Now we shall look at why atoms combine the way they do and why elements and compounds have the properties they have. We will answer questions like:

Why do Hydrogen and Oxygen combine as H₂O to form water and not H₃O or HO₂?

Why is metalic Sodium (Na) highly reactive while Neon (Ne) is inert?

Why is Common Salt (a compound of Sodium and Chlorine, NaCl) a crystaline solid while Carbon Dioxide (a compound of Carbon and Oxygen, CO₂) a gas?

So, let's begin our journey into the structure of the atom.

The protons and neutrons are found in the nucleus of the atom. They will not be important for this essay. Protons have a a single positive charge. This is called the Atomic Number of an atom.

Electrons have a single negative charge. Normally, atoms are electrically neutral so that the number of electrons is equal to the number of protons. Electrons orbit around the nucleus.

The table below shows Atomic Numbers for the first 20 elements.

In the above table, the Atomic Number tells us the number of electrons that the atom contains. It is these electrons that determine the properties of the atom and the way it combines with other atoms to form specific compounds.

Because of considerations due to Quantum Mechanics, electrons cannot orbit the nucleus of an atom in any orbit. The electrons are restricted to specific paths called orbitals or shells.

According to an atomic rule that goes by the name of the Pauli Exclusion Principle, Each shell can only hold a certain number of electrons. When a shell is full, no more electrons can go into that shell.

Please note that the whole subject of electron shells is very complex. For the purposes of this essay we will look at the first three shells and simplify the discussion. This is only an introduction to give the reader an idea of what is happening.

The electron shells are given letters. The table below has the details we will use in this discussion.

Note that the d and f shells can actually hold more electrons in the larger atoms but that information is not needed here.

Hydrogen has only one electron so that will go into the s shell.

Helium has two electrons. Both of these will go into the s shell.

After Helium, we have Lithium with three electrons. Two go into the s shell. The s shell can only hold two electrons, so the third electron goes into the p shell.

Following these rules we can build up the electron shell structure of all the atoms. This can best be illustrated in the table below.

The key to the properties of atoms is the electrons in the outer shell. Note that Helium, Neon and Argon all have complete outer shells. All three are gases and all are inert, forming no or very few compounds with other atoms.

A complete outer shell of electrons is a very stable condition for an atom.

Hydrogen, Lithium, Sodium and Potasium all have a single electron in their outer shells. Hydrogen is a special element with many unique properties. Lithium, Sodium and Potasium, however, are highly reactive metals. They are all so reactive that they react with water! It is that single outer electron that makes these atoms reactive.

Fluorine and Chlorine have seven electrons in their outer shell. Both are highly reactive gases not found as elements in the natural state and forming crystaline salt-like compounds with metals. These elements are one electron short of a complete outer shell. This makes them reactive.

The next section will introduce the concept of Valency, or the combining power of atoms.

Valency can be simply defined as the number of Hydrogen atoms that an element can combine with. In the above table, Helium, Neon and Argon have a valency of 0. They do not normally form compounds.

Lithium, Sodium and Potasium have a valency of 1 because they combine with one Hydrogen atom. Beryllium, Magnesium and Calcium all have a valency of 2: they combine with two Hydrogen atoms. Note that the valences of all these atoms are equal to the number of outer electrons that these elements have.

Boron and Aluminium combine with three Hydrogen atoms - their valences are 3 - and they have three outer electrons.

Carbon and Silicon combine with four Hydrogen atoms. The valency of these elements is 4. It will come as no surprise that they both have four outer electrons.

What about Nitrogen and Phosphorus? They have five outer electrons. But they normally only combine with three Hydrogen atoms. Their valences are 3. Note that 3 is 5 less that 8. These atoms are three electrons short of a full shell.

Just a note to say that both Nitrogen and Phosphorus can also have a valency of 5. Some atoms are capable of having more than one valency. That will confuse the issue so I will talk of normal valency.

Now to Oxygen and Sulphur. Both have six outer electrons. Six is two short of a full shell. Their normal valences are 2 and they combine with two atoms of Hydrogen. Water is H₂O! Sulphur can also have a valency of 6 (or even 4) in some of its compounds.

Finally, Fluorine and Chlorine - seven outer electrons. This