this post is just a systematic list and explanation of chemical bonds because I needed to learn them for chemistry class and at the same time, why not make a post from it?
Chemical bonds are bonds between two atoms. There are various ways in which they can “connect”, and that is what this post is about:
Covalent bond is bond between two atoms which causes sharing of electron pairs. The difference between electronegativity is less than 1.67. If it would be bigger than the bond would be called ionic bond. Covalent bond is always going to be between two same atoms, for example H-H or C-C and so on.
EDIT (18.2.2016): Also if the difference is less than 0.4 the bond is nonpolar while if it is between 0.4 and 1.67 it is polar bond. This is important when you want to dissolve some stuff because only polar things with polar dissolve and nonpolar and nonpolar while not polar and nonpolar. For example water has polar covalent bond because the difference is roughly 1.2 or so.
Ionic bond is extreme case of covalent bond since the difference between electronegativity of two atoms is greater than 1.67. This means that one atom is going to attract the other electron so much that the first atom wont even hold it anymore. Examples of such cases are: Na+ and Cl– or Li+ and F– .. this “plus” means that the electron was moved a lot towards the other atom while “minus” means that the atom has a lot greater electronegativity and is able to catch this electron causing the atom to be negatively charged.
Above you can see how atom of fluorine takes one electron from lithium under his control (red arrow). Such molecules are easily dissolved in water and they are called non-polar molecules.
Dipolar bond is very similar to covalent bond and it can create when one of the atoms do not have any free electrons while the other one has two in one orbital. One of them then may move to the first atom and create bond with the other electron which is alone now.
Because bond can always create from electrons of opposite spin, with dipolar bond you can be sure that it will create because both the electrons came from one orbital so they have to have opposite spin.
An example of such a bond is NH4. Nitrogen has three orbitals ready to be connected with hydrogen which happens. Then the last hydrogen is actually H+ so he does not have any electron but nitrogen can share both his electrons from 2s2 orbital as you can see on the next picture. Here nitrogen is called “donor” and hydrogen “acceptor“.
Metallic bond is very often seen on metals and it is the reason for their thermal and electric conductivity. As you can see below atoms are all positively charged, which means they are cations while their electrons keep flying around making what is called “electron gas” (as you can see on the next picture), that is why the current so easily “flows” through metals.