 Firstly, covalent compounds. What is a covalent compound? Well, we've already dealt with ionic compounds briefly and we said that they're formed when a cation, a positively charged ion, and an anion attract each other. And usually this happens between a metal and a non-metal. Well, if you look at the compounds here, you'll notice that they're all made of non-metal elements joined together. When non-metal atoms bond, they do it by sharing electrons rather than by forming ions. The details of how the bonding occurs, we can leave for later. For now, you need to remember only that covalent compounds are made from non-metal atoms only and they don't involve ions. So let's look through them. All of these turn out to be gases. Carbon monoxide is the colourless, toxic gas that's produced in exhaust and it's an indication that fuel isn't burning completely. Carbon dioxide is what you get when fuel does burn completely, that means when there's plenty of oxygen around. It's not toxic but it's suffocating, but it's a fuel source for plants and it is an important greenhouse gas in our planet's atmosphere. Dynitrogen monoxide is laughing gas. It has an anesthetic effect and is often used by dentists or as pain relief for women giving birth. Sulfur trioxide is a gas pollutant and one of the compounds that produces acid rain when it combines with rainwater. Ozone's another toxic gas but the layer of it that exists high in the atmosphere fulfills a vital role. It absorbs UV radiation from the sun before it gets down to us on the surface. Hydrogen sulfide is rotten egg gas. Methane is the primary component of what we call natural gas and it's also produced by bacteria and in farts. Detecting methane is one of the tasks of the curiosity rover that NASA sent to Mars. It's up there now since it's thought that this will give us a clue as to whether there is or was any forms of life on Mars. An ammonia is often dissolved in water and used as a cleaning product. It has a very distinctive pungent sharp smell and most of the world's production of ammonia is used to produce the fertilizer ammonium nitrate. You may also notice if you look closely that the naming's not unlike ionic naming except for one thing. Whereas in the ionic names we simply gave the names of the ions without mentioning how many ions were present. In covalent compounds we use prefixes like mono, di and tri to specify how many atoms there are. So for instance CO is carbon monoxide meaning 1 oxygen monoxide and CO2 is carbon dioxide, di meaning 2. The reason this is necessary is that non-metal atoms are often able to combine in different ratios, CO and CO2 for instance, whereas ions can't do this. There are no hard and fast rules to learn here. I'd simply advise you to become familiar with these compounds because they will turn up.