 So let's go back to hydrogen fluoride. Fluorine has a valence of minus one. Being in group seven it has seven valence electrons so it needs to gain one electron in order to complete its outer shell which makes it energetically stable. One way fluorine can gain an extra electron is by becoming an ion and this is what happens if it reacts with a metal. But if it's pairing with another non-metal like hydrogen it does it by covalent bonding. When it forms a covalent bond fluorine donates one electron to the bond and the other atom here its hydrogen also donates one. This forms the pair of bonding electrons that make up a single bond. Fluorine has access to the extra electron from the hydrogen which means it has eight electrons in its outer shell and the hydrogen has access to the extra electron from fluorine so it has two electrons in its outer shell which for hydrogen is a full shell. Let's imagine for a second that fluorine was to form a second bond with another hydrogen atom. It would have to donate one of its eight outer electrons and it would gain an extra one from the other atom. But that would mean that it now had nine valence electrons and the second energy level can only hold eight. So the extra electron would have to go up into the next level, the third electron level. This would mean the fluorine atom no longer had a full outer shell which is energetically unfavorable and means the molecule would be unstable and would fall apart. So fluorine forms an ion with a charge of minus one when it's ionic and it forms one covalent bond when it's covalent. Let's take another example carbon. Carbon is in group four which means it has four valence electrons. It needs to gain four electrons to have a full outer shell. When it does form an ion it accepts four electrons to form an ion with a charge of minus four. C4 minus is called the carbide ion and there are a number of metal carbides such as titanium carbide and tungsten carbide. When it forms a covalent compound it forms four bonds. We can see this in the methane molecule. The carbon donates all four of its valence electrons and receives four electrons from the four hydrogens. This gives the carbon access to eight valence electrons, a full shell, and the hydrogens each to two, a full shell for them. If carbon formed fewer than four bonds it would not have access to enough electrons to make a full shell. If it formed more than four bonds it would have too many electrons to fit in its outer shell. So carbon almost always forms four bonds. So by looking at a non-metal's place on the periodic table you could have a good guess at how many bonds it likes to form. And it is those covalent bonds that hold atoms together to make molecules that are as simple as methane or as complex as DNA. It's because of the fact that atoms can form a limited number of covalent bonds that most covalent substances do form separate molecules although some of those molecules are very big. However there are a number of covalent substances that form lattice structures like metals and ionic compounds do. Diamond and graphite which are both forms of carbon are examples of covalent lattice structures. You can find out more about these in another video. For your task for this video I'd like you to look at these five elements here and determine how many covalent bonds you would expect the atoms to typically form. And then in question two I'd like you to use your knowledge of how many bonds each of the atom types form and then use that to figure out what the molecule might look like, how the atoms get joined together with bonds. For instance we already know that in methane the carbon is joined by four single bonds and we can draw that out like this. Do these three molecules in the same style.