The atomic number of an element is the number of protons present in the nucleus of one atom of that element.
- It is represented by the symbol Z.
- Since atoms are electrically neutral, the number of electrons in a neutral atom is also equal to the atomic number.
- The atomic number determines the identity of an element — no two different elements have the same atomic number.
- It also decides the position of an element in the periodic table and its chemical properties, because these depend on the arrangement of electrons.
Examples:
- Hydrogen (H) has 1 proton → Atomic number = 1.
- Carbon (C) has 6 protons → Atomic number = 6.
- Oxygen (O) has 8 protons → Atomic number = 8.
In short:
Atomic number (Z) = Number of protons ( = Number of electrons in a neutral atom).