What an atom is
- An atom is the basic building block of all matter. Atoms are tiny (about 1 × 10⁻¹⁰ m in radius), so around 100 million atoms could sit side by side across a thumbnail
- Every atom has two parts:
- a tiny dense nucleus at the centre
- a cloud of electrons orbiting the nucleus
- The nucleus is about 10 000 times smaller than the whole atom but contains almost all of the mass. If the atom were scaled up so the nucleus was a marble in the middle of a football stadium, the electrons would be orbiting the seats in the back row
The three sub-atomic particles
| Particle | Location | Relative charge | Relative mass |
|---|
| Proton | In the nucleus | +1 | 1 |
| Neutron | In the nucleus | 0 | 1 |
| Electron | Orbiting the nucleus | −1 | 1/2000 (effectively zero) |
- Together protons and neutrons make up the nucleus and are called nucleons
- In a neutral atom, the number of electrons equals the number of protons, so the positive nuclear charge is exactly cancelled by the negative electron charge
- If an atom loses or gains electrons, the charges no longer balance and the atom becomes an ion: positive if electrons have been lost, negative if extra electrons have been gained
Atomic and mass numbers
- Atomic number (Z) = the number of protons in the nucleus. This single number defines which element an atom is:
- Hydrogen: Z = 1
- Carbon: Z = 6
- Sodium: Z = 11
- Uranium: Z = 92
- Mass number (A) = the total number of nucleons (protons + neutrons) in the nucleus
- The number of neutrons in an atom can therefore be found by subtraction:
number of neutrons = mass number − atomic number = A − Z
Nuclear notation
- A nucleus is described by writing its element symbol together with its mass number and atomic number:
ᴬ_Z X
- where:
- A = mass number (top, total nucleons)
- Z = atomic number (bottom, protons)
- X = the element's chemical symbol
- Some examples:
- ¹₁H is hydrogen (1 proton, 0 neutrons, 1 electron)
- ²³₁₁Na is sodium (11 protons, 12 neutrons, 11 electrons)
- ²³⁸₉₂U is uranium-238 (92 protons, 146 neutrons, 92 electrons)
Example — an atom of gold has the nuclear notation ¹⁹⁷₇₉Au. How many protons, neutrons and electrons does it have?
- Number of protons = atomic number = 79
- Number of neutrons = mass number − atomic number = 197 − 79 = 118
- Number of electrons = number of protons (atom is neutral) = 79