Label the parts of an atom:
proton, electron, neutron

electron is the - circles around the atom, protons are the + circles inside the atom, neutrons are the blank circles inside the atom

Explanation:

Electrons have negative charge ( - ) and also rotate around the atom - they are not in the nucleus

Protons are in the nucleus but have a positive charge ( + )

neutrons have no charge and are in the nucleus

a:

by the combination of an unlimited number of elements