Olivia: Have you ever looked carefully at the intro for this show? I mean, really carefully. If you have, you might have noticed that there’s a diagram of an atom, with little electrons orbiting the nucleus. But here’s the thing: atoms don’t actually look like that.

Over the years, scientists have come up with different atomic models based on what we know about how they work. The atomic model that’s in the SciShow intro was one of them, and it has a lot of history behind it, but the most accurate atomic models are a little more complicated. Because atoms are complicated.

By the start of the 20th century, scientists knew that atoms were made up of negatively-charged electrons, plus some sort of positive charge. The tricky part was figuring out how these charges fit together. The running theory was that the electrons were embedded in a positive sphere, which was called the Plum Pudding Model because it looked like a traditional Christmas pudding.

But that all changed around 1911, when a scientist named Ernest Rutherford, along with his team at Manchester University, published the results of the famous gold-foil experiment. Rutherford and his colleagues fired alpha particles, which are positively charged, at thin gold foil. According to the plum pudding model, the alpha particles should’ve just passed straight through the foil, because atoms would be mostly empty space with some charges scattered around.

And atoms are mostly empty space, so most of the alpha particles did pass straight through the foil. But, to Rutherford’s surprise, some alpha particles were deflected. By a lot. He concluded that an atom’s positive charge was concentrated in a tiny central nucleus, and these nuclei were deflecting alpha particles that bounced off of them.

He also predicted that the electrons were orbiting around the nucleus, kind of like how planets orbit the sun. That’s why his model is sometimes called the Planetary Model.

Rutherford was right about protons being in the middle, with electrons around them, and you’ll still see his model used today to explain the very basics of the atom. It’s the one in the SciShow intro!

But there was one major problem with the Planetary Model: it predicted that orbiting electrons would lose energy in the form of radiation, which would make them spiral inward and eventually crash into the nucleus. This implied that all atoms would eventually collapse. But we know that stable atoms do exist, so there had to be something missing.

Just two years later, in 1913, Danish scientist Niels Bohr proposed an adjustment to the Rutherford model that solved this problem. Bohr’s model predicted that electrons orbit at very specific energy levels, which he called orbits. The electrons could only orbit at precisely those levels, and so they couldn’t spiral inwards. An electron could switch levels, if it absorbed or released some energy -- but only specific, discrete levels were allowed, and electrons couldn’t go below the lowest level. That explained why stable atoms didn’t just collapse.

Bohr’s model quickly became the most popular model of an atom, and it’s often used today to show the basic way that an atom is arranged. But it still wasn’t totally right.

One breakthrough was in 1932 when English physicist James Chadwick discovered that neutrons exist. Neutrons weren’t electrically charged, and they helped explain why the nucleus was so heavy.

Another breakthrough involved quantum mechanics, and the idea that electrons don’t necessarily orbit the nucleus at all. In fact, electrons aren’t even really in a specific place at any given time. Instead, they’re kind of in lots of different places at once within a bigger area. Then, when you actually measure an electron, suddenly it’s in one specific spot within that area. It’s a weird concept that’s very different from the way we normally experience the world, but that’s quantum mechanics for you.

The area where you might find it if you tried to measure it is called the electron cloud. In diagrams, normally the cloud is drawn darker where there’s a high probability of the electron being there when you measure it. With the most basic atoms -- like hydrogen and helium -- this cloud looks kind of like a big sphere. And it turns out that electrons have the highest probability of being in one of Bohr’s orbits, which is why you can use Bohr’s model to simplify things. But when you get to bigger and bigger atoms, with more and more electrons, these clouds begin to interfere with each other, and start to have weirder shapes.

So, the electron cloud model is the most up-to-date model of an atom, and it’s used by scientists around the world. But that doesn’t make the other models useless. Like, Bohr’s model can be helpful if you need to focus on energy levels and radiation. But if you’re studying chemical bonds, you might need the electron cloud model to know where the electrons are. And if you want a model that shows off the fundamentals and still looks pretty cool, you might want to go for the Planetary Model.

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