Atomic Structure Basics

Atoms are the smallest units of matter that maintain the properties of an element. Each atom contains three main subatomic particles: protons, neutrons, and electrons. These tiny building blocks determine what makes one element different from another.

The atomic number equals the number of protons in an atom's nucleus and identifies the element. Meanwhile, the mass number is calculated by adding the number of protons and neutrons together. For example, oxygen has 8 protons, so its atomic number is 8, and with 8 neutrons, its mass number is 16.

Elements can exist as different forms called isotopes. In isotopes, the number of protons (atomic number) stays constant, but the number of neutrons may vary. This is why scientists like J.J. Thompson, John Dalton, Ernest Rutherford, and Niels Bohr made important discoveries about atomic structure.

Quick Tip: Remember that protons never change in an element—if the number of protons changes, you get a completely different element!

Let's look at some examples: Nitrogen (N) has 7 protons, 7 neutrons, and 7 electrons with a mass number of 14. Oxygen (O) has 8 protons, 8 neutrons, and 8 electrons with a mass number of 16. Elements can also form ions by gaining or losing electrons, changing their overall charge.