Look around you. Everything you see, including you, is made of the same stuff—elements. Each of those elements has its own unique characteristics, but all elements are made of atoms—the smallest unit of an element that still has the characteristics of the element.

Scientists used to think there was nothing smaller than an atom.

Today, we know the atom is made of smaller particles, and those are made of even smaller particles.

Atomic Structure

Nucleus

The nucleus is made of protons and neutrons; each has the same mass: 1 amu (atomic mass unit).

Protons and neutrons aren’t exactly alike, though; protons have a positive charge while neutrons don’t have a charge.

Electrons

Electrons are so small that they have nearly no mass at all. A single neutron has only 1/1836 amu. They are also negatively charged.

Organizing the Elements

All of the known elements are organized on the periodic table of the elements. They are arranged by atomic number, from smallest to largest, and labeled with their element symbol, atomic number, and atomic mass.

Standard Nuclear Notation

To easily communicate information about the elements, scientists use standard nuclear notation.

Nuclear notation is formed by writing an elemental symbol preceded by a subscript indicating its atomic number—the number of protons—and a superscript indicating its mass number—the number of protons and neutrons combined.

For example: Carbon has 6 protons, so it’s atomic number is 6.

Carbon's mass number is 12. How many neutrons does it have?

The mass number of an element is a round number; the atomic mass usually isn't. Atomic mass is an average mass of all of the isotopes of an element. We use the mass number, which is always a round number, to make calculations easier.

Isotopes

Think about clover. Clovers can have three, four, or even more leaves. The four-leaved clovers are rare, but they are still clovers. In a similar way, two atoms of an element can have different numbers of neutrons. Because they still have the same number of protons, though, they are the same element. These “varieties” of the same element are called isotopes.

Learn more about radioactivity

Atomic Mass