Inside the atom

Everything around you — your desk, the air you breathe, even your own body — is made of incredibly tiny particles called atoms.

Atoms are almost impossibly small. An adult human body contains roughly 10 thousand million million million million atoms — and about 98% of those are just three types: hydrogen, oxygen, and carbon.

The idea that matter is made of atoms goes back to Ancient Greek philosophers, who reasoned that if you kept cutting something into smaller and smaller pieces, you would eventually reach a piece that could not be cut any further. They called this the atomos.

In the early 1800s, the British chemist John Dalton (1766–1844) developed this idea into a proper scientific theory. He proposed that:

1. All matter is made of atoms, which are indivisible (cannot be split).
2. Atoms are indestructible — they cannot be created or destroyed in chemical reactions.
3. All atoms of the same element are identical.

For a long time, Dalton's model was accepted. However, in 1896, the French scientist Henri Becquerel made a startling discovery: certain materials (like uranium) spontaneously gave off radiation all by themselves — a phenomenon later called radioactivity. Nobody had asked uranium to do anything; it just did it on its own.

This was a huge clue that atoms must be made of even smaller parts. If atoms were truly indivisible and indestructible, how could they be spitting out energy and particles? Later work by Ernest Rutherford and Frederick Soddy (around 1901–1902) showed that radioactive decay actually involves one element transforming into a completely different element — something Dalton's model could never explain.