What Is an Atom? The Basics of Atomic Structure

Everything around you — the screen you're reading this on, the air you're breathing, you yourself — is built from atoms. An atom is the smallest unit of an element that still behaves like that element.

The three particles

Every atom is made of three kinds of particles:

The nucleus — protons and neutrons packed tightly together — sits at the atom's center. Electrons occupy the much larger space around it, arranged in shells at increasing distances from the nucleus.

What makes an element an element

The number of protons in an atom's nucleus is its atomic number, and it's what defines the element. Every atom with exactly 6 protons is carbon — no exceptions. Change the proton count and you've changed the element entirely.

A neutral atom (no overall charge) always has an equal number of protons and electrons. The number of neutrons can vary between atoms of the same element — these variants are called isotopes — which is why an element's atomic mass on the periodic table is usually not a whole number: it's a weighted average across all the naturally occurring isotopes.

Electron shells

Electrons don't orbit randomly — they fill shells in a specific order, starting with the shell closest to the nucleus. The first shell holds up to 2 electrons, the second up to 8, and it continues from there. The electrons in the outermost shell (called valence electrons) are what determine how an atom bonds with others — which is the whole basis of chemistry.

Try it yourself: open the Atom Viewer in Science Lab and pick any element from Hydrogen to Krypton. You'll see its actual shell structure animated, plus its proton, neutron, and electron counts.