The Atom Factory

Why it matters

Every atom in the universe is built from just three parts: protons, neutrons, electrons. Change the recipe and you change the element. Today you run the factory.

Predict first

What single thing decides whether an atom is sodium and not, say, oxygen?

the proton count (atomic number) is the element's identity card. 11 protons = sodium, always — no matter how many neutrons or electrons tag along.

• Its number of protons — correct
• Its number of electrons
• Its total mass
• Its colour and hardness

What you do

Challenge: build a neutral sodium atom — 11 protons, 12 neutrons and 11 electrons (mass number 23). Watch the shells fill 2, 8, 1 — and notice the lonely outer electron.

Check yourself

An atom has 11 protons, 12 neutrons and 11 electrons. Its mass number is…

Mass number = protons + neutrons = 11 + 12 = 23. Electrons are ~1/1836 the mass of a proton — too light to count.

If sodium loses its one outer electron, what does it become?

It still has 11 protons, so it is still sodium — but now 11+ and 10− leaves a charge of 1+. Identity comes from protons; charge from the electron balance.

Two atoms both have 17 protons, but one has 18 neutrons and the other has 20. They are…

Same protons (17 ⇒ chlorine), different neutrons ⇒ isotopes: Cl-35 and Cl-37. Dalton's 'identical atoms' idea was updated by exactly this discovery.

• 11
• 12
• 23 — correct
• 34
• A neon atom
• A sodium ion, Na⁺ — correct
• A different isotope of sodium
• A magnesium atom
• two different elements
• an atom and its ion
• isotopes of chlorine — correct
• impossible — atoms of an element are identical