Science · Grades 8–11
Atomic number and mass number
Atomic number counts protons; mass number counts protons plus neutrons.
The key idea
Atomic number counts protons; mass number counts protons plus neutrons.
neutrons = mass number − atomic number
Worked examples
In a simplified isotope exercise, atomic number Z = 27 and mass number A = 62. How many neutrons?
62 − 27 = 35
35
In a simplified isotope exercise, atomic number Z = 4 and mass number A = 11. How many neutrons?
11 − 4 = 7
7
Try it yourself
In a simplified isotope exercise, atomic number Z = 8 and mass number A = 27. How many neutrons?
In a simplified isotope exercise, atomic number Z = 23 and mass number A = 44. How many neutrons?
Check your understanding
Answers and marking notes
- 19
27 − 8 = 19
- 21
44 − 23 = 21
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Learning preview · teacher review required. Grade bands are approximate, not official curriculum alignment.