Atomic Structure

Inside the atom there are three very important particles: protons, neutrons and electrons.

In the very middle of the atom is the nucleus which contains protons and neutrons. The proton carries a positive charge, which is equal and opposite to the negative charge of the electron. The neutron is an uncharged particle with a mass slightly greater than a proton.

In a neutral atom, the number of protons in the nucleus is equal to the number of electrons outside the nucleus. Compared to the mass of protons and neutrons, electrons are very small. See the table below for a comparison in properties of the three subatomic particles.

Properties of Subatomic Particles
Relative Charge
Mass (amu)
Outside the nucleus

* amu = atomic mass units

Atomic Properties

The atomic number is the number of protons in the nucleus of an atom. Hydrogen, for example, has an atomic number of one because it has one proton. Carbon (atomic number 6) has six protons.

Elements that have atomic numbers of up to 92 can be found in nature; those over 92 are created in laboratories.

The atomic mass is the number of protons and neutrons in the nucleus of an atom. Although all atoms of the same element have the same number of protons, they sometimes have more neutrons. Such atoms are called isotopes.

Normally, atoms are electrically neutral. But sometimes they crash into other atoms, causing them to gain or lose electrons. An atom in which there is an unequal number of protons and electrons is called an ion.


Masterton & Hurley, Chemistry Principles and Reactions, Third Edition, (1997, 1993, 1989 Saunders College Publishing)