Atomic structure
Inside the atom
- Every atom has a tiny, dense nucleus with electrons moving in shells around it.
- The nucleus holds protons and neutrons.
| Particle | Relative mass | Relative charge |
|---|---|---|
| proton | 1 | $+1$ |
| neutron | 1 | $0$ |
| electron | almost 0 | $-1$ |
- An atom has no overall charge — equal numbers of protons and electrons.
Practice
Match each particle to its relative charge.
Protons are +1, neutrons 0, electrons -1; protons and neutrons have a relative mass of 1.
Proton number and mass number
- The proton number (atomic number) = the number of protons — it tells you which element it is.
- The mass number (nucleon number) = protons + neutrons.
- So neutrons = mass number − proton number.
Practice
An atom has proton number 17 and mass number 35. How many neutrons does it have?
Neutrons = mass number − proton number = 35 − 17 = 18.
Practice
Which number tells you which element an atom is?
The proton number (atomic number) is unique to each element.
You've got it
Key idea
- atom = nucleus (protons + neutrons) + electrons in shells; no overall charge
- proton number identifies the element; mass number = protons + neutrons
- neutrons = mass number − proton number