Ionic Bond

Picture two people meeting on the street. One is carrying heavy bags in both hands. The other has empty hands. Once the bags change hands, both feel lighter and more comfortable. Atoms do almost exactly the same thing.

Atoms have an instinct to fill their outermost shell with 8 electrons, because that makes them stable. Chemists call this the octet rule. A metal atom like sodium finds it much easier to get rid of its one extra electron. A nonmetal atom like chlorine finds it much easier to pick up the one electron it is missing. So sodium hands its electron to chlorine without a second thought.

Sodium, having given up an electron, becomes a positively charged cation (+). Chlorine, having taken one, becomes a negatively charged anion (-). These two particles now carry opposite charges, and a strong force called electrostatic attraction snaps them together like magnets.

To be fair, pulling an electron off sodium actually costs energy. The trade still works because the new cation and anion attract each other strongly and release a lot of energy. That pull is what locks the two together in a bond that is very hard to break.

Ionic substances pull on their neighbors so strongly that they have very high melting points. At room temperature they are hard solids. That is why the salt crystals on your dinner table never melt, no matter how hot the day gets.

Yet even a hard grain of salt crumbles easily if you tap it with a hammer. When force hits the crystal, one layer of ions slides over by a single position. Now plus sits next to plus, and minus sits next to minus. The moment like charges meet, a repulsive force that pushes them apart kicks in and splits the crystal in two.

Ionic substances also have an unusual way of handling electricity. As a solid, the cations and anions hold each other so tightly that no electricity flows at all.

Dissolve the crystal in water, or melt it with high heat, and everything changes. The ions break free of their shackles and move around freely. Now they can carry an electric current.

We write salt as 'NaCl', so it is easy to picture one sodium atom holding hands with one chlorine atom, like a separate pair. After all, we call a fixed number of atoms paired up and floating around on their own a molecule, like water or carbon dioxide.

To be more precise, an ionic solid does not exist as separate molecules. Countless cations and anions (more than a quintillion in a single grain of salt) line up in alternating order in every direction, weaving a three-dimensional net. This is a giant crystal lattice.

Inside one grain of salt, sodium and chlorine are packed tightly together in 3D. There is no separate unit you could point to and say, 'this is one piece of salt.'

So 'NaCl' is not the name of a molecule. It is the simplest ratio, telling you that sodium and chlorine sit in that giant crystal net in a 1-to-1 proportion.