Understand the relationship between the proton number (atomic number) and the relative electric charge of a nucleus.
Key Concepts
The nucleus is the central part of an atom and contains protons and neutrons.
Protons carry a positive electric charge; neutrons are electrically neutral.
The number of protons in the nucleus is called the atomic number, denoted by \$Z\$.
The total charge of the nucleus is the sum of the charges of all its protons.
Relationship Between Proton Number and Relative Charge
The elementary charge carried by a single proton is \$e = 1.602 \times 10^{-19}\ \text{C}\$. For a nucleus containing \$Z\$ protons, the relative charge \$Q\$ is given by
\$Q = Z\,e\$
Since \$e\$ is a constant, the relative charge of a nucleus is directly proportional to its proton number.
Examples
Element
Proton Number (\$Z\$)
Relative Charge (\$Q\$) in units of \$e\$
Absolute Charge (\$Q\$) in coulombs
Hydrogen‑1
1
+1 \$e\$
+\$1.602 \times 10^{-19}\$ C
Helium‑4
2
+2 \$e\$
+\$3.204 \times 10^{-19}\$ C
Carbon‑12
6
+6 \$e\$
+\$9.612 \times 10^{-19}\$ C
Uranium‑238
92
+92 \$e\$
+\$1.474 \times 10^{-17}\$ C
Implications for Chemical Behaviour
The relative charge determines the element’s identity; changing \$Z\$ changes the element.
Atoms seek electrical neutrality. An atom with \$Z\$ protons will attract \$Z\$ electrons to form a neutral atom.
Ions are formed when the number of electrons differs from \$Z\$, giving a net charge of \$(Z - Ne)e\$ where \$Ne\$ is the number of electrons.
Common Misconceptions
Misconception: The nucleus has a “total charge” that varies with the number of neutrons.
Clarification: Neutrons have no charge; only protons contribute to the nuclear charge.
Misconception: All atoms of the same element have the same charge.
Clarification: Atoms of the same element have the same \$Z\$, but they may be neutral or form ions with different net charges.
Suggested Diagram
Suggested diagram: A schematic of a nucleus showing \$Z\$ protons (each labelled +) and \$N\$ neutrons (neutral), with the surrounding electron cloud containing \$Z\$ electrons for a neutral atom.
Summary
The relative charge of a nucleus is directly proportional to its proton number. This simple linear relationship, \$Q = Z e\$, underpins the identity of elements, the formation of ions, and the overall electrical neutrality of atoms.