element
An element is a pure substance made of just one kind of atom — the simplest building blocks the everyday world comes in. Gold is nothing but gold atoms; oxygen is nothing but oxygen atoms. What makes one element different from another is a single number: how many protons sit in the center of each atom. Hydrogen has one proton, carbon has six, gold has seventy-nine, and that count is the atom's permanent name tag — change it and you've changed the element itself.
About 118 elements are known, and roughly the first 94 occur in nature; the rest are made for fleeting moments in laboratories. Together they are the alphabet of all matter: your body, the air, the oceans, and the stars are all just these few dozen letters spelled out in different combinations. Chemists line them up by proton count in the periodic table, where the pattern reveals which elements behave like cousins.
Here's the part people often miss: an element can't be broken down into anything simpler by ordinary chemistry. You can burn it, dissolve it, or react it, but each oxygen atom stays oxygen. (Only nuclear processes, which crack the atom's core, can turn one element into another.) Don't confuse an element with a compound like water — water can be split into hydrogen and oxygen, but those two are the end of the line.
Hydrogen (1 proton), carbon (6), oxygen (8), iron (26), gold (79) — same idea, just a different proton count each time.
The proton count alone decides which element an atom is.
The word "element" comes from the Latin elementum, meaning a first principle or basic constituent. For two thousand years people thought everything was made of four "elements" — earth, water, air, and fire — until Antoine Lavoisier in 1789 redefined an element as a substance that chemistry cannot break apart, putting the idea on its modern footing.