The common definition of matter is anything that has both mass and volume (occupies space). All objects we see with the naked eye are composed of atoms. For example, a chair would be said to be made of matter, as it occupies space, and has mass.
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons. Thus, the nucleus is the very dense region consisting of protons and neutrons at the center of an atom. Nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The only exception is hydrogen-1, which is the only stable nuclide with no neutrons.
A model of the atomic nucleus showing it as a compact bundle of the two types of nucleons: protons (red) and neutrons (blue)
The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other by chemical bonds based on the same force, forming a molecule. Molecule is a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound.
The proton is a subatomic particle with the symbol p and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom. The neutron is a subatomic particle that has the symbol n, no net electric charge and a mass slightly larger than that of a proton.
An atom containing an equal number of protons and electrons is electrically neutral, otherwise it is positively or negatively charged and is known as an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determines the isotope of the element.