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the answer is that it is called a atomic number.
The atomic number is equal to the number of the protons in the atomic nucleus.
Atomic weight in atomic mass units = the number of protons + the number of neutrons.
The number of protons is your atomic number. Subtract that from the atomic weight for the number of neutrons.
this elemnt has an atomic number that is double the atomic number of silicon?
Subtract the atomic number from the atomic weight.