Ionic compounds

Leon has been swimming in the sea in salt water. The salt found in the sea, is the same substance we use in cooking – table salt. Table salt consists of two elements, sodium and chlorine. The chemical name of the compound is sodium chloride. Sodium and chlorine are both dangerous substances to come in contact with.

Metallic sodium can cause corrosive burns on your skin and if you inhale chlorine gas you can damage your lungs. However, when sodium and chlorine have formed a chemical compound sodium chloride they are harmless. Why is that? It has to do with the atoms of the substances. The atoms in sodium are keen to get rid of electrons.

This means that sodium easily reacts with other substances. It is reactive. When each atom lets one electron go it becomes a sodium ion. Chlorine is reactive because its atoms want to pick up more electrons. A chlorine atom that gets one extra electron becomes a chloride ion.

Sodium ions and chloride ions are happy with the number of electrons they have. So, in ionic form neither sodium nor chlorine are reactive. The sodium ions that lost one electron are positive and the chloride ions that picked up one electron are negative. Positive and negative charges attract but as long as the salt is dissolved as in this sea water the ions move around without bonding to each other. Salt water is a sodium chloride solution.

As the water evaporates, like it does here on Leon’s arm, the ions will get closer to each other. So close, that positive and negative ions will start attaching to each other. Here one sodium ion is attached to one chloride ion. Does that satisfy them? Not at all.

The positive charge of the sodium ions continues to attract negatively charged chloride ions from all directions. And the chloride ions attract more sodium ions from all directions. The ions attach in a repeating pattern. In a crystal lattice. Chemical compounds made up of ions are called ionic compounds.

In a sodium chloride crystal, the ions are arranged like this, with alternating sodium and chloride ions. When the crystal has grown to contain some hundred quadrillion sodium ions and some hundred quadrillion chloride ions it’s large enough to be visible. A grain of salt. For each sodium ion there is one chloride ion. That means there is the same number of both.

With chemical symbols, the substance is written like this: NaCl This is the chemical formula for sodium chloride. In the chemical formula of an ionic compound, the positive ion is written first and the negative ion last. The plus and minus charges cancel each other out so they are not included in the chemical formula. When we use the word “salt”, we usually mean table salt. However, in chemistry, “salts” are a whole group of substances.

There are thousands of different salts. But since sodium chloride is the most common of the salts in the sea and in the kitchen we usually just call it: salt.