What is the difference between an ion and an atom?

An atom can be an ion, but not all ions are atoms. The difference between an atom and an ion has to do with net electrical charge. ... The identity of an element is determined by the number of positively charged protons in the atom's nucleus. A stable atom contains the same number of electrons as protons and no net charge.
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This constancy of the chemicals involved denotes that no particular reaction is taking place on the cells involved. For instance, during photosynthesis it would require CO2, H2O and photons to initiate the process.