write the mass number, atomic number,number of protons, neutrons,and electrons for He, Si, K+, N3-

Answer 1

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.