Question

Why is formaldehyde an aldehyde?

Answer 1

Carbonyl is on the terminal carbon of the chain.

Explanation:

All aldehydes are characterized by possessing a carbonyl group.
http://chemistry.elmhurst.edu/vchembook/700carbonyls.html

Formaldehyde is considered an aldehyde because its carbonyl reside on a carbon at the end of chain where a hydrogen is present, rather than in the middle of a chain (that would be a ketone). Since formaldehyde has only 1 carbon TOTAL, the carbonyl group is on the "final carbon in the chain", technically it's on both the first and the last carbon
:-)

https://simple.wikipedia.org/wiki/Formaldehyde
Formaldehyde structure.