Define proton number/atomic number as the number of protons in the nucleus of an atom

What is an Atomic Number?

The atomic number (denoted by \$Z\$) is the number of protons in the nucleus of an atom. It uniquely identifies an element.

Think of an element like a book series. Each book (atom) has a title number (atomic number) that tells you which book it is. No two books share the same title number.

Protons, Neutrons & Electrons

• Protons – positive charge, counted by \$Z\$.
• Neutrons – neutral, counted by \$N\$.
• Electrons – negative charge, usually equal to \$Z\$ in a neutral atom.

The mass number \$A\$ is the sum: \$A = Z + N\$.

Atomic Number in the Periodic Table

In the periodic table, each element’s atomic number is displayed above its symbol. It tells you how many protons (and usually electrons) the element has.

Analogy: The Element “Family Tree”

Imagine each element as a family. The atomic number is the family’s surname number. All members (atoms) in that family share the same surname number, but they can have different numbers of children (neutrons) and pets (electrons).

Exam Tip Box 📚

1. Remember: Atomic number = number of protons.
2. When given a chemical symbol, you can find its atomic number in the periodic table.
3. For isotope questions, use \$A = Z + N\$ to find missing neutrons.
4. Practice converting between \$Z\$, \$N\$, \$A\$, and electron count for neutral atoms.