Atomic Number
Definition
The number of protons in an atom's nucleus. It is the atom's ID: it tells you exactly which element you have.
Symbol: ·
Technical definition
The atomic number (Z) is the number of protons in an atomic nucleus. It uniquely identifies a chemical element and determines its position in the periodic table, as well as its electron count when the atom is neutral.
Example
Oxygen's atomic number is 8, so every oxygen atom has exactly 8 protons and, when neutral, 8 electrons.
Common confusion
Atomic number counts protons, not total particles. It says nothing about neutrons (that is the mass number) or about the atom's charge.