Atoms, Nuclei and Radiation – Nucleon Number vs Proton NumberAtoms, Nuclei and Radiation
Distinguishing Nucleon Number and Proton Number
In nuclear physics two fundamental integer quantities are used to describe a nucleus:
- Proton number (also called atomic number) – the number of protons in the nucleus. It determines the chemical element and is denoted by \$Z\$.
- Nucleon number (also called mass number) – the total number of nucleons (protons + neutrons) in the nucleus. It is denoted by \$A\$.
The relationship between these quantities can be written as
\$A = Z + N\$
where \$N\$ is the neutron number. The three numbers \$Z\$, \$N\$, and \$A\$ uniquely identify a specific isotope of an element.
Key Differences
| Aspect | Proton Number (\$Z\$) | Nucleon Number (\$A\$) |
|---|
| Definition | Count of protons in the nucleus | Count of all nucleons (protons + neutrons) |
| Symbol | \$Z\$ | \$A\$ |
| Determines | Chemical element (e.g., carbon, oxygen) | Isotope of the element (e.g., \$^{12}\$C vs \$^{13}\$C) |
| Typical Range | 1 – 118 (known elements) | 1 – \overline{300} for naturally occurring nuclei |
| Conservation in Reactions | Conserved in nuclear reactions that do not involve transmutation | Conserved in all nuclear reactions (mass number is conserved) |
Practical Examples
- For the nucleus of a carbon‑12 atom:
- \$Z = 6\$ (six protons)
- \$A = 12\$ (six protons + six neutrons)
- For the nucleus of a uranium‑235 atom:
- \$Z = 92\$ (ninety‑two protons)
- \$A = 235\$ (ninety‑two protons + one hundred forty‑three neutrons)
Why the Distinction Matters
Understanding the difference between \$Z\$ and \$A\$ is essential for:
- Identifying isotopes and predicting their stability.
- Balancing nuclear equations, where both \$Z\$ and \$A\$ must be conserved.
- Interpreting decay modes (e.g., \$\alpha\$ decay reduces \$A\$ by 4 and \$Z\$ by 2).
Suggested diagram: A schematic nucleus showing protons (red) and neutrons (blue), with labels for \$Z\$, \$N\$, and \$A\$.