Option 1: Rutherford’s alpha-particle scattering experiment did not lead to the discovery of electrons. Instead, it led to the discovery of the atomic nucleus. Rutherford`s experiment involved firing alpha particles at a thin gold foil and observing their scattering patterns. This led to the conclusion that most of the atom`s mass and positive charge is concentrated in a small, dense region called the nucleus.

Option 2: J J Thomson suggested the existence of electrons, but he did not propose that the nucleus of an atom contains protons. He proposed the "plum pudding" model of the atom, where electrons are embedded in a positively charged matrix.

Option 3: This statement is correct. The atomic number of an element is equal to the number of protons in the nucleus of its atom. Each element has a unique atomic number, which determines its position in the periodic table.

Option 4: The mass number of an atom is not equal to the number of electrons in its shells. The mass number is the sum of the number of protons and neutrons in the nucleus, while the number of electrons determines the atom`s charge and chemical behavior.

Therefore, the correct statement among the given options is option 3.