I believe you may have flipped which numbers apply where. Since each proton and neutron equal a mass of 1 amu (atomic mass units), the mass cannot be lower than the atomic number.
So if the atomic number is 18, and the atomic mass is 40, that means that the number of neutrons is the difference.
[tex]atomic mass = m_p+m_n[/tex]
[tex]m_p= mass of protons[/tex]
[tex]m_n= mass of neutrons[/tex]
so, you have
[tex]40= 18+m_n[/tex]
[tex]40-18=m_n[/tex]
[tex]m_n= 22 amu[/tex]
each neutron weighs 1 amu so you should have 22 neutrons.
Answer: 22 Neutrons
