Atoms 101

Atoms 101: A Guide to The Elements

Who Discovered Atoms?

Ancient Greek philosophers were the original scientists, asking pivotal questions like, "what's it all about, eh?"

Without a whiff of technology at his disposal, Leucippus came to the conclusion that all matter is composed of tiny bits. He named them atomos; Greek for indivisible.

This was a great starter hypothesis, although we now know the name is a total misnomer. Modern science brought us to the more robust conclusion that atoms are indeed divisible. What's more, all kinds of quantum quirks lurk within.

What Do Atoms Look Like?

Although it's only a symbolic representation, Niels Bohr devised a way of conceptualising atoms that's so useful it's still taught in school today. His conceptual model offers three basic components to help us understand how atoms work.

The mass of an atom comes mostly from a cluster of protons and neutrons at the centre. This is orbited by much smaller electrons that balance the overall charge.

Here's an example of how Bohr conceptualised helium (He):

Helium Atom Illustration

  • Protons in the nucleus carry a positive charge (+1)
  • Neutrons in the nucleus carry a neutral charge (0)
  • Electrons in the orbitals carry a negative charge (-1)

The number of protons, neutrons, and electrons of an atom determines its element, which is why we pay due attention to the atomic number (proton count) and the atomic mass (protons plus neutrons).

Atomic Element Diagram

With just two protons, helium is one of the lightest elements in the universe. Contrast this with heavy elements, such as uranium (U) which has 92 protons. We visualise the elements in the iconic periodic table, which also reveals their common properties.

Periodic Table of Elements

The Periodic Table of Elements

The heaviest element in nature is uranium. Physicists have created a further 24 man-made elements in nuclear reactors and particle accelerators. Many are named after famous scientists, such as rutherfordium (Rf) and einsteinium (Es). However, elements with more than 99 protons have no use outside of scientific research, having extremely short half-lives and breaking down rapidly.

Now let's take a closer look at those zappy little electrons.

What Are Electrons?

Electrons are tiny subatomic particles that carry a negative charge. They act as the primary carrier of electricity in solids.

Electrons move around in their energy shells, playing a crucial role in determining how atoms react chemically to one another.

While the innermost energy shell (orbit) can hold up to two electrons, the second shell can hold up to eight. The third shell holds up to 18.

There's a general formula to this: any given shell (n) can hold up to 2(n2) electrons.

Although they're miniscule by comparison, each electron still holds an equal and opposite charge to one proton. So when an atom contains the same number of protons and electrons, it's electrically neutral—or stable.

How Do Atoms Bond?

Bonds allow atoms to join together and form molecules. Bond can occur between atoms of the same or different elements to form molecules.

For instance, two oxygen atoms bonded together produces a molecule of oxygen, or O2, which is very handy for all animal life on Earth. Atoms are social creatures and bond whenever the opportunity arises. Let's look at two types of bonds now.

Covalent Bonds

Covalent bonding is the most common way for atoms to get together. It involves sharing electrons between their outermost shells.

Take oxygen for example. Oxygen has six electrons in its outermost shell, with space for two more. It's like that incomplete feeling you get when only six guests turn up to your dinner table reservation of eight.

Oxygen Atom Illustration

The easiest way to resolve this is to invite two more electrons to the table. But where can they come from?

Hydrogen steps up. Hydrogen has only one electron in its outer shell. Just one, lonely diner who really wants a date. And so a hydrogen atom is always on the lookout for an extra electron.

Now, this is where my dinner table analogy gets weird. Because the solution is to have the waiters push two tables closer together—and have diners hop continuously back and forth between tables so the respective hosts feel good about themselves.

How Water Molecules form with Covalent Bonds

What's really happening here? The electrons zap around between both conjoined shells, sometimes completing oxygen with eight outer electrons, and sometimes completing hydrogen with two.

See how the electrons hang out in pairs. They also maintain the furthest distance away from other pairs as possible, repelled by their charges. This is what gives molecules distinct and predictable shapes.

Covalent bonds create a friendly alliance between many elements in nature. And when I say nature, I mean the entire universe.

