Atomic mass is the given by the sum of the number of protons and number of neutron in the nucleus of the atom.
A = P + N
where A = atomic mass of the atom
P = number of protons in the nucleus of the atom
N = Number of neutrons in the nucleus of the atom
Given that :
A = atomic mass = 13
P = number of protons in the nucleus = 6
N = ?
Using the equation
A = P + N
13 = 6 + N
N = 13 - 6
N = 7
hence the isotope has 7 neutrons
