Excited States and Electron Transitions

Introduction

An electron configuration usually means a ground-state arrangement unless the question says otherwise. Energy input can produce an excited state with an electron in a higher available state. When the atom later loses energy, it may emit a photon whose energy matches a difference between allowed states.

Core explanation

The ground state is the lowest-energy state of the specified atom or ion under stated conditions. An excited state lies above it. Excitation can occur by absorbing radiation, collisions in an electrical discharge, or other energy transfer. An excited atom still has the same proton number, and ordinary electronic excitation keeps the same electron count; it redistributes energy or electron occupancy rather than ionizing the atom.

In a simple one-electron picture, hydrogen's n = 1 state can absorb a matching photon and move to n = 2. The atom's energy rises by about 10.2 eV. A return from n = 2 to n = 1 can emit a photon of that energy. An excited atom may also follow a sequence through intermediate states, emitting multiple lower-energy photons whose energies together account for the total drop.

Multi-electron excited configurations can be written by promoting an electron while respecting Pauli and the total electron count. For instance, a simplified excited helium arrangement 1s¹2s¹ has two electrons but lies above the 1s² ground arrangement. Configuration notation alone may not uniquely specify its energy because spin coupling and interactions can produce distinct states with the same subshell occupancies.

Not every apparent energy difference yields a strong optical line. Quantum transition selection rules affect which photon processes are allowed or likely, and state populations affect line intensity. Some excited states can persist longer than others. The basic relation Eγ = Einitial − Efinal remains necessary for photon energy but is not sufficient to predict every observed line.

Ionization is distinct from bound-state excitation. An ionizing photon supplies enough energy to remove an electron to an unbound state; the species' electron count changes. A sub-threshold excitation keeps the electron bound in a higher state. In a real gas, collisions and other processes can accompany these events, so a measured spectrum reflects the full experimental setting.

Step-by-step reasoning

1. Identify the species and its ground-state electron count. 2. Decide whether energy input makes a bound excited state or removes an electron. 3. For a transition, subtract allowed state energies. 4. Assign absorption to upward change and emission to downward change, considering selection rules.

Visual explanation

Draw three energy levels with an upward absorption arrow from the bottom to top. Draw a two-step downward cascade producing two photons whose energies add to the original energy gap.

Real-world analogy

A ball lifted to a higher shelf has gained potential energy and can later descend in one drop or several steps. Atomic transitions are quantized and involve photons, unlike the continuous classical shelf motion.

Real-world example

An electric discharge excites atoms in a gas lamp. Their subsequent transitions produce spectral lines characteristic of the gas and the populated atomic states under operating conditions.

Why?

Why can one excited atom emit several photons on its return to ground state? It may pass through intermediate allowed states, releasing a photon at each downward energy step.

Common misconception

“Excitation always changes the element into an ion.” Bound-state excitation keeps the electron count; ionization specifically removes an electron and changes net charge.

Worked example

Suppose an atom has allowed energies 0, 2.0 and 5.0 eV measured above ground. Exciting it from 0 to 5.0 eV requires 5.0 eV. It can return directly and emit a 5.0 eV photon, or pass through 2.0 eV and emit 3.0 eV followed by 2.0 eV. Both routes conserve the 5.0 eV energy difference, if the transitions are allowed.

Quick check

1. Does a bound excited state necessarily contain more electrons than the ground state? Answer: No. It usually has the same electron count with a higher-energy arrangement.

Exam focus

Separate excitation from ionization. Use positive photon energy from level differences and mark transition direction independently.

Advanced insight

Excited-state lifetimes depend on transition probabilities. A state with a weak radiative transition may be relatively long-lived, allowing collision or other pathways to influence observed spectra.

Summary

Excitation raises an atom to an allowed higher-energy state without necessarily changing electron count. Downward transitions can emit one or more photons whose energies match allowed state gaps.

Practice questions

1. What is the difference between excitation and ionization? Answer: Excitation can keep an electron bound; ionization removes it and changes electron count. 2. What photon energy accompanies a downward change from 5.0 to 2.0 eV? Answer: 3.0 eV, the positive difference. 3. Must every calculated gap appear as a strong spectral line? Answer: No. Transition rules and initial-state population also affect observation.