The ancient Greeks were not nearly as stupid as they looked, despite draping themselves in bed sheets for every occasion. It was a philosopher called Leucippus in the 5th century BC who proposed the idea of the atom, named for atomos which means indivisible. He said that everything is made of either tiny atoms or voids in between them. Full or empty. Solid or not solid.
Today, we know the atom is largely comprised of empty space, and houses even smaller particles, some of which are completely theoretical. So the name is a misnomer: atoms are indeed divisible, and what quantum quirks lurk within are especially unsettling.
What Do Atoms Look Like?
Tricky one, this. We use representative models to convey the structure of atoms, because trying to actually draw them is like riding a bicycle with the handlebars on backwards while counting in thirty-sevens. Neils Bohr devised the model you learned in school, which we’ll look at first, in the form of the element helium.
In the centre of the atom is the nucleus, home to protons (positively charged particles) and neutrons (particles with no electrical charge). Orbiting the nucleus in various energy shells are electrons (negatively charged particles).
These three features tell us a lot about the species of atom we’re looking at (for example, whether it’s carbon or hydrogen) and its natural behaviours (for example, how it bonds with other atoms).
Now, look at this table:
I’ve listed the first seven elements to illustrate a point. Why seven? Well, it’s all I happened to fit on the canvas. Also, it’s said that geniuses favour the number seven. I can’t remember who said that. It might have been me.
The point I was making is that we distinguish elements from one another by their number of protons. Imagine a staircase, where each step represents the increasing number of protons in the various elements. Let’s climb.
On the first step, the lightest atom is hydrogen with one proton. Hydrogen is unique in that it has zero neutrons, so its has an atomic mass of just one. It has one electron too, but let’s not worry about electrons at this point.
On the second step, we greet helium. It has two protons and two neutrons, which add up to an atomic mass of four.
In nature, the staircase has 92 steps, finishing around uranium. However, physicists have figured out how to make new elements, thereby adding an extra 26 steps to the staircase, for a total of 118 elements. Interestingly, all man-made elements are radioactive and many are named for historical scientists, such as the rather awkward-sounding rutherfordium and einsteinium.
Now consider the electrons in their orbits, which play a crucial role in determining how atoms react to each another. As you’ll recall, the protons in the atomic nucleus create a positive electrical charge. Similarly, the negative charge of electrons in the orbits balance them out. Although electrons are relatively minuscule, each one still holds an equal and opposite charge to one proton, making the entire atom electrically neutral.
Atom Fiddling: Changing The Number of Neutrons and Electrons
New terminology sucks. It doesn’t stick until you understand the topic, yet you can’t really understand the topic until you know the terminology. It’s about as frustrating as trying to fold a fitted bed sheet, or at least wearing one as a tunic – just ask Leucippus.
Anyway, it’s necessary, so here are some new words for your chemical vocab:
- Covalent bonds. These are links which form when two atoms buddy-up and share electrons in their outer orbit, thereby forming a molecule.
The inner orbit of an atom is full with two electrons. Each orbit thereafter is full with eight electrons. So an atom of oxygen – with eight protons, eight neutrons, and eight electrons – has a problem. Although it’s electrically neutral, it has only six of eight spaces filled in its outer electron orbit.
This sucks for oxygen. It dreams of filling its outer shell. The easiest way do this is to share electrons with other atoms, and that is exactly what it does. For example, oxygen loves to bond with hydrogen, which also seeks to complete its outer shell.
When an oxygen atom shares electrons with two hydrogen atoms, we have a water molecule. And that’s great. One of reality’s best ideas so far. Water makes up 90% of your body. It’s the medium for all the chemical reactions in your cells. It makes up plant sap (which is a big deal because plants are the original source of food for all animals on the planet). Water anyone? Yes please.
Ok, here’s the next bit of terminology.
- Ions. When an atom gains or loses an electron, it’s known as an ion. Its overall electrical charge has changed.
Ions are formed as a result of ionic bonds, such as the one that makes table salt. The picture below may help. Sodium has just one electron in its outer orbit, so rather than try to make seven covalent bonds for a full house, it’s easier just to give one single electron away completely, and drop a shell.
Chlorine is in the opposite position, where it needs just one more electron to feel whole. It could form a covalent bond but if sodium’s lurking around, it’s quick to take advantage and go ionic.
What’s important to realise is these atoms aren’t bonded by shared electrons like the covalent bonds in water. Instead, they are pulled together by electrical attractions between the now-oppositely charged atoms (ions) of sodium (Na+) and chlorine (Cl-).
Here’s one last bit of atomic trivia, then we’re going quantum.
- Isotopes. If an atom gains or loses a neutron, it is known as an isotope. Its overall mass has changed because the weightiness of an atom is the sum of the heavy protons and neutrons at its heart.
In nature, this happens when cosmic rays smack into carbon atoms in the atmosphere. Carbon, which usually has six neutrons in its centre, now has eight neutrons and becomes a carbon-14 isotope. The 14 is representative of its abnormal atomic weight.
So we’ve fiddled with the electrons and neutrons in atoms and seen how that creates ions and isotopes. But what happens if you add or remove a proton from an atom? In this case, you get the creation of a completely different element. My fellow countryman, Ernest Rutherford, was the first to do this in 1919 in the lab. He bombarded nitrogen atoms with energetic particles, which collided with the atomic nucleus to produce oxygen and hydrogen nuclei.
The Bohr model of atoms suggests electrons orbit the nucleus of an atom in the same way the moon orbits the Earth. Not only is this over-simplistic, it’s also completely wrong. Sorry about that.
Quantum physics tells us the electron orbit (better described as 3-dimensional energy shells, not merely 2-dimension orbits as shown above) is a mathematical function. It describes the probability of the electrons being present at a particular location at any particular moment.
To visualise this, the scientist-slash-deviant-artist DarkSilverflame created a program to plot the volumes in which electrons can be found in a hydrogen atom. The results are beautiful. In the usual Science Me tradition, I’ve cartoonised my favourite, where coloured areas depict the probability of an electron being found at that location.