An ion is formed when an atom

Ions are formed whenatoms lose or gain electrons in order to fulfill the octet rule and have full outer valence electron shells. When they lose electrons, they become positively charged and are named cations. When they gain electrons, they are negatively charged and are named an ions.

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for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.