Use the nuclide notation _Z^A X

What is a nucleus?

The nucleus is the tiny, dense core at the centre of an atom. Think of it as the “heart” of the atom, holding most of its mass while the electrons orbit around it like planets around the sun.

Nuclide notation

Nuclides are written as a superscript and subscript around the element symbol:

$_{Z}^{A}\text{X}$

• $Z$ – number of protons (atomic number)
• $A$ – total number of protons and neutrons (mass number)
• X – element symbol

Example: The most common isotope of carbon is written as $_{6}^{12}\text{C}$.

Protons, neutrons, and electrons

🔬 Protons carry a positive charge and determine the element.

🧪 Neutrons are neutral; they add mass but no charge.

⚡ Electrons orbit the nucleus and carry a negative charge. In a neutral atom, the number of electrons equals the number of protons.

Formula for mass number: $A = Z + N$, where $N$ is the number of neutrons.

Isotopes and mass number

Isotopes are atoms of the same element (same $Z$) but different $A$ because they have different numbers of neutrons.

Analogy: The Atom as a Solar System

Imagine the nucleus as the sun – heavy and central. The electrons are like planets orbiting around it. Just as the sun’s mass dominates the solar system, the nucleus holds most of the atom’s mass.

Just as planets can have moons (neutrons), the nucleus can have varying numbers of neutrons, giving rise to different isotopes.

Exam Tips

1. When given a nuclide, identify $Z$ (protons) and $A$ (mass number). Then calculate $N = A - Z$.
2. Remember that the element symbol tells you the element; the subscript is the atomic number.
3. Use the analogy of a solar system to explain why the nucleus is so small yet contains most of the mass.
4. Practice converting between different notations: e.g., $^{12}\text{C}$, $_{6}^{12}\text{C}$, and “Carbon-12”.
5. Check your answers by verifying that the number of protons matches the element’s position on the periodic table.