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We defined 2 protons to be helium.
>why did we decide that the number of protons defines the element?
Protons do not directly affect any of an atoms chemical properties. The chemical properties of an atom is dependent on the number and arrangement of electrons in the neutral atom. Electrons in an atom are arranged in different energy levels called "shells" with the innermost, lower energy orbitals always being filled up first. (Though in reality it can be more complicated.) The electrons in the outermost (valence) shell contribute most to the chemical properties of the element.
Since neutrons are uncharged, the number of electrons and therefore the arrangement of electrons in a neutral atom are determined by only the number of protons that the atom possesses.
For example if you changed the number of:
>neutrons
the electron number and configuration in the neutral atom is unchanged so the atom still has the same chemical properties as before, and therefore is the same element.
>protons
the electron number and configuration in the neutral atom is changed so you get a completely different element with different chemical properties.
So if electrons play such a big part in determining the chemical properties of an atom, why can't we define an element by it's electron number?
When an atom undergoes chemical reactions, it can either share, donate or receive electrons in the process of forming bonds. But even after a chemical reaction has taken place the atom is still of the same element, because if you break apart the compound formed you would get back the same element as before.
Due to this reason it would suck to define an element by it's electron number.
