Define the terms proton number (atomic number) Z and nucleon number (mass number) A and be able to calculate the number of neutrons in a nucleus

5.1.2 The Nucleus

Objective: Define the terms proton number (atomic number) \$Z\$ and nucleon number (mass number) \$A\$ and calculate the number of neutrons in a nucleus.

Key Terms

• \$Z\$ (Proton Number) – the count of protons in the nucleus. It determines the element’s identity. ⚛️
• \$A\$ (Nucleon Number) – the total number of protons plus neutrons. It tells us how heavy the nucleus is. 📦
• \$N\$ (Neutron Number) – the count of neutrons, calculated as \$N = A - Z\$. 🧪

Neutron Calculation

To find the number of neutrons, simply subtract the proton number from the nucleon number:

\$N = A - Z\$

Example: For carbon‑12, \$Z = 6\$ and \$A = 12\$ → \$N = 12 - 6 = 6\$ neutrons. 🌱

Isotope Comparison Table

Analogy: The Nucleus as a City

Imagine the nucleus as a bustling city.

- Protons (\$Z\$) are the city’s houses – they give the city its name (the element).

- Neutrons (\$N\$) are the parks and public spaces – they add mass but don’t change the city’s identity.

- The total mass number (\$A\$) is the total number of buildings and parks combined.

Just like counting houses and parks gives you the total number of structures, \$A = Z + N\$.

Exam Tip: When you see a question like “How many neutrons are in \$^{18}\text{O}\$?”, remember that \$^{18}\text{O}\$ means \$A = 18\$ and \$Z = 8\$ (oxygen). So \$N = 18 - 8 = 10\$. Quick mental subtraction saves time! ⏱️

Practice Question: Calculate \$N\$ for \$^{27}\text{Al}\$.

Answer: \$Z = 13\$, \$A = 27\$ → \$N = 27 - 13 = 14\$ neutrons. ??