Covalent bonds are formed when electrons are shared. You might wonder why chlorine only ‘comes as a molecule’ of chlorine, Cl2.
Both atoms have one space to fill so they both have the same desire to collect an electron. We say that they have the same ‘electronegativity’. Ionic bonds are formed when electronegativities are very different. Covalent bonds are formed by atoms having similar electronegativities. The only way round this is if each chlorine shares one each of its outermost unpaired electrons.
If we imagine the electrons like to be paired, (examiners like you to draw them as pairs, incidentally) there’s three pairs in each outermost shell of a chlorine atom and one lone electron left over, which can be shared with another lone electron from another chlorine atom, making a covalently bonded molecule.
This Word document is a very comprehensive handout and you need to learn the major points thoroughly.
Print it off , add it to your notes. Here’s a dot-cross diagram showing the structure of methane. Four hydrogen atoms each share their one electron with a carbon atom having four spaces
Try drawing dot-cross diagrams to show the following covalent bonds, hydrogen, fluorine, carbon dioxide, carbon tetrafluoride.
Here are the solutions… Please learn them
It’s easy to name covalent compounds. This link is really easy to follow and there’s some useful stuff on properties also.
There’s also a section in the document on giant covalent structures, in particular diamond, graphite and silicon dioxide. ‘Giant’ here dosn’t mean’big’ – it just means lots and lots of the same repeating structure – having high melting and boiling points.