Posted by Alex on Thursday, February 9, 2012 at 4:59pm.
#protons = atomic number from the periodic table.
# electrons = # protons.
# p + #n = mass number
For 40Ar, atomic number is 18.
#p = 18, #e = 18
#p + #neutrons = 40
# neutrons = 40-18 = 22
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