Atomic Structure (Years 8–11) — Science

Everything you can see, touch or breathe is built from atoms — objects so small that lining up a million of them would barely cross a full stop. Yet each one has a hidden architecture: a dense central nucleus orbited by a haze of electrons in layers called shells.

Why this matters: once you can count the three particles inside an atom, you can predict how it behaves — what it weighs, what charge it carries and which other atoms it will react with. This is the foundation of all chemistry.

Shells fill up 2, then 8, then 8…

Meet the three particles

Heavy protons and neutrons sit in the nucleus; light electrons orbit outside.

An atom has just three building blocks, and the easiest way to keep them straight is to picture the nucleus as the Sun and the electrons as tiny planets whizzing around it at a distance.

Protons — sit in the nucleus, carry a positive (+) charge, and are heavy.
Neutrons — also in the nucleus, carry no charge (neutral), and are heavy too.
Electrons — orbit in shells, carry a negative (−) charge, and are almost weightless.

Particle	Charge	Relative mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	almost 0	Shells

Two numbers that describe any atom

The larger number on top is the mass number; the smaller below is the atomic number.

Chemists label every atom with two whole numbers:

Atomic number = the number of protons. This is the atom's identity card — carbon always has 6 protons; change it and it stops being carbon.
Mass number = protons + neutrons. Because electrons weigh almost nothing, only the nucleus counts towards mass.

An atom is electrically neutral overall, which means the positives balance the negatives exactly. So the number of electrons equals the number of protons.

How the electrons stack up

Shells fill inside-out: 2, then 8, then 7 for chlorine's 17 electrons.

Electrons do not pile in randomly. They fill the shells from the inside out, like filling the front row of a cinema before starting the next row. The pattern is 2, then 8, then 8: the first shell holds up to 2 electrons, and the next two hold up to 8 each.

Worked reasoning. An atom has 17 protons and 18 neutrons. Ask three questions in order. Mass number? Add the nuclear particles: 17 + 18 = 35. Electrons? Neutral atom, so electrons = protons = 17. How are they arranged? Fill 2, then 8, leaving 7 for the third shell: 2, 8, 7. That single arrangement tells a chemist this is chlorine and that it reacts fiercely.

Common mix-up: students swap atomic number and mass number, or add the electrons to the mass. Remember: only protons and neutrons have mass. Electrons are so light they never count towards the mass number.

Memory hook: "PEN" — Protons = atomic number, add Electrons balance the protons, and Neutrons + protons = mass number. Shells fill 2, 8, 8.