E.1.2—Nuclear notation

Read the symbol

Nuclear notation is written as ${}^{A}_{Z}X$ . The chemical symbol $X$ identifies the element, the proton number $Z$ is written below, and the nucleon number $A$ is written above.

Count the nucleus

The nucleus contains $Z$ protons and $N=A-Z$ neutrons. The number of electrons is not encoded by $A$ and $Z$ ; for a neutral atom it equals $Z$ , while an ion has gained or lost electrons.

Compare nuclides

Atoms of the same element have the same $Z$ . Isotopes have the same $Z$ but different $A$ , so they contain different numbers of neutrons.

Common trap

Do not use the electron count as the proton number for an ion, and do not confuse $A$ with the number of neutrons. Subtract $Z$ from $A$ to find the neutron number.

How it is assessed

Questions ask learners to identify Z or construct nuclear notation from proton, neutron and electron counts.

Command terms

Identify / Write

What earns marks

Place the proton number below the symbol, calculate A as protons plus neutrons, and keep the electron count separate when the species is an ion.

Watch for

Using the electron count as Z for an ion or placing the neutron number directly as A.

Retrieve the evidence chain

Rutherford scattering supports a small positive nucleus; nuclear notation separates protons, neutrons and electrons; line spectra show discrete energy differences; and $E_\gamma=hf=hc/\lambda$ connects transitions to photons.

Check the model

When reading a spectrum, identify the transition, use the energy difference rather than an absolute level, and compare characteristic lines with known spectra to identify elements.

E.1.2—Nuclear notation

Read Nuclear Notation

E.1.2 Exam Analysis

Assessment in practice

Retrieve the SL Atomic Model