Covalent Bond

Picture two friends on a rainy day, each without an umbrella. Instead of walking alone and getting soaked, they stand shoulder to shoulder and hold one umbrella together. Now both stay dry. It's the same idea as lending each other things so that both of you come out ahead.

Many nonmetal atoms in the world are in a very similar spot. Most atoms settle into a low-energy, comfortable state when their outermost shell holds 8 electrons. Chemistry calls this the octet rule (the 8-electron rule). The exception is that very small atoms like hydrogen are different: they become stable with just 2 electrons in the outer shell (the duet rule).

But atoms like oxygen, carbon, and hydrogen can't fill that shell on their own. They don't want to simply lose their electrons to someone else, and it's hard for them to snatch electrons away by force. So each one puts its electron into the middle, and they form a bond in which they share each other's electrons.

The moment atoms share electrons, as if joining hands, each one feels as though it has a full set of its own. It's the most peaceful chemical compromise: each fills in what the other is missing.

Imagine two people each gripping one end of a single tug-of-war rope, pulling it taut. The harder both pull, the more firmly the two are tied together at a steady distance, with the rope between them.

At the center of each atom sits a nucleus with positive (+) charge, and the electrons the two atoms contribute together carry negative (-) charge. Just as opposite poles of magnets stick strongly together, an electrostatic attraction (a pulling force) acts between positive and negative charges.

In the end, both nuclei pull on the shared electron pair at the same time, and that pull locks the two atoms in place. Instead of forcing electrons from one atom to the other and making ions, the atoms are held together by a common treasure between them. It's a clever way to bind.

This bond is much stronger than you might expect. But note that what is strong is the covalent bond that ties atom to atom. The forces holding finished molecules to each other are far weaker. When water boils at 100 degrees, the water molecules are separating from one another. The covalent bonds inside H₂O are not breaking. That is why the frame of a molecule hardly ever falls apart, and why it makes a sturdy foundation for the material world we live in.

To be a bit more precise, atoms do not always share electrons in a perfectly fair 50-50 split. That's because each kind of atom pulls electrons toward itself with a different strength (its electronegativity).

The water (H₂O) we drink every day is a classic example. The oxygen atom pulls electrons much harder than the hydrogen atoms do. As a result, the electron cloud shifts toward the oxygen, so oxygen gets a slight negative charge and hydrogen a slight positive charge. This is a polar covalent bond.

Atoms can also share more than one pair when they need to: one pair (a single bond), two pairs (a double bond), or three pairs (a triple bond). The more electron pairs shared, the shorter the bond length and the more tightly the atoms are held together.

Thanks to covalent bonds, which join hands in so many flexible ways, an enormous variety of complex molecules has come into being on Earth, from the DNA and proteins that make up our bodies to diamond and cutting-edge plastics.