The Lewis Definition Focuses on Lone Pairs

Looking back to the previous examples of bases, we see that a lone pair of these bases is accepting the positively charged proton, as illustrated here. Note how the curved arrows indicate the movement of electrons  

In this case, the water is behaving as a base because its lone pair seeks out and accepts a positive charge (the proton) while the hydrochloric acid, HC1, behaves as an acid because it donates a positive charge to the lone pair. 

An example of an acid-base reaction that doesn t involve the transfer of a proton is the formation of carbonic acid from water and carbon dioxide as shown here  

How is it possible for carbon dioxide, C02, to behave as an acid when it has no hydrogen ions to donate  

Was this your answer A molecule behaves as an acid when it donates a positive charge, which is usually a proton, but not always. Carbon dioxide behaves as an acid when it donates the slightly positive charge of its carbon atom to the lone pair on the oxygen of a water molecule. 

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