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A single sip of water contains roughly 102410^{24} individual molecules — far more than all the stars in the observable universe.
Because atoms and molecules are unimaginably tiny, chemists cannot count them one by one. Instead, we bundle them into a standard counting package called the mole.
How big is this counting unit, and where did it come from?

The Mole and Avogadro's Number

In the SI system, a mole (abbreviated mol\text{mol}) is defined as exactly 6.02214076×10236.02214076 \times 10^{23} representative particles.
We call this value Avogadro's number (NAN_A), rounded to 6.022×10236.022 \times 10^{23} for standard chemistry calculations.
In 1811, Italian scientist Amedeo Avogadro deduced that equal volumes of gases contain equal numbers of particles, though French physicist Jean Perrin actually calculated the constant in 1909 and named it in Avogadro's honor.
What counts as a 'particle' depends entirely on what kind of chemical substance you are looking at.

Representative Particles

A representative particle is the smallest freestanding unit of a pure substance that retains its chemical identity.
Elements made of single atoms (like iron, Fe\text{Fe}) have atoms as their particles; molecular compounds (like water, H2O\text{H}_2\text{O}) have molecules; and ionic compounds (like table salt, NaCl\text{NaCl}) have formula units.