Why is the smallest unit that retains the properties of an element?
The smallest unit that retains the properties of an element is an atom.
If you change the atom by changing the number of protons you change the element to another element.
Each element has unique properties based on the behavior of the valence electrons. If the number of protons changes the number of electrons also changed in the neutral atom. This is because in a neutral atom the electrons always equals the number of protons
So if atom is changed the properties of the element changes. In fact it is no longer the same element.