Aldehydes and ketones both share a common structural feature called the carbonyl group: a carbon atom joined to an oxygen atom by a double bond. This shared structural feature is exactly what makes both aldehydes and ketones reactive in broadly predictable ways, since the carbonyl group's particular electronic arrangement creates a site that's especially open to attack by certain other molecules. What actually distinguishes an aldehyde from a ketone comes down to a comparatively small difference: exactly what's attached to the carbonyl carbon on either side.
The carbonyl carbon's own electronic imbalance drives its reactivity
Oxygen pulls shared electrons toward itself more strongly than carbon does, and in a carbonyl group's carbon-oxygen double bond, this pull leaves the carbon atom with a partial positive electrical character and the oxygen atom with a partial negative one. This electronic imbalance makes the carbonyl carbon an attractive target for molecules that carry a surplus of negatively charged electrons looking to bond, which is exactly the underlying reason carbonyl-containing compounds participate so readily in a wide range of well-studied, broadly predictable chemical reactions.
What's attached on either side of the carbonyl carbon separates the two categories
An aldehyde has its carbonyl carbon attached to at least one hydrogen atom, alongside one other carbon-containing group, or in the simplest case, two hydrogens. A ketone instead has its carbonyl carbon attached to two separate carbon-containing groups, with no hydrogen directly on that carbon at all. This structural difference, though it might look small on paper, meaningfully affects each category's specific chemical behaviour and reactivity, which is exactly why organic chemistry treats aldehydes and ketones as related but genuinely distinct functional groups, sharing the same core reactive carbonyl feature while differing in their surrounding structural context.
What we're still unsure about
The carbonyl group's basic structure and electronic character, and the resulting reactivity patterns of aldehydes and ketones, are precisely defined, extremely well established organic chemistry, confirmed across an enormous body of experimental and industrial application. What's more genuinely an ongoing area of applied research is developing more selective, efficient catalytic methods for carrying out specific carbonyl-related reactions at industrial scale, since achieving high selectivity for one particular desired product while avoiding unwanted side reactions remains a genuinely active area of chemical engineering and catalysis research, rather than a fully solved problem for every possible carbonyl transformation chemists might want to carry out.
This sits inside Carbonyl Chemistry (Aldehydes & Ketones), one of eight topics in Organic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.