The Story of the Atom

● Try it: Atoms as tiny solid balls

Dalton imagined atoms as hard, indivisible spheres — like microscopic billiard balls, constantly moving and bouncing. Different elements = different sized balls.

Add atoms, switch elements (notice the size changes!) and crank up the heat to watch them race around. Atoms of the same element are identical — atoms of different elements differ.

What Dalton got right

• All matter is made of atoms — tiny particles too small to see.
• Atoms of one element are identical, and different from atoms of other elements.
• Atoms combine in fixed whole-number ratios to form compounds (like H₂O — always 2:1!).
• Atoms can't be created or destroyed in chemical reactions — only rearranged.

● Try it: Electrons in a positive "pudding"

Thomson discovered electrons — tiny negative particles nearly 2000× lighter than the whole atom. He pictured them stuck inside a ball of positive charge, like plums in a pudding.

Drag the electrons around inside the pudding — they're free to move, but can't escape the positive sphere. Then run the actual experiment that discovered them!

The discovery that changed everything

• The cathode ray experiment: a mysterious glowing beam inside a vacuum tube bent toward the positive plate — proving the beam was made of negative particles.
• Atoms are divisible after all! Electrons are pieces of atoms — Dalton's "indivisible" ball was broken open.
• Atoms are neutral overall, so positive charge must exist to balance the negative electrons.

● Try it: A tiny nucleus in empty space

Rutherford pictured the atom as mostly empty space: a tiny, dense, positive nucleus at the centre, with electrons whirling far around it. He proved it by firing positive alpha particles at ultra-thin gold foil — if Thomson's pudding were right, they should all sail straight through.

This is Rutherford's nuclear model. Notice how tiny the nucleus is compared to the whole atom — and it's still drawn thousands of times too big! Now run the experiment that revealed it.

What the experiment proved

• Atoms are mostly empty space — that's why almost all alpha particles passed straight through.
• A tiny, dense, positive nucleus sits at the center — only a direct hit made particles bounce back.
• Scale check: if an atom were a football stadium, the nucleus would be a marble at the center field!
• Electrons orbit somewhere in the empty space around the nucleus.

● Try it: Make the electron jump!

Bohr's fix: electrons can only ride on fixed orbits (energy levels), like rungs of a ladder. They never spiral in — they can only jump between levels by absorbing or releasing light.

You can also click directly on any orbit to send the electron there. Falling from higher levels releases photons of different colours — this is exactly why neon signs and fireworks glow in specific colours!

Why this model was genius

• Electrons live on fixed energy levels (n = 1, 2, 3…) — they can't exist in between.
• Jump up = absorb energy. Fall down = emit light of an exact colour.
• It explained the hydrogen spectrum perfectly — the exact colours hydrogen gas glows had puzzled scientists for decades.
• Atoms are stable because an electron on the lowest rung (n = 1) has nowhere lower to fall.

● Try it: Hunt the electron

In the modern model, an electron has no fixed path. We can only know the probability of finding it somewhere. The fuzzy cloud below shows where the electron is likely to be — denser cloud = higher chance.

Press "Measure" repeatedly — the electron appears in a different random spot each time, but almost always inside the dense part of the cloud. That IS the orbital: a map of probability, not a path.

The model we use today

• Electrons behave like waves, not tiny planets. Schrödinger's equation describes their wave.
• Heisenberg's Uncertainty Principle: you can never know an electron's exact position and speed at the same time.
• Orbitals replace orbits — 3D regions (s, p, d, f) where the electron is most likely to be found.
• It works for every element — and powers all of modern chemistry, lasers, LEDs and computer chips.