Use the nuclide notation _Z^A X

5.1.2 The Nucleus

Learning Objective

Students will be able to write and interpret nuclide notation \$_Z^A\text{X}\$, where:

• \$Z\$ = atomic number (number of protons)
• \$A\$ = mass number (total number of nucleons = protons + neutrons)
• \$\text{X}\$ = chemical symbol of the element

Key Concepts

The nucleus is the tiny, dense centre of an atom. It contains two types of sub‑atomic particles:

1. Protons – positively charged, define the element (its atomic number \$Z\$).
2. Neutrons – neutral, add to the mass but not to the charge.

The number of neutrons (\$N\$) can be found from the relationship:

\$N = A - Z\$

Writing Nuclide Notation

To write the nuclide notation for a given atom:

1. Identify the element’s chemical symbol (\$\text{X}\$).
2. Determine the atomic number \$Z\$ (number of protons).
3. Determine the mass number \$A\$ (protons + neutrons).
4. Place \$Z\$ as a subscript, \$A\$ as a superscript, and the symbol \$\text{X}\$ to the right: \$_Z^A\text{X}\$.

Examples

Isotopes

Atoms of the same element that have different numbers of neutrons are called isotopes. They share the same \$Z\$ but have different \$A\$ values.

Example: Carbon has two stable isotopes:

• \$_6^{12}\text{C}\$ (6 neutrons)
• \$_6^{13}\text{C}\$ (7 neutrons)

Practice Questions

1. Write the nuclide notation for an atom that has 15 protons and 16 neutrons.
2. How many neutrons are in \$_{26}^{56}\text{Fe}\$?
3. Identify the element and mass number for the nuclide \$_{11}^{23}\text{Na}\$.
4. Two isotopes of chlorine are \${17}^{35}\text{Cl}\$ and \${17}^{37}\text{Cl}\$. What is the difference in the number of neutrons between them?
5. Explain why the chemical properties of isotopes of the same element are almost identical.