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Quantum Physics and Bohr

We’ve had lots of progress since Dalton’s sphere-atoms. We found out that electrons exist, then found out about the nucleus. We saw different masses for atoms of the same elements, called isotopes. Then we figured out that protons exist too. And finally we added neutrons in, which completed the picture:

But, as we’ve already seen, our model still doesn’t explain a couple of issues. So we have no choice but go with a new, different model- the one proposed by Mr. Niels Bohr of Denmark.

Here’s what we need to fix up

The Rutherford model hypothesized that electrons move around the nucleus in an orbit due to gravity, kind of like planets revolving around the sun. The big problem with this was that if this was true, the electrons would be emitting electromagnetic radiation, meaning their charge would decrease and they would eventually collapse into the nucleus, which can’t be a good thing for anyone.

The second thing is that if electrons emit radiation while orbiting the nucleus, the charge and thus the radiation would decrease continuously. So if, for example, we would look at an emissions spectrum, it should have a continuous frequency and show up as a cloud or smear of colourful light.

But evidence showed that luckily, the world doesn’t work that way. Atoms aren’t collapsing all around us (clearly, otherwise… well, it’s hard to imagine life with collapsing atoms, but just picture a mash-up playdough universe that’s constantly changing in an icky way). The emission spectrum also work differently, as we’ll discuss later.

Traveling back- origins of quanta

Let’s discuss Max Planck for a bit. In the late 1800s, Planck was studying electromagnetic waves and light emissions. He found out that emission depended on frequency rather than amplitude of waves, and after complex thinking he came to an interesting idea- energy is not continuous. It’s discrete.

This was a new concept, and it became known as quanta, since it suggested energy travels at “quantum”, or packets. This was only a mathematical concept at the time though.

In 1905, a young Albert Einstein (yeah, same Einstein we all know) applied Planck’s mathematical concept to real life. He explained the Photoelectric Effect (discovered by Heinrich Hertz), which basically had to do with using electromagnetic radiation on metals and causing electrons to be emitted. The electron emission depended on frequency rather than intensity of light, which meant light doesn’t move in waves but in “photons”, which are discrete quantities of energy. Cool!

So, quanta + atom = ?

Niels Bohr did this math. In 1913, the incorporation of quantum physics into the atomic model began. Bohr’s idea was this- electrons don’t just orbit the nucleus like whatever. They surround the nucleus in specific distances, and this distance is proportional to their energy. Each electron is associated with a certain amount (quanta) of energy.

This meant that, since electrons have discrete quanta of energy, they couldn’t lose energy continuously and just collapse into the nucleus. Electrons can only make “quanta jumps”, in which they basically jump from their orbit to a higher or lower one, releasing or absorbing a specified, discrete, amount of energy in the form of a photon. This explains why the spectra looks like this:

Bohr’s model was successful in explaining the emission spectra phenomenon of hydrogen- when the electrons of hydrogen are collided with electromagnetic waves, the photon’s energy from the electromagnetic waves transfer the exact quanta of energy to the electron which causes it to jump a level. When the electron jumps back down to normal level, a photon of a specific frequency associated with that level of energy is released, resulting in a discrete spectral line rather than a continuous blur.

A few final developments

In 1924, Louis de Broglie proposed that particles exhibit wave-like motion. 2 years later Erwin Schrödinger applied this to the atom’s electrons and came up with an equation that describes an electron as a wave function rather than a tiny orbiting dot. This helped clean up the spectral phenomena that Bohr’s model didn’t fully explain, specifically anything above the Hydrogen spectral lines.

Later though, came a guy named Werner Heisenberg, and declared an Uncertainty Principle, stating that you can never know both the position and the momentum of a particle (electron) at the same time, since measuring one must affect the other. This meant that Bohr’s model was a bit off, since electrons don’t actually occupy orbits at all, but they can be symbolized using the probability of their existence in a certain place- an atomic orbital.

This basically means that with all of our attempts to find a small but nice explanation to what makes up the universe, we discovered that even the tiniest things can be extremely complex. Things are never what they seem, even if they’re too small for us to see, and atoms are cool.

We’ve come a long long way from Democritus and Kanada and their wacky theoretical ideas based on pretty much hunches. We still don’t know everything on what atoms look like, how electrons really move, and other stuff like that, but maybe one day someone will out. And hey, that someone just might be you!

The End (for now)