TL;DR: Atoms form molecules by donating or sharing electrons, which binds them together in a similar way to opposite poles of a magnet. This creates a prodigious variety of new substances out of which we are made.
If you are an eccentric billionaire 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. It includes helium, carbon, and uranium, for a total of 118 types. All 118 atoms 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 combinations of these 118 atoms is nearly endless, in the same way that the number of possible combinations of 26 letters to form words in the English language is nearly endless.
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
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 layers of ‘clouds’.
The other thing to know is that each layer of electrons has a certain capacity it can hold before it begins to fill up the next layer. For example the first layer can hold two electrons, and the second layer 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, and the second row has eight.
But here’s the kicker. Outermost layers of electrons… are usually unstable. Despite the number of protons they have, atoms want to either fill the layers 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 (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 magnet-like bonds. Let’s take a quick look at the two types.
Forming ionic molecules
Atoms form ionic bonds when they donate electrons to, and accept electrons from other atoms.
In the GIF above, a sodium (Na) atom has only one electron in its outer layer, and fluorine (F) needs one electron to complete it’s own. If the two atoms come into contact under the right conditions, they will exchange that 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. For sodium it’s the reverse, with more protons than electrons, giving it a positive charge.
The changed atoms are now called ‘ions’, and the oppositely charged atoms attract each other, and 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 that are ionically bond tend to join together in a lattice structure like crystals. When you put them in water they tend to dissolve, as water molecules are often more attractive to the ions than each other.
Forming covalent molecules
Covalent bonds come from sharing electrons rather than donating them. Sharing also completes outer electron layers, but locks the atoms into an even closer relationship.
In the above example, an oxygen atom has six electrons in its outer electron layer and wants to gain two more to complete it. It can share two electrons with another atom (in this case, another oxygen atom). Atoms joined with covalent bonds tend to form independent molecules, rather than crystal lattices. They can get extremely long and complex.
Putting them together
Every one of the 118 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 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. The discipline of the design and discovery of these new materials is called materials science. 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.