TL;DR: Atoms form molecules by donating or sharing electrons, which binds them together like magnets. This creates a prodigious variety of new substances out of which we are made.
If you you are an eccentric billionaire nerd like Bill Gates, you might have one of these in your office.
It’s a giant periodic table, containing samples of every type of atom in the universe. There’s helium, gold, carbon, and uranium, for a total of 118 of them. All 118 have unique characteristics, but on their own they’re nowhere near enough to create the stunning complexity of the world and its life around us.
For the world as we know it to exist, we have to thank the ability of atoms to bond to each other, which creates new building blocks called ‘molecules’. The number of different combinations of these 118 atoms is nearly boundless, in the same way that the number of possible combinations of words in the English language is nearly boundless.
How atoms bond to form molecules is a trick of chemistry, and it’s one of the most important phenomena in the universe.
Atomic building blocks
A diagram of a simple atom. Svdmolen/Jeanot, Public Domain
Just like how magnets have positive and negative poles, atoms have internal parts with a positive and negative ‘charge’.
Atoms are a mix of three components: protons which are positive, electrons which are negative, and neutrons which have no charge. A typical atom has an equal number of protons and electrons, which cancel each other out, giving the entire atom a neutral charge.
But the components aren’t all in the same place. Protons and neutrons stick together in the centre of the atom, while electrons buzz around the outside in multiple levels of ‘clouds’.
The diagram of a monster-sized atom called hassium. Electrons buzz around the outside in levels. Note that this is just a diagram, atoms actually look like beautiful mathematical patterns. Image source: Technology UK
The other thing to know is that each level of electrons has a certain capacity it can hold before it begins to fill up the next level. For example the first level can hold two electrons, and the second level can hold eight. This concept is why the periodic table on Bill’s wall is arranged the way it is, at least in part. The first row has two entries, the second row has eight, etc.
But here’s the kicker. Outermost levels of electrons… are usually unstable. Atoms want to either fill the levels up to capacity or to get rid of them. They feel this need so strongly that they will donate or accept another atom’s electrons (called an ‘ionic bond’), or share them (a ‘covalent bond’) until they’re satisfied.
This exchange of electrons and the bond it creates causes the charged attraction between atoms that form molecules. Chemistry is the art of building molecules with these tiny magnets. There are two types of bonds in a chemist’s toolkit.
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Forming ionic molecules
Sodium (Na) giving an electron to Fluorine (F), which creates a charged attraction. This forms sodium fluoride.
Atoms form ionic bonds when they donate electrons to, and accept electrons from other atoms.
Let’s look at the above example. A sodium atom has one electron to lose to drop its outer level, and fluorine has one electron to gain to complete it. If the two atoms come into contact under the right conditions, they will exchange that single electron, and both atoms will be happy.
But the exchange comes with a consequence. With the number of electrons changed, each atom no longer has a neutral charge.
Fluorine’s electrons now outnumber its protons, giving the atom a slight overall negative charge. Sodium now has more protons than electrons, giving it a positive charge. The changed atoms are now called ‘ions’, and the opposite charges attract each other. They stick together in an ‘ionic bond’. A molecule of sodium fluoride is formed, which is the blue stuff that dentists put on your teeth.
Atoms joined with ionic bonds tend to join together in a lattice structure like crystals. When you put them in water they tend to dissolve, as the special attraction of water molecules is often more attractive to the ions than each other.
Ions tend to self-organise together to form a giant lattice. Image source: Get Revising UK
Forming covalent molecules
Two oxygen atoms sharing electrons in a covalent bond. Source: Nathaniel S at ThingLink
Covalent bonds come from sharing electrons rather than giving them. Sharing also completes outer electron levels, but it locks the atoms into a very close relationship.
In the above example, an oxygen atom has six electrons in its outer electron levels and want to gain two more to complete it. What it can do is share two electrons with another atom, including another oxygen atom. Atoms joined with covalent bonds tend to form independent molecules, rather than crystal lattices. They can get extremely long and complex.
Diagrams like this one show the covalent bonds between atoms, where one line shows one shares electron, two lines are two shared electrons, etc.
How atoms and molecules shape our world
Every one of the atoms on Bill Gate’s wall has different attributes that come from the number of electrons, protons, and neutrons they have. But when they start combining, we end up with an immense variety of molecules, each one with its unique behaviour that comes from their atoms and how they relate to each other.
You can then mix molecules to form a mind-blowing number of mixtures, from concrete to chlorinated water. Each mixture has its own attributes that come from the interplay of its molecules, even if they’re not bonded together.
Through billions of years of trial and error, life on Earth discovered that certain molecules and mixtures could form brains, nervous systems, hearts, eyes, and DNA when combined in just the right way.
Likewise, our technology has allowed us to create mixtures of metals, called alloys, that can withstand extraordinary temperatures, ceramics that can withstand spaceflight, and silicon chips that can perform computations.
But even with the prodigious variety of life and technology on Earth, we know that we have only begun to scratch the surface of what atoms and molecules can do.