What is the difference between a molecule and a molecular ion?  

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A molecule is made up of atoms of one or more elements. Some examples of molecules are oxygen gas (O2), Hydrogen gas (H2), sulfuric acid (H2SO4), carbon dioxide (CO2), water (H2O), among others. In a molecule, the atoms are combined in a certain fixed ratio. For example, in water (H2O), hydrogen and oxygen atoms combine in a 2:1 ratio. 

A molecular ion is formed when electrons are added to or removed from a molecule. When electron/s are removed, the ion gains a net positive charge, while addition of electrons will give a net negative charge to the molecular ion. For example, H2+ is a molecular ion and is formed when a molecule of hydrogen gas loses an electron. 

Another difference is that molecules are neutrally charged, that is, they do not have any charge. In comparison, a molecular ion is either positively charged or negatively charged, depending on whether it gained or lost electron/s.

Hope this helps. 

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