Atomic structure, isotopes and relative atomic mass
Everything in chemistry comes back to three particles. Once you can count them from the periodic table, isotopes, relative atomic mass and electron configurations all follow.
Part 1 of 3: Learn it
In short
- Protons and neutrons are in the nucleus; electrons are in shells around it.
- The atomic number counts the protons, the mass number counts protons and neutrons together, and the difference between them is the number of neutrons.
- Relative atomic mass is the mean mass of an element's isotopes, weighted by how common each one is.
Where this is in your specification
Spec points: AQA 4.1.1.3 to 4.1.1.7, Edexcel Topic 1, OCR C1.2 and C2.1
| Board | Topic: Atomic structure and the periodic table |
|---|---|
| AQA 8462 | 4.1 (8462); 5.1 (8464) |
| Edexcel 1CH0 | Topics 1, 2 and 6 |
| OCR J248 | C1 and C2.1 |
The three particles
| Particle | Relative mass | Relative charge | Where |
|---|---|---|---|
| Proton | 1 | +1 | nucleus |
| Neutron | 1 | 0 | nucleus |
| Electron | very small (about 1/1840) | −1 | shells around the nucleus |
An atom has the same number of electrons as protons, so it has no overall charge. An ion has gained or lost electrons.
Reading the periodic table
Each element has two numbers. The atomic number (the smaller one) is the number of protons. The mass number (the larger one) is protons plus neutrons.
Sodium has atomic number 11 and mass number 23, so an atom has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
Isotopes and relative atomic mass
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They react the same way, because chemistry depends on the electrons.
Relative atomic mass (Ar) is the average mass of the atoms in a natural sample, taking account of how much of each isotope there is:
Electron configurations
Electrons fill the lowest energy shell first. For the first 20 elements the first shell holds up to 2 electrons and the next two hold up to 8 each. Sodium (11 electrons) is 2, 8, 1. The number of shells gives the period, and the number of outer electrons gives the group.
How many neutrons are in an atom of potassium-39 (atomic number 19)?
Show the answer
39 − 19 = 20.
Part 2 of 3: See it worked
Worked examples
Example 1
Chlorine is 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass.
- Ar = (35 × 75 + 37 × 25) ÷ 100
- = (2625 + 925) ÷ 100
- = 3550 ÷ 100 = 35.5
Answer: 35.5, which is the value on the periodic table.
Example 2
Give the number of protons, neutrons and electrons in a magnesium ion, Mg²⁺ (atomic number 12, mass number 24), and its electron configuration.
- Protons = 12; neutrons = 24 − 12 = 12
- The atom has 12 electrons; a 2+ ion has lost 2, so 10
- Atom: 2, 8, 2. Ion: 2, 8
Answer: 12 protons, 12 neutrons, 10 electrons, configuration 2, 8.
Common mistakes
- Swapping atomic number and mass number.
- Saying isotopes have different numbers of protons. Then they would be different elements.
- Forgetting to change the electron count for an ion.
- Putting 8 electrons in the first shell.
What is the electron configuration of oxygen (atomic number 8)?
Show the answer
2, 6.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
How many neutrons are in an atom of potassium-39 (atomic number 19)?
39 − 19 = 20.
What is the electron configuration of oxygen (atomic number 8)?
2, 6.
Gallium is 60% gallium-69 and 40% gallium-71. Find Ar to 1 decimal place.
(69 × 60 + 71 × 40) ÷ 100 = 69.8.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses