I'll do two and show you how.
He. Atomic number is 2 from the periodic table, look at the periodic table and round the atomic mass to a whole number. In most cases that will give you at least one of the isotopes with that mass number. For He that will be 4. That would be written as 24He.
# protons = atomic number
# electrons = # protons
# protons + # neutrons = mass number which allows to you calculate the number of neutrons.
Si. I would go with 1428Si
# protons = 14
# electrons = 14
# neutrons = 28-14 = 14
For K^+ take away one electron.
For N^-3 add three electrons.
write the mass number, atomic number,number of protons, neutrons,and electrons for He, Si, K+, N3-
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