Experiment 5 - Emission Spectra & Electronic Structure of Atoms

Emission

The process in which an excited atom or electron releases energy as light when it falls from a higher energy level to a lower one.
→ Produces bright spectral lines.

Absorption

The process in which an atom or electron takes in energy (light) and moves from a lower energy level to a higher one.
→ Produces dark lines in a continuous spectrum.

Wavelength (λ)

The distance between two consecutive wave peaks (or troughs) of light.

• Usually measured in nanometers (nm)

• Shorter wavelength = higher energy

Wavenumber (ṽ)

The number of waves per unit distance, equal to the reciprocal of wavelength.

• Units: cm⁻¹

• Higher wavenumber = higher energy

When to use the Rydberg equation

Use it ONLY when:

• You’re dealing with hydrogen or hydrogen-like atoms (H, He⁺, Li²⁺)

• The question involves emission or absorption lines

• An electron is moving between energy levels (n values)

🚫 Don’t use it for multi-electron atoms (unless stated hydrogen-like).

atomic emission spectra

When electrons drop to lower energy levels, atoms emit light at specific wavelengths

determine a value for the Rydberg constant

Measure the wavelengths of hydrogen emission lines, then use the Rydberg equation to calculate RRR.

Wavelength → Wavenumber

1. Convert nm → cm

1 nm=1×10−7 cm

2. Take the reciprocal

Example:
500 nm →
500×10^−7=5.0×10^−5cm

Wavenumber → Wavelength

1. Take the reciprocal

2. Convert cm → nm

energy level diagram drops reading

Matching to spectra

• Each arrow corresponds to one spectral line

• Different arrows = different wavelengths

• Multiple arrows ending at same level = a series (e.g., Balmer ends at n = 2)

Incandescent (tungsten) spectrum

• Produced by a hot solid metal filament

• Gives a continuous spectrum (smooth rainbow)

• Light comes from thermal energy

• No distinct lines

Memory: Hot solid → continuous

Fluorescent lamp spectrum

• Produced by excited gas atoms (mainly mercury)

• Gives a line spectrum (bright colored lines)

• Each line = specific electron transition

• Often appears with a weak background

Memory: Excited gas → lines

Emission spectrum

• Bright lines on a dark background

• Occurs when atoms emit light as electrons fall to lower levels

• Characteristic of each element

Memory: Energy out → bright lines

Absorption spectrum

• Dark lines on a continuous background

• Occurs when atoms absorb specific wavelengths

• Same wavelengths as emission lines for that element

Memory: Energy in → missing lines