Atoms contain subparticles, electeons, protons, and nuetrons. Electrons are negativity chrarged while protons are positively charged. In an atom there are equal numbers of electrons and protons unless they become an isotope where it looses or adds one or more electeons to become stable. That's when the atom can be positively or negativity chrarged. If an electronic is taken away it has a positive charge. If it receives electrons then it is negativity chrarged.