A Prediction Experiment

Inside the Atom

Everything around you is built from atoms — and atoms are built from just three tiny parts. Today you'll learn those parts, then you will predict what happens when we change one of them.

Before we start — you've heard that everything is made of atoms — but have you ever wondered what an atom itself is made of?

An atom has a centre and a cloud

At the centre is the tiny, heavy nucleus, packed with protons and neutrons. Whizzing around it, far lighter, are the electrons — arranged in shells.

Quick check, together: Which two parts sit in the nucleus, and which part moves around the outside?

Answer: Protons and neutrons sit in the nucleus; the electrons move around the outside in shells.

Each part carries a charge

A proton is positive (+), an electron is negative (−), and a neutron is neutral (0). When an atom has the same number of protons and electrons, the + and − cancel out — overall charge 0.

Quick check, together: An atom has 6 protons and 6 electrons. Do the charges cancel? What is its overall charge?

Answer: Yes — 6 positive protons and 6 negative electrons cancel out, so the overall charge is 0 (neutral).

The protons name the element

The number of protons is the atomic number, and it decides which element the atom is. 1 proton is always hydrogen, 6 is always carbon, 8 is always oxygen. Change the protons and you change the element itself.

Quick check, together: An atom has 8 protons. Which element must it be — and could it ever be a different element while keeping 8 protons?

Answer: It must be oxygen. With 8 protons it is always oxygen — change the proton count and you change the element.

What did changing the atom teach us?

Protons set the element · neutrons set the isotope · electrons set the charge.