INTRODUCTION

When it comes to capturing a little bit of nature in our homes, we aquarium keepers have managed to pick the most difficult environment to reproduce. The difficulty arises from the numerous chemical parameters that must be controlled. These include dissolved ions, dissolved gases, pH, and waste. In an open, aquatic system these parameters are naturally controlled through a complex system of self-regulating feedback based control mechanisms. However, in a closed aquatic system the self regulation exists only very tenuously at best and can be easily overwhelmed if the system is not adequately maintained. Proper maintenance requires our intervention where Nature normally would step in. This maintenance involves the application of simple chemical principles. Although one can maintain a tank without understanding the principles being applied, a basic understanding can help to take some of the mystery out of what we do and why we do it. Additionally, the more one understands, the more likely it is one will be able to avoid a disaster or achieve a greater level of success.

ATOMS

Before proceeding with any discussion of the chemistry that occurs in an aquarium it is necessary to briefly touch upon some rudimentary general chemistry. Without this basic knowledge, any further discussions would be of little benefit. All matter (living organisms, soil, air, water, asteroids, stars, etc.) are composed of atoms. An atom is the smallest unit of matter that still retains the unique properties of its element. An element is matter that is composed of only one type of atom. Examples of elements include carbon, oxygen, nitrogen, and iron. Atoms themselves are composed of even smaller parts: electrons, protons, and neutrons. Protons carry a positive charge, electrons a negative charge, and neutrons are neutral. Every atom is composed of these three basic components (only hydrogen lacks a neutron). For example, a proton in an atom of gold is exactly the same as a proton in an atom of oxygen. What distinguishes elements is the number of protons found in their respective atoms. For example, oxygen has 8 protons, while gold has 79. Each atom houses its protons at its core, along with a certain number of neutrons. This core is called the nucleus of the atom. In a neutral atom the nucleus is surrounded by the same number of electrons as protons. This can be simplistically visualized as equivalent to planets orbiting a star although a more accurate picture would be akin to bees buzzing about a hive: random, unpredictable movement in a spherical swarm.

IONIZATION OF ATOMS

Some atoms are more stable than others. This stability is related to the number and location of electrons orbiting the nucleus. Some atoms try to be “like” their more stable cousins by giving up or gaining electrons so that their configuration of electrons is identical to that of their nearest most stable cousin.

In Figure 1 we see that the central configuration is the most stable and is represented by a nucleus surrounded by an orbital shell containing four pairs of electrons. There are multiple shells but only the outermost one is shown for clarity. These stable elements are commonly called “noble gases”. They are so stable that they do not normally react with any other elements in any way.

In Figure I (left side) we see that the halogens (elements with one less proton in their nucleus than their nearest noble gas neighbor) will gain an electron so their electron configuration is the same as their nearest noble gas neighbor. Because they have an extra electron they are negatively charged and called negative ions (anions). An atom is in a neutral state when the number of electrons is the same as the number of protons. Likewise (Figure 1, right), an element with one more proton than its nearest noble gas neighbor (alkali metal) will give up an electron so that its electron configuration is the same as its nearest noble gas neighbor (the outer shell is empty and can be ignored here). Because it is lacking an electron it has a positive charge and is called a positive ion (cation).

MOLECULES

When atoms combine they form larger structures called molecules. Each molecule of a substance is identical to every other molecule of that substance. The atoms in a molecule are held together by an energetic association known as a bond. A bond can most simply be characterized in terms of its strength. The three basic bond types (strongest first) are covalent, ionic, and coordinate (or dative). Covalent bonds are difficult to break and cannot be broken by dissolving in water. Ionic bonds are much weaker and are typically easily broken in water solution. Coordinate bonds are the weakest and actually are not a true physical bond but rather more of a strong association based on opposite charges.

In general, if an element is bonded to a halogen (fluorine, chlorine, bromine, iodine) it has an ionic bond. If an element is bonded to carbon, nitrogen, or phosphorous it has a covalent bond.
Molecules can have a mixture of different bonds. Some parts of a molecule may be held together by covalent bonds while other parts are held together by ionic bonds. For example, sodium bicarbonate has three oxygen atoms bonded to a central carbon atom (see Figure 9). Two of the oxygens are ionically bonded to hydrogen ion and sodium ion respectively. Ionic bonds are considered to be highly polar. This means that there are predominantly positive and negative areas to the bond (similar to the poles of a magnet). These “opposites” attract each other quite strongly, forming the ionic bond.

In the presence of a solvent that possesses some polar character (such as water) these ionic bonds will break, releasing the individual components which are then stabilized by the overwhelming number of polar solvent molecules. This helps us to understand why certain compounds dissolve in water and why they dissolve in the manner that they do (e.g. sodium bicarbonate yields sodium cation, hydrogen cation, and carbonate anion, but no carbon or oxygen ions).