Section 1 of 7Properties of sub-atomic particles
| Particle | Relative charge | Relative mass | Where found |
|---|---|---|---|
| Proton | +1 | 1 | In the nucleus |
| Neutron | 0 | 1 | In the nucleus |
| Electron | −1 | Very small (about 1/2000) | Orbiting the nucleus |
Section 2 of 7Two key numbers
Atomic (proton) number
- Atomic (proton) number This is the number of protons in the nucleus and is written as the lower number.
- It identifies the elementhydrogen has 1 proton, sodium has 11 and uranium has 92.
Mass (nucleon) number
- Mass (nucleon) number This is the total number of protons plus neutrons in the nucleus and is written as the upper number.
Neutral atoms
- Neutral atoms The number of electrons equals the number of protons, so the total charge of the atom is zero.
Section 3 of 7Finding the numbers in an atom
1
Read the symbol of the element and note the mass number (top) and atomic number (bottom).
2
The number of protons equals the atomic number.
3
The number of electrons in a neutral atom also equals the atomic number.
4
The number of neutrons = mass number − atomic number.
5
Worked example: sodium, mass number 23 and atomic number 11, has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
Section 4 of 7Worked examples
| Atom | Mass number | Atomic number | Protons | Neutrons | Electrons |
|---|---|---|---|---|---|
| Hydrogen | 1 | 1 | 1 | 0 | 1 |
| Sodium | 23 | 11 | 11 | 12 | 11 |
| Lithium | 7 | 3 | 3 | 4 | 3 |
| Gold | 197 | 79 | 79 | 118 | 79 |
| Uranium | 238 | 92 | 92 | 146 | 92 |
Section 5 of 7Why it matters
Nuclear notation
Identifying isotopes
- Identifying isotopes Two isotopes of an element have the same atomic number but different mass numbers, such as carbon-12 and carbon-14.
Radioactive decay
- Radioactive decay Notation lets us write nuclear equations in which the mass numbers and the atomic numbers balance on each side.
Charge
- Charge An ion has a different number of electrons from protons, so its notation has to show its charge as well.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Atomic number | The number of protons in the nucleus of an atom. |
| Mass number | The total number of protons and neutrons in the nucleus. |
| Nucleon | A particle in the nucleus, either a proton or a neutron. |
| Nuclear notation | A way of writing an atom's symbol with its mass number above and atomic number below. |
| Isotope | Atoms of one element with different numbers of neutrons. |
Section 7 of 7Exam technique
1
Swapping the two numbers — the larger one on top is the mass number, and the smaller one below is the atomic number.
2
Finding neutrons by adding the numbers; you must subtract atomic number from mass number.
3
Forgetting that electrons are not counted in the mass number because their mass is so small.
4
Assuming the number of electrons always equals the number of protons — this is only true for neutral atoms, not ions.