Energy level diagram

Essential knowledge 15.3.A.4.iii gives the diagram its place in one line: energy level diagrams are commonly used to visually represent the energy states of an atom. Topic 15.3's first suggested skill is 1.A, creating diagrams and schematics, so drawing one is examinable work rather than a textbook decoration.

Read the axis before anything else. It is energy, usually in electron volts, and it runs upward. Zero is placed where the electron is just free of the atom, so every bound energy level sits below zero as a negative number. Negative means bound, and the ground state at the bottom is the level furthest from escape, which is 15.3.A.5's point about ionisation costing the most from the ground state.

Only the gaps are physical. The position of zero is a convention; no experiment measures a level, only the difference between two of them. Statement 15.3.A.2 says energy is absorbed or emitted only when it matches the difference between two states, and 15.3.A.3 makes each transition a single frequency and so a single wavelength.

So an arrow between two lines is a photon. Its length is $\Delta E$, upward for absorption and downward for emission, and $\Delta E = hf$ converts it. On a hydrogen diagram the lines crowd together toward zero going up, which is a feature of that element's spectrum rather than a rule: 15.3.A.4 says each element has its own unique set of allowed levels.

The Topic 15.3 boundary statement is short and binding: in AP Physics 2, only energy level diagrams of single-electron atoms will be considered.