How Does Water Hold Itself Together?
The starring roles in water chemistry, apart from water molecules themselves, are played by the dissolved CO₂ from the atmosphere and the calcium and magnesium salts from the bedrock. To understand properly how these substances dissolve into your water — and how they sometimes undissolve — you need to understand how water molecules hold themselves together.
An atom consists of a positively charged nucleus surrounded by negatively charged electrons. A key to the way an atom behaves in a chemical reaction is the number of electrons it has. The electrons in an atom are arranged in a series of ‘shells’ — energy levels that can contain a fixed number of electrons. The first, innermost shell contains up to two electrons, and the outer shells can contain up to eight.
Atoms are most stable when the outermost electron shell is full. Helium, for example, with just two electrons, has a complete shell, so it is very stable and does not react easily with other atoms. Other atoms, such as hydrogen (the H in H₂O), which only has one electron, have extra space for more electrons. Oxygen atoms have eight electrons. That means two fill up the inside shell and six go on the outside; this leaves space for two more electrons. This situation means that pure oxygen and hydrogen are very reactive. In order to fill up their outer shells with electrons, oxygen and hydrogen atoms will react with other atoms.
The water molecule, H₂O, can form if two hydrogen atoms each share their one electron with one oxygen atom. This forms what is called a covalent bond. (Note that ‘molecule’ is the name for a collection of atoms bonded together.) In this bond, the oxygen shares one electron with each hydrogen, which means the outer shell of both atoms is full. This creates a stable molecule: water.
In this depiction of a water molecule,