Know the relationship between the proton number and the relative charge on a nucleus

5.1.2 The Nucleus

Key Concepts

⚛️ The nucleus is the tiny, dense centre of an atom that contains protons and neutrons.
💡 Protons carry a positive charge, while neutrons are electrically neutral.
📚 The relative charge of a nucleus is simply the total positive charge contributed by its protons.

Proton Number (Z) and Relative Charge

The proton number (also called the atomic number) is denoted by $Z$ and counts how many protons are in the nucleus. Each proton contributes a charge of $e = 1.602 \times 10^{-19}\,\text{C}$. Therefore the relative charge of a nucleus is:

$$q = Z \, e$$

Example: A carbon nucleus (Z = 6) has a relative charge of $$q = 6 \times 1.602 \times 10^{-19}\,\text{C} = 9.612 \times 10^{-19}\,\text{C}$$.

Important Relationships

• Relative charge $q$ is directly proportional to the proton number $Z$.
• Neutrons do not affect the relative charge because they have zero charge.
• The relative charge is independent of the mass number $A$ (total number of protons + neutrons).
• For any element, the relative charge in elementary charge units is simply its proton number: $q/e = Z$.

Sample Table: Elements and Their Relative Charges