A. Subtract the atomic number from the atomic mass.
Explanation:
Atoms are identified by using two numbers:
- Atomic number: it is equal to the number of protons in the nucleus of the atom. It is labelled with the letter Z, so that
Z = p (1)
Where p is the number of protons.
- Mass number: it is equal to the number of protons + neutrons in the nucleus of the atom. It is labelled with the letter A, so that
A = p + n (2)
where n is the number of neutrons.
When using atomic mass units (a.m.u.), the mass of each nucleon (proton or neutron) is 1 a.m.u, so the mass number also corresponds to the mass number.
From eq.(2), we can rewrite the number of neutrons as:
n = A - p
And substituting A from eq.(1), we find
n = A - Z
So the correct choice is
A. Subtract the atomic number from the atomic mass.
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