The Bohr Model

Bohr model energy level of electrons cartoon

In 1913 Niels Bohr proposed a model to describe how energy levels are organized around an atom. While his model is now known to be incorrect, it is useful for understanding energy levels as Bohr’s model closely resembles planets orbiting the sun (a visual that we are familiar with).

According to the theory, electrons orbit the nucleus in rings (what we refer to as shells or energy levels).

  • The innermost ring electrons (lowest energy) are bound the most tightly as they are very close to the nucleus and don’t have any electrons shielding them.
  • Rings further from the nucleus are less tightly bound because they are both further away from the nucleus and also have other electrons in between them, shielding them from the pull of the positive nucleus.
  • The further away from the nucleus, the higher the energy level.

Energy levels

It may seem contradictory that the higher the energy level, the less energy that’s required to pull the electron from the atom. However, higher energy should be thought of more like less stability or higher volatility, because the outermost electrons are what contribute to the atom’s reactivity.

If an electron were to absorb energy, would it go down in energy level, stay the same, or go up? Does this sound like something we’ve learned before?

The answer is the electron would move up in energy level when absorbing energy and that this is awfully similar to ionization energy. Recall that ionization energy is the amount of energy a valence electron would have to absorb to be ejected from the atom. What we’re talking about now is like ionization energy but from a lower shell to a higher one.

When an electron absorbs enough light energy, it travels up an energy level. However, it may quickly become unstable and start to lose energy. When this happens, the electron moves back down an energy level. This is called decay and the electron emits a photon of light.

In this theory, energy levels are given as n = 1, n = 2, n = 3 and n = 4.

  • If an electron moves from n=1 to n=3, the amount of energy absorbed is equal to the difference in energy between level 3 and level 1.
  • If it goes from level 4 to 3, then it loses the amount of energy equivalent to difference between level 4 and level 3.

Don’t confuse energy levels with orbitals. Energy level describes the shell or ring and covers an entire row on the periodic table. Orbital describes space where electron pairs are most likely to be found. One energy level can cover multiple orbitals.

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