The compact, positively-charged nucleus of an atom contains two main types of subatomic particles: protons and neutrons. Orbiting the atom's nucleus are negatively-charged subatomic particles called electrons. The mass of an atom is concentrated in the nucleus.
The three primary subatomic particles that constitute matter include neutrons, protons and electrons. Protons carry positive charges, electrons carry negative charges and neutrons are electrically neutral. The net positive charge of the nucleus is the result of the positively-charged protons within the central core. Protons and neutrons are almost identical in mass while electrons are so light that their masses are sometimes considered negligible.
A chemical element is composed of the same type of atoms. An atom of a particular element retains all the chemical properties of that element. In a periodic table, an element is characterized by an atomic symbol, atomic number and mass number. The elements are organized in increasing order of atomic numbers, since the atomic number is unique to each element.
The atomic number indicates the number of protons present in the nucleus, which is also equal to the number of electrons in a neutral atom. The mass number refers to the total number of protons and neutrons in the nucleus. While all atoms of an element are identified by the same atomic number, this is not always the case for the mass number. One form of atom, known as an isotope, contain the same number of protons but vary in the number of neutrons.