What happens if you add one proton to the nucleus of an atom?
The proton number increases by one changing the element of your atom.
The only factor that determines the identity of your element is the proton number.
Hence if I have any element, let's say Lithium, and I add one proton to it, you will get Beryllium.
Extra: Your new atom would not actually be an atom if you only add one proton and nothing else. Taking the previous example, you would get an isotope of Beryllium that is an ion, as it has one less electron than the protons. Thanks!