Explain how the number of protons, neutrons and electrons in an atom determines its atomic number, mass number and overall charge.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An atom of an element has an atomic number of 11 and a mass number of 23.
Model answer (4 marks)
The atomic number equals the number of protons in the nucleus.
The mass number equals the total number of protons plus neutrons.
The number of neutrons is 23 − 11 = 12.
Because the number of protons (11) equals the number of electrons (11), the atom has no overall charge.
The mass number equals the total number of protons plus neutrons.
The number of neutrons is 23 − 11 = 12.
Because the number of protons (11) equals the number of electrons (11), the atom has no overall charge.
Examiner tips
- State the definition of atomic number and mass number. Show the calculation for neutrons. Explain the charge balance using protons and electrons.
Common mistakes
- Confusing mass number with atomic mass. Forgetting to subtract atomic number from mass number to find neutrons. Assuming the atom is ionised without evidence.
Mark scheme (4 marks)
- The atomic number equals the number of protons (in the nucleus)
- The mass number equals the total number of protons plus neutrons
- The number of neutrons is 12 (mass number minus atomic number / 23 − 11)
- An atom has no overall charge because the number of protons equals the number of electrons (both 11 in this atom)
Key terms in this question
atomic number · mass number · proton · neutron · electron