the two are isotopes of chlorine.
Section 4
How are electrons arranged in an atom?
Electrons occupy the lowest available energy levels, called shells, as close to the nucleus as possible.
The electronic structure of the first 20 elements can be shown as a number (e.g. 2,8,1 for sodium) or as a diagram with electrons arranged in shells around the nucleus.
The maximum number of electrons in each of the first three shells is 2, 8, 8.
Sodium has 11 electrons, arranged as 2,8,1 — two full inner shells and one electron in the outer shell.
Must Know
- Protons: +1 charge, mass 1; neutrons: 0 charge, mass 1; electrons: −1 charge, negligible mass
- Number of protons = number of electrons in a neutral atom; number of protons = atomic number
- Mass number = protons + neutrons; isotopes have the same protons but different neutrons
- Electrons occupy the lowest available energy levels (shells)
- Know the order of historical atomic models: solid sphere → plum pudding → nuclear → Bohr's shells → protons/neutrons identified
That's the notes covered.
Carry on to the next subtopic.