By the 1930s it became generally accepted that the basic building blocks of the universe were protons, neutrons, and electrons, rather than atoms. Each element's characteristic physical and chemical properties are determined by the number and arrangement of these three elementary particles. The positive charge of an atom, as well as almost all its mass, is concentrated at the center of the atom in its nucleus. The nucleus is a tightly bound cluster of protons and neutrons. There is one important difference between neutrons and protons.
Every proton carries a definite positive electric charge, whereas neutrons are neutral.
While most of the atom is empty space, each atom contains a definite number of electrons which orbit in that space. The identity of an element is determined by the number of protons in the nucleus of the atom. For example, hydrogen atoms have one proton, helium atoms have two protons, and so on. The number of neutrons determines what specific isotope of an element is present. For example hydrogen atoms can have zero, one, or two neutrons.