Flame Tests & Atomic Spectra

What you will investigate

This virtual flame test lab joins the color of a flame with the lines seen through a spectroscope. Compare metal salts, inspect emission spectra, and match an unknown sample to reference features. Changing the anion while keeping sodium as the metal component provides a controlled comparison: the investigation distinguishes the metal's characteristic light from the identity of the complete salt. The spectroscope then supplies more detailed evidence than a single color name.

The course also includes discharge-tube spectra, absorption, and a hydrogen energy-level model. These investigations share the relationship between light and atomic energy, but they are not interchangeable. A flame's overall appearance can combine several features, and some metal-containing flames include molecular emission. A hydrogen energy-level calculation should therefore not be used as a complete model for every flame color.

Reading spectral evidence

Read separated bright features against the spectroscope's wavelength scale and compare sets of lines. One matching feature is useful evidence, but a set provides a stronger identification than color alone. Mixtures combine features from their components. The course distinguishes continuous feature centers from integer-nanometer records, so the displayed number can be a rounded representation of a merged spectral feature.

Reference feature	Continuous model center	Ideal integer record
Sodium feature	589.1940000 nm	589 nm
Lithium red feature	670.7810000 nm	671 nm
Hydrogen red line	656.279 nm	656 nm
Hydrogen blue-green line	486.135 nm	486 nm

These are the course validation values, not new laboratory measurements. Record wavelength as well as appearance. A small reading difference should be interpreted with the scale's resolution and the selected mode in mind. In Real mode, the scale can shift and estimated readings vary; repeated readings help distinguish a consistent offset from random variation.

From wavelength to photon energy

Photon energy follows E = hν = hc/λ. Convert nanometers to meters before using SI constants to calculate energy in joules. Shorter wavelength corresponds to greater photon energy. Using the course's recorded 656 nm hydrogen line gives a displayed energy of 3.03 × 10⁻¹⁹ J, or 1.89 eV. The recorded 486 nm line gives 4.09 × 10⁻¹⁹ J, or 2.55 eV. These examples use rounded measured wavelengths, so they need not match every digit of an energy computed from an unrounded line center.

The hydrogen model uses Eₙ = −13.6/n² eV. Its first four levels are −13.6, −3.4, approximately −1.5111, and −0.85 eV. Visible hydrogen transitions end at n = 2 in this investigation. Use an energy-level difference to predict emitted photon energy, then compare the corresponding wavelength with the spectroscope evidence. A larger drop releases a more energetic photon; it does not give a longer wavelength.

Emission, absorption, and common mistakes

Emission occurs when an excited atom makes a transition to a lower energy level and releases a photon. Absorption requires a photon energy that matches an available transition from an occupied starting level. That starting condition matters: cold hydrogen in its lowest state does not automatically absorb every visible Balmer wavelength merely because those wavelengths occur in a hydrogen emission spectrum.

Do not describe a line spectrum as a continuous rainbow. Do not identify every salt only by flame color, or treat a bright flame as proof of a greater single-photon energy. The grating separates light by wavelength, while photon and energy-level reasoning explains why particular wavelengths can be emitted or absorbed. Those two descriptions answer different parts of the same observation.

Standards, model boundaries, and classroom use

Course topics connect to AP Chemistry light and electronic transitions, AP Physics 2 atomic spectra, and selected NGSS and New York topics. The quantitative hydrogen model is identified as AP material; the NGSS alignment does not imply that every quantum calculation is part of the NGSS assessment boundary. The Bohr picture is a simplified single-electron teaching model, and the course does not present it as a complete account of multi-electron atoms.

A discussion can ask students to identify an unknown using a set of recorded lines, then explain one wavelength using energy reasoning. Compare Ideal and Real records without replacing student observations with reference answers. This virtual investigation avoids handling flame-test chemicals but does not certify practical laboratory technique. Continue with electrochemical cells or ripple tank waves for another investigation involving energy or wavelength.