Hey there!
The atomic mass is the sum of the protons and electrons.
The charge of an atom is the difference between protons and electrons.
So, the number of protons is p, and p - 11 = -1.
Add 11 to each side and get p = 10. The atom has 10 protons.
The atom has 10 protons and 8 neutrons, the sum of the two is the atomic mass.
10 + 8 = 18
The atom has an atomic mass of 18.
Hope this helps!
