Bohr Model & Atomic Spectra

So the important things about the Bohr model of the atom are:

  1. Electrons orbit the nucleus in discrete orbits called energy levels

  2. Electrons can move up energy levels if they absorb the EXACT right amount of energy to do so.  Too much or too little energy and the electron won't move.

  3. Electrons can move back down to a lower energy if they emit the EXACT right amount of energy.  

  4. These discrete amounts of energy are given off as photons or light.  These photons are quantized.  They can only have specific energies.  Thus leading to the discrete lines in the spectrum.

Emission spectrum of hydrogen

The more certain we are of an electron's position, the less certain we are of it's energy and vice versa. This is known as the "Heisenberg Uncertainty Principle"

An Energy Level is a specific amount of energy which an electron in an atom can possess.  

The idea of energy levels comes from the Bohr Model. n refers to the energy-level number. The greater the value of n, the farther the electron is from the nucleus and the greater it's energy.  In the diagram below, an energy level of n=5 is greater in energy and further from the nucleus than n=1. 

If an electron gained the exact amount of energy, it could "jump up" or "be excited" to a higher energy level. It would then return to it's original energy level and release that same amount of energy in the form of light (photons). This idea that electrons exist at specific energy levels and can transition only between levels was very important. It meant that the energy of electrons is "quantized", meaning that energy exists in discrete packets known as photons.

Hydrogen atom and energy levels in a linear fashion

Electrons fall down to lower rungs, emitting light at the specific energy of the difference between the rungs. This allows different atoms to emit different colors of light. Sodium’s spectrum does not look like nitrogen’s spectrum — nor like the spectrum of any other element.

All elements absorb and emit specific wavelengths of light that correspond to those energy levels.

  • An absorption spectrum is a spectrum of light transmitted through a substance, showing dark lines or bands where light has been absorbed by atoms, causing a dip in the spectrum.

  • An emission spectrum is made by electrons falling down the energy ladder. It’s what you get when you look at hot gas, which is heated by something out of the line of sight.


comparison of continuous, emission and absorption line spectrum

Atomic Emission Spectrum: A series of coloured lines that correspond to photons of specific energies emitted by a heated element.

In order for an electron to emit a photon, it must be relaxed to a lower energy state.

Around 1857 the German chemist Robert Bunsen invented a special gas burner that produces a clean, colourless flame. 

  •  Certain chemicals are easy to identify by the distinctive colours emitted when bits of the chemical are sprinkled into the flame of the Bunsen burner -- the flame test.

Kirchhoff sometimes saw dark spectral lines, called absorption lines, among the colours of the rainbow.  In other experiments, he saw bright spectral lines, called emission lines, against an otherwise dark background.  By the early 1860s, Kirchhoff had discovered the conditions under which these different types of spectra are observed.  These are now known as Kirchhoff's laws of spectroscopy.

  • A Continuous Spectrum

    Kirchhoff's First Law states that a hot solid, liquid, or dense gas produces a continuous spectrum. A continuous spectrum is a complete arrangement of colors, like that of the rainbow, devoid of spectral lines.

  • An Emission Line Spectrum

    Kirchhoff's Second Law states that a thin, hot gas, produces an emission line spectrum. An emission line spectrum is a spectrum with bright spectral lines on a dark background.

An excited gas will emit photons from its excited atoms. These photons come out as bright lines of a specific wavelength unique to the atom that's producing it, on an emission line spectrum.


Spectroscopy is the study of the spectra produced when material interacts with or emits light.

image of a spectroscope and how it works

“In the early days of spectroscopy, experiments revealed that there were three main types of spectra. The differences in these spectra and a description of how to create them were summarized in Kirchhoff’s three laws of spectroscopy:

  1. A luminous solid, liquid, or dense gas emits light of all wavelengths.

  2. A low density, hot gas seen against a cooler background emits a BRIGHT LINE or EMISSION LINE spectrum.

  3. A low density, cool gas in front of a hotter source of a continuous spectrum creates a DARK LINE or ABSORPTION LINE spectrum.” - source thing