No, an atom with one less proton will not be the same element. The number of protons in an atom determines its atomic number, which defines the element. Each element on the periodic table has a unique number of protons in its nucleus.
The identity of an element is determined by its atomic number because the number of protons defines the fundamental properties of the atom, including its chemical behavior and the arrangement of its electrons. Changing the number of protons changes the entire nature of the atom.
For example, hydrogen, the simplest element, has one proton in its nucleus. If we remove one proton from a hydrogen atom, it would no longer have any protons and would essentially become a particle called a neutron. Neutrons, however, do not exhibit the same chemical behavior as hydrogen atoms.
Therefore, altering the number of protons in an atom changes its atomic number, and as a result, it becomes a different element with distinct chemical properties. The number of protons uniquely identifies an element and defines its place on the periodic table.