Atom Electrons

At the heart of every atom is a nucleus, around which dance tiny particles with negative electric charge called electrons.

Hydrogen, the simplest atom has just one electron, appearing as a fuzzy cloud where the electron is likely to be:

atom hydrogen quantum

Unlike planets orbiting the sun in predictable paths, electrons move in ways that are less certain. We can only predict the regions where they are most likely to be found, which is what we call the "electron cloud".

But for clarity we often use a simplified diagram:

simple hydrogen atom model

In the diagram, the nucleus (labeled "H" for "Hydrogen") is in the center, and the electron is shown around it.

An atom's identity comes from the number of protons in its nucleus.

For example, hydrogen has 1 proton, carbon has 6.

In a neutral atom there is one electron for each proton, so let's just focus on the electrons here.

Next is helium with its two electrons:

simple helium atom model

Helium behaves differently from hydrogen because of its electron arrangement.

Atoms mostly interact through their outermost electrons. And electrons love to pair up.

Moving on to lithium:

simple lithium atom model

Lithium has a third electron, but it has to go in another layer (called a "shell" because an atom is really a 3D thing).

Like hydrogen, lithium is eager to bond with other atoms due to its single electron in the outer shell.

Adding more electrons starts filling up this second shell:

simple beryllium atom model
simple boron atom model
simple carbon atom model

The second shell is not full yet (it can hold 8 electrons), but let's look at Carbon.

Every atom has its story, and its behavior and chemical properties are mostly formed by its outer electrons.

Electrons and Shells

Why 2 electrons in the first shell, but 8 in the second shell?

It is due to orbitals (not orbits)

Orbitals are regions within an atom where an electron is most likely to be found.


Electrons fill orbitals in a way that minimizes the energy of the atom. This leads to some interesting patterns in how the shells get filled.

It is like packing things into a box, the items find their lowest energy locations.

Each orbital can hold 2 electrons (each with opposite "spin"):

Find out more at Atom Orbitals.


Atoms are both beautifully simple and fascinatingly complex, and can mostly be understood by how their electrons are arranged.