Atoms join together to form molecules, but how are they held together?
Atoms are made up of a very small positive nucleus and shells of electrons rotating around them. When atoms join together to form molecules they form chemical bonds. The commonest types of bonds are ionic and covalent bonds.
Nucleus
Each element has a defined number of protons in its nucleus. For example Hydrogen (H), the element with the smallest nucleus (and hence lowest atomic weight) has just one proton in its nucleus. Helium (He), the next heaviest, has two protons and two neutrons. Further up the scale, Sodium (Na) has eleven protons and twelve neutrons and Chlorine (Cl) has seventeen protons and (usually) eighteen neutrons.
A neutral atom of any element must have the same number of electrons as it does protons. So a single, neutral hydrogen atom would have one electron, a single sodium atom, eleven; and a single Chlorine atom, seventeen. When we think about the bonds that these atoms may form, it helps to know how the shells are arranged.
Electron Shells
In this article we will use the simplest way of explaining the electron shells. The shell closest to the nucleus can hold up to two electrons. So, hydrogen has one electron in this shell and helium has two, the shell is now full. The next shell is bigger and has space for eight electrons. So the next element, Lithium (Li), has two electrons in its first shell and one in its second. This can be written Li (2,1). Sodium, with eleven electrons, overflows the second shell and must have one in the third shell, Na (2,8,1) and chlorine has seven in this third shell, Cl (2,8,7). The third shell also has room for eight electrons. Further shells gradually have more room for more electrons, but we will limit our discussion in this article to the first three.
Ionic Bonds
When Sodium and Chlorine form a compound together they form Ionic Bonds. The compound formed is Sodium Chloride, which we know as table salt. How does the ionic bond form? One useful principle to remember when thinking about chemical bonds is that an atom with a full outer shell of electrons is more stable. This is why the so-called noble gases are so un-reactive because they all have full outer shells of electrons.
When sodium and chlorine react together one electron of sodium is taken by the chlorine so that they both now have a full outer shell. The nuclei of the atoms, however, have not changed. So sodium now has one more proton than electrons and has an overall positive charge: Na+. Chlorine, on the other hand has one fewer proton than electrons and so has an overall negative charge: Cl-. These new atoms are called ions because they have a charge. The ions stick together very well because they attract each other due to the opposite charges. This is an ionic bond.
Covalent Bonds
When two chlorine atoms bond together, however, a different type of bond is formed called a Covalent Bond. Another important principle to be added here is that the electrons are grouped in pairs. The two atoms both have one space on their outer shells. So a covalent bond is formed because the unpaired electron in the outer shell of one chlorine atom is paired with the unpaired electron in the other chlorine atom. Now both atoms have full outer shells, and the atoms are held together very strongly by the shared electrons. A two atom, chlorine molecule is formed. Covalent bonds formed in this way are very strong.
Much more needs to be said about these types of bonds and other kinds of bonds and attractive forces, but this is a simple introductory overview of the phenomena.
