When I described the early universe, I discussed the cosmic microwave background, the oldest radiation in the cosmos, and how fluctuations exist in this radiation. Cosmologists think this variation dates to the very early universe, when it was so small its wave function was larger than it was. Highs and lows in this quantum wave function were amplified as the early universe rapidly expanded, resulting in variations in the distribution of matter. In time, these differences in the density of matter resulted in galaxies containing billions of stars and areas of star-free space between them. The large-scale structure of our vast universe is thought to have resulted from its quantum behaviour while it was still extraordinarily small.
The Two-Slit Experiment
The quantum behaviour of atoms is deeply peculiar, but I have not yet explained how it helps us understand the way in which atoms interact. Thinking back to my chemistry lessons at school, I was unable to find the answer to this question. We weren’t taught it. In fact, I realized only three things remain with me from these lessons. First, I recall a kid called Gary managing to fill the classroom with an acrid-smelling gas that made everyone cough and led to the whole science block being evacuated. Gary spent the rest of his school chemistry career attempting to repeat the feat, but failed. Second, I remember telling my friend Mike that if he attached the Bunsen burner to the water tap it would shoot a powerful jet of water that would probably go as far as the teacher about fifteen feet away. Not believing me, he attached the Bunsen to the tap, but he didn’t have the nerve to try to drench the teacher. Instead, he just turned the water tap on with the burner sitting on the bench. The water hit the ceiling and we got dripped on for the rest of the lesson. Third, we were taught that atoms are a bit like the solar system, with the nucleus at the centre, being orbited by particles called electrons that behaved like planets. There would have been better things for me to remember than the first two, and the third ‘fact’ we were taught is largely wrong.
One aspect of the solar system analogy that is correct is that most of an atom is empty space, but that’s about as far as it goes. The simplest atom is that of hydrogen, consisting of one proton and one electron. If we were to expand the proton to be the size of the sun, the electron’s most likely closest position would be ten times further away than Pluto is from the sun. That’s a very long way. Mercury, the closest planet to the sun, would be closer to the nucleus than where we would expect to find any electron if the solar system were to be shrunk down to the size of an atom.
Talking in terms of probability by saying things like ‘the electron’s most likely closest position’ sounds odd, particularly when we think of planets. It would be much easier to write that the electron is ten times further away than Pluto is from the sun, rather than that is its most likely position. Unfortunately, though, in the same way that we do not know where an atom is when it leaves the gun and before it hits the screen in the two-slit experiment, we do not know where the electron is with respect to its atomic nucleus. We just know where it is more or less likely to be, and this means we have to talk in terms of chance. Given this, we cannot describe an electron as being in orbit around an atom’s nucleus. Its wave function is smeared around the nucleus and the electron can appear anywhere within this wave function if coerced to show itself. To describe this complexity, chemists use the word ‘orbital’ rather than ‘orbit’. Electrons form orbitals around their nuclei, and there is a probability that they are at any point in their orbital. If planets formed orbitals rather than orbits, they would appear as a mist smeared across the night sky. Some bits of the mist would be thicker than others, with the thick bits denoting where the planet would most likely be. Probability adds complexity, but we do all sometimes think in terms of probability in our everyday lives.
Mine is not a family that tracks each other on our mobile phones, so as I write this in my office I cannot tell you where my wife, Sonya, is, nor the whereabouts of my three children Sophie, Georgia and Luke. I can put odds on Sonya being in her office at work, out shopping or having a coffee with friends, but to spare my marriage I won’t list those chances. Luke will likely be at college, but if the weather is nice and he doesn’t have a lecture he could either be at the skatepark or out taking pictures for his photography project. Electrons live less exciting lives than members of my family, but chemists can assign them odds of being at any location around their nucleus much in the same way I can assign where my family members are likely to be.
A typical atom of hydrogen consists of one proton that forms the nucleus, and one electron. The wave function that describes where the electron could be can place it anywhere around the nucleus, but says it is much more likely to be between a range of intermediate distances, rather than being close in or further out. The electron’s energy level determines its wave function, its orbital around the nucleus, and consequently the distance the electron is likely to be from the proton. Electrons with more energy are found in more distant orbitals than those with less energy.
Image of a Hydrogen Atom
Electrons are attracted to the nucleus, and they like to be in the closest possible orbital to it. To do this, they need to have as little energy as possible. An atom of hydrogen consisting of a proton and a highly energetic electron in a distant orbital loses energy, and the electron disappears from the distant orbital and appears in a closer-in one. The energy the electron produces is carried away from the atom by a photon. As this happens, the electron vanishes from the outer orbital and appears in a more inner one.
Electrons have a property called spin, and this creates patterns in how multiple electrons arrange themselves in atoms of elements with more than one electron. Spin is all about the way an electron moves, but its definition is mathematically quite complicated. There are two types of spin – spin up and spin down – and a good analogy is to think of a spin up electron as spinning clockwise and a spin down electron as spinning anticlockwise. But like ‘charm’ and ‘colour’ in quantum theory, ‘spin’ doesn’t have its ordinary English meaning. Electrons don’t really spin like a cricket ball.
Each orbital can contain a maximum of two electrons, but two electrons with the same spin cannot share an orbital. Orbitals are arranged around atoms in structures called shells. The shell closest to the nucleus contains one orbital, the second contains four orbitals, the third nine, the fourth sixteen, the fifth twenty-five and the sixth thirty-six. In terms of numbers of electrons, shells one through six can respectively hold a maximum of two, eight, eighteen, thirty-two, fifty and seventy-two electrons.
Hydrogen has one electron in its first shell, while helium, the second-lightest element, which consists of a nucleus of two protons and two neutrons, has two electrons in its first shell – one spin up electron, and one spin down. Lithium, the next-heaviest element, has a nucleus of three protons and four neutrons, and consequently has three electrons, because each atom has the same number of protons and electrons. It contains two electrons in its first shell, and one in its second. Oganesson has two electrons in its first shell, eight in its second, eighteen in its third, thirty-two in its fourth, thirty-two in its fifth, eighteen in its sixth, and eight in its seventh. At least, that is what chemists predict. No atom of oganesson has existed for sufficiently long for its electron configuration to be accurately determined.