In the last post, we talked about how to determine the valence electrons. We talked about how chemical bonding is caused by attractions between electrons and protons. We also talked about how the most favorable electron configuration are those of the noble gases, which have all their orbitals filled. Now we are going to talk about how chemical bonding occurs.
In ionic bonding the valence electrons are not shared between the individual atoms. Instead they are either lost by or gained by the other atom. It is then called an ion. If a molecule gains an electron it becomes a negatively charged ion, called a anion. If a molecules gives up its extra electron it becomes a positively charged ion, called a cation. The atoms that most readily form ions are those that either have one or two electrons more than their nearest noble gas or one or two fewer electrons that their nearest noble gas.
The alkali metals on the far left-hand column of the Periodic Table all have only one valence electron. This means that all of these atoms have only one electron more than their nearest noble gas. They will easily give up their extra electron to become more like the configuration of the noble gas, and therefore become a positively charged ion, called a cation.
Halogens, on the far right-hand side of the Periodic Table, have one fewer electron than the nearest noble gas. These, then want to gain an electron to become a negatively charged ion, called a anion. You can see how these most easily come together to form molecules.
Covalent bonding, on the other hand, occurs when two atoms share their electrons. Neither gives up its electrons and but instead their orbitals merge into a larger molecular orbital. Since atoms with covalent bonds actually share an electron valence between them, their bonding is stronger than ionic bonding. The differences between ionic and covalent bonds can be seen most in how they behave.
The next post in this series: Molecular Geometry.