atom - The smallest particle that an element can be broken down into and still maintain its unique identity. atomic number - The number of positive charges or protons in the nucleus of an atom. Core of an atom. The nucleus contains both positive (protons) and neutral (neutrons) subatomic particles. - O donor atoms - Pentavalent atoms that give up electrons to the conduction band in an N type semiconductor material. neutral atom - An atom in which the number of negative charges (electrons in orbit) is equal to the number of positive charges (protons in the nucleus).