Atomic structure and isotopes
Protons, neutrons and electrons
An atom has a tiny central containing and , surrounded by in shells.
| Particle | Relative mass | Relative charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | Very small | −1 |
Atoms have no overall charge because they contain the same number of protons and electrons.
Atoms are very small: their radius is about (1 × 10⁻¹⁰ m). The nucleus is about 1/10 000 of that, yet it contains almost all of the atom's mass.
Key terms
Atomic number and mass number
The is the number of . Every atom of the same element has the same atomic number, and it is what makes an element that element.
The is the total number of .
So for any atom:
- protons = electrons = atomic number
- neutrons = mass number − atomic number
Example: sodium has atomic number 11 and mass number 23. It has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
Key terms
Isotopes and relative atomic mass
are atoms of the with the same number of protons but . They have the same atomic number but different mass numbers. Chlorine-35 and chlorine-37 are isotopes of chlorine.
The of an element is the average mass of its atoms, taking the abundance of each isotope into account. It is measured on a scale where an atom of carbon-12 has a mass of exactly 12.
Aᵣ = sum of (isotope mass × abundance) ÷ sum of abundances
Example: chlorine is 75% chlorine-35 and 25% chlorine-37.
Aᵣ = (35 × 75 + 37 × 25) ÷ 100 = 35.5
Key terms
Electronic structure
Electrons fill the shells . For the first 20 elements:
- 1st shell: up to 2 electrons
- 2nd shell: up to 8 electrons
- 3rd shell: up to 8 electrons (then the 4th shell starts)
The is written as numbers separated by commas:
| Element | Electrons | Electronic structure |
|---|---|---|
| Carbon | 6 | 2,6 |
| Sodium | 11 | 2,8,1 |
| Chlorine | 17 | 2,8,7 |
| Calcium | 20 | 2,8,8,2 |