---
title: "Excited State — AP Physics 2 Definition & Exam Guide"
description: "An excited state is any atomic energy level above the ground state, reached by absorbing a photon. Key to emission and absorption spectra in AP Physics 2 Unit 15."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/excited-state"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit 15"
---

# Excited State — AP Physics 2 Definition & Exam Guide

## Definition

In AP Physics 2, an excited state is any atomic energy level above the ground state; an atom reaches it by absorbing a photon whose energy exactly matches the gap between two energy levels, and it can spontaneously emit a photon to drop back down.

## What It Is

An excited state is any [energy](/ap-physics-2-revised/unit-15/6-compton-scattering/study-guide/OoE2k26dtiHSsZEf "fv-autolink") level of an atom that sits above the [ground state](/ap-physics-2-revised/key-terms/ground-state "fv-autolink") (the lowest possible energy). The CED models the atom as a system of a nucleus and an electron, and that system can only hold certain specific amounts of energy. To get into an excited state, the atom must absorb a photon whose energy *exactly* matches the difference between two of those allowed levels. Close doesn't count. A photon with too much or too little energy just passes by unabsorbed.

Once the atom is excited, it doesn't stay there. It spontaneously emits a photon and drops to a lower [energy state](/ap-physics-2-revised/unit-15/3-emission-and-absorption-spectra/study-guide/QvJsunuWugAm3bOk "fv-autolink"), and the emitted photon carries away exactly the energy difference between the two levels. That's the whole engine behind emission and absorption spectra. Every bright line in an emission spectrum is one specific downward jump from an excited state, and every dark line in an absorption spectrum is one specific upward jump into one.

## Why It Matters

Excited states live in **Topic 15.3 (Emission and Absorption Spectra)** in **[Unit 15](/ap-physics-2-revised/unit-15 "fv-autolink"): Modern Physics**, supporting learning objective **15.3.A**: describe the emission or absorption of photons by atoms. This is where [AP Physics 2](/ap-physics-2-revised "fv-autolink") forces you to think quantum. Energy in an atom isn't a dial, it's a ladder with fixed rungs. The excited state concept is what makes line spectra make sense. If atoms could absorb any energy, spectra would be smooth rainbows. Because they can only sit on specific rungs, each element produces its own fingerprint of discrete lines. Quantization of atomic energy is one of the central ideas of the modern physics unit, and excited states are how you see it in action.

## Connections

### [Ground State (Unit 15)](/ap-physics-2-revised/key-terms/ground-state)

The ground state is the bottom rung of the energy ladder, and an excited state is any rung above it. Every absorption starts the climb and every emission heads back down, so you can't define one without the other. For hydrogen, the ground state is -13.6 eV, and absorbing a 10.2 eV [photon](/ap-physics-2-revised/key-terms/photon "fv-autolink") lifts the electron to the n=2 excited state at -3.4 eV.

### [Line Spectrum (Unit 15)](/ap-physics-2-revised/key-terms/line-spectrum)

A [line spectrum](/ap-physics-2-revised/key-terms/line-spectrum "fv-autolink") is the visible evidence that excited states exist. Each emission line is one specific downward transition from an excited state, with photon energy equal to the gap between levels. An electron in mercury's n=5 level has multiple possible drops (to n=4, 3, 2, or 1), so a single excited state can feed several different spectral lines.

### [Ionization (Unit 15)](/ap-physics-2-revised/key-terms/ionization)

[Ionization](/ap-physics-2-revised/key-terms/ionization "fv-autolink") is what happens when you over-excite. Pump in enough energy to lift the electron past the top of the ladder and it escapes the atom entirely. The key difference is that excitation requires an exact photon energy, while ionization just requires at least the binding energy, with any extra becoming the freed electron's kinetic energy.

## On the AP Exam

Excited states show up mostly in multiple-choice questions that test whether you understand quantization. A classic stem gives you a hydrogen atom with ground state energy -13.6 eV absorbing a 10.2 eV photon and asks for the final energy level. The math is just E_final = -13.6 + 10.2 = -3.4 eV, which corresponds to n=2. Other questions ask *why* only photons of one specific energy get absorbed (answer: atomic energy levels are quantized, so the photon energy must match the gap exactly), or ask you to count the possible emission lines when an electron in a high excited state can cascade down through multiple levels. No released FRQ has used the term verbatim, but you should be ready to draw or interpret an energy level diagram and connect transitions to photon energies using E = hf.

## excited state vs ground state

The ground state is the single lowest energy level an atom can have, and it's where atoms naturally sit. An excited state is any level above it. The confusion comes from energy signs. Ground state energies are the most negative (like hydrogen's -13.6 eV), and excited states are less negative (like -3.4 eV at n=2). So a 'higher' energy state has a smaller magnitude but a larger (less negative) value. An atom in an excited state will spontaneously emit a photon and head back toward the ground state; an atom already in the ground state can't emit anything because there's nowhere lower to go.

## Key Takeaways

- An excited state is any atomic energy level above the ground state, and an atom reaches it by absorbing a photon whose energy exactly equals the difference between two allowed levels.
- Atoms can only absorb or emit photon energies that match gaps between energy levels, which is why each element produces a discrete line spectrum instead of a continuous rainbow.
- An atom in an excited state spontaneously emits a photon and drops to a lower state, and the photon carries away exactly the energy difference between the two levels.
- For hydrogen, absorbing a 10.2 eV photon from the -13.6 eV ground state puts the electron at -3.4 eV, which is the n=2 excited state.
- Energy values for bound states are negative, so an excited state has a less negative energy than the ground state, not a positive one.
- An electron in a high excited state can take multiple downward paths, so one excited level can produce several different emission lines.

## FAQs

### What is an excited state in AP Physics 2?

An excited state is any energy level of an atom above the ground state. An atom enters one by absorbing a photon whose energy exactly matches the gap between two allowed energy levels, and it leaves by emitting a photon and dropping to a lower level.

### Can an atom absorb a photon with more energy than the gap and just keep the extra?

No. For a bound-state transition, the photon energy must exactly equal the difference between two energy levels or it isn't absorbed at all. The exception is ionization, where a photon with at least the binding energy frees the electron and any extra energy becomes the electron's kinetic energy.

### How is an excited state different from the ground state?

The ground state is the single lowest energy level (for hydrogen, -13.6 eV), while excited states are all the levels above it (like -3.4 eV at n=2). Atoms in excited states spontaneously emit photons to move lower; an atom in the ground state has nowhere lower to go.

### Why are excited state energies negative?

The energies are negative because the electron is bound to the nucleus, with zero energy defined as a free electron infinitely far away. An excited state is less negative than the ground state, so moving up the ladder means the energy value increases toward zero.

### How long does an atom stay in an excited state?

Not long. The CED's key point is that emission is spontaneous, meaning an excited atom drops to a lower state on its own, releasing a photon with energy equal to the gap between the levels. That spontaneous emission is what produces the bright lines in an emission spectrum.

## Related Study Guides

- [15.3 Emission and Absorption Spectra](/ap-physics-2-revised/unit-15/3-emission-and-absorption-spectra/study-guide/QvJsunuWugAm3bOk)

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