In this video we will look at covalent bonds in methane, ammonia, water and hydrogen fluoride. They are small, covalently-bonded molecules. The atoms within them share electrons because they have half full or more than half full valence shells of electrons: they are non-metals.
Methane is a fuel, ammonia is used in household cleaners, water is a drink and the essence of life, and hydrogen fluoride is used to etch glass.
The bonding in methane, ammonia, water and hydrogen fluoride shows a pattern: methane is carbon bonded to four hydrogen atoms; ammonia is nitrogen bonded to three hydrogen atoms; water is oxygen bonded to two hydrogen atoms, and hydrogen fluoride is fluorine bonded to just one hydrogen atom.
Carbon, nitrogen, oxygen and fluorine appear in the periodic table in this order, moving along the second row from left to right.
Carbon has four out of eight electrons in its outer shell, so makes four covalent bonds. Nitrogen has five out of eight electrons in its outer shell, so can make three covalent bonds to make the shell full. Oxygen has 6 electrons in its outer shell. It can bond with two hydrogen atoms to share 2 more electrons. It now has a full outer shell of 8 electrons.
Ammonia has two electrons, called a lone pair of electrons, occupying the fourth position. These electrons take up space. Because electrons are negatively charged, lone pairs repel bonds even more strongly than bonds repel each other. This makes ammonia less symmetric than methane. The water molecule is bent in shape. Oxygen has two lone pairs. Negatively charged lone pairs are slightly attracted to the hydrogen atoms, so there is a weak attraction between molecules. Forces between molecules are a little stronger in water than in ammonia or methane.
Water is liquid at room temperature and pressure, whilst ammonia a gas that is easily liquefied, and methane is a gas.
Intermolecular forces are normally very weak in covalent compounds, but in water they are just strong enough to keep it liquid. A bit more energy is needed to overcome these forces and boil it. If water were not a liquid, life as we know it would be completely different!
Ethanol contains carbon and oxygen bonding. The carbon atoms always form four bonds and the oxygen forms two. Remember, carbon forms 4 bonds, nitrogen forms 3 bonds and has one lone pair of electrons, and oxygen forms two bonds and looks bent.
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