Overall a simple nucleophilic addition can be represented with curly arrows as follows:

The reactivity of aldehydes and ketones can be easily rationalized by considering the important resonance contributor which has charge separation with a +ve C and -ve O.
Question: Why is this resonance structure more important than the one with a -ve C and +ve O ?

In general the reactivity order towards nucleophiles is : aldehydes > ketones (see below) The substituents have two contributing factors on the reactivity at the carbonyl C:
Size of the substituents attached to the C=O. Larger groups will tend to sterically hinder the approach of the Nu. The electronic effect of the substituent. Alkyl groups are weakly electron donating so they make the C in the carbonyl less electrophilic and therefore less reactive towards nucleophiles.

These trends are supported by the trends in the equilibrium data for the formation of hydrate (see later)
 
 

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