Question

Why, in terms of atoms, are `CH_3CH_2OH` and `CH_3CHO` not isomers of each other?

Answer 1

Because they don't follow the definition of an isomer

Explanation:

Here is the Oxford Dictionary definition of an isomer
"Each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties."

`CH_3CH_2OH` has one more `O` and `H` than `CH_3CHO`

An example of an isomer is `C_4H_8`, as shown in this picture

It has the same amount of C and H atoms, but they're just arranged differently.