Question 11
The atomic number of aluminium is 13 and its mass number is 27. How many neutrons are in that aluminium atom?
The right answer is the one marked above.
14. The difference between the mass number and the atomic number gives the neutrons:
neutrons = A − Z = 27 − 13 = 14
13 is the proton count — and, in a neutral atom, the electron count too. In standard notation this atom is written 2713Al.