Water is one of the universe's best ideas so far. It's great for life on Earth, and makes up 90% of your body, providing an excellent medium for yet more chemical reactions to take place in your cells.

Water also makes up plant sap. This is critical for animals because plants are, ultimately, our original food source. Whether you're a herbivore, an omnivore, or a hard-out carnivore, there's a humble plant at the bottom of the food chain.

Ionic Bonds

Instead of politely sharing electrons, some atoms outright steal electrons altogether. This is fine though, because their willing victims have excess electrons to give away. There's no foul play in physics.

To make table salt, a sodium (Na) atom gives up the one and only electron in its third orbit to an atom of chlorine (Cl), which is so nearly complete with seven. This creates sodium chloride (NaCl).

Sodium Chloride Cartoon

Chlorine can form covalent bonds with other elements in this situation. But if sodium's lurking nearby, it goes for the ionic bond.

Unlike covalent bonds, which is instrumentalised by the sharing of electrons, ionic bonds are retained by the residual charges of entire atoms. That's because sodium is now one electron short of balancing its positively charged protons. The atom becomes an ion with an overall positive charge of +1.

Likewise, chlorine now has an overall negative charge produced by the extra electron, so we call it a chloride ion.

Electrically unbalanced atoms are called ions—hence the term ionic bonds.

But it's all good. The electron shells are full, which is the primary concern here. And the positive sodium ion is attracted to the negative chloride ion, forming a stable molecule. Because there's more than one type of elements involved, the molecule can also be called a compound.

What Are Isotopes?

We've seen that changing the number of protons gives you a different element. And changing the number of electrons occurs by bonding to create molecules. So what happens if you change the number of neutrons?

Your atom becomes an isotope. In the atmosphere, cosmic rays smack into carbon atoms and provide them with extra neutrons. Carbon can go from having six neutrons to seven or even eight neutrons, creating carbon isotopes.

Carbon Isotopes Diagram

The standard carbon-12 atom becomes a carbon-13 isotope when you add one more neutron, or a carbon-14 isotope when you add two neutrons. As a result, different isotopes of the same element have different atomic weights.

Carbon-14 is radioactive, meaning its unstable. This is useful for carbon dating, where we determine the age of a fossil based on the decay of carbon-14 atoms.

So What Do Atoms REALLY Look Like?

Earlier, using the Bohr model, we imagined electrons orbiting the nucleus of an atom in the same way we envision planets orbiting the sun. In reality, not only is this conceptual model over-simplistic, it's also completely wrong. Sorry about that.

A more accurate way to think about it is that electrons pop in and out of existence, somewhere within their designated three-dimensional energy shells.

While electron behaviour can seem pretty random, it can be roughly predicted by mathematical functions of quantum theory. Why roughly? Because nature only allows us to know the probability of an electron being at any particular location at any particular moment.

Probability functions can also be visualised pretty beautifully:

Hydrogen Electron Orbitals

Visualising the probability density of hydrogen electron orbitals

But this is still a conceptual representation based on mathematics. Is it even possible to see an atom and know what actually looks like?

It is. In 2018, David Nadlinger at the University of Oxford took this long-exposure photo of a positively charged strontium ion.

An Actual Photo of an Atom

Oh, you can't see it? Let's zoom in.

Photo of an Atom Zoomed

And that's about as close as we can get. The photo, titled Single Atom in an Ion Trap, was taken with a DSLR camera.

"The idea of being able to see a single atom with the naked eye had struck me as a wonderfully direct and visceral bridge between the miniscule quantum world and our macroscopic reality. A back-of-the-envelope calculation showed the numbers to be on my side, and when I set off to the lab with camera and tripods one quiet Sunday afternoon, I was rewarded with this particular picture of a small, pale blue dot." - David Nadlinger

Nadlinger illuminated the strontium ion with a blue-violet light, and held it motionless with an electric field emanating from the metal electrodes either side. It's the first time we've been able to see a single atom with the naked eye.

Of course, you can see atoms any time you want. The whole world is make of them. What you see, to the best of your optical capacity, is the accumulation of countless atoms making up every single thing in the known universe. Atoms have been staring us in the face all along.

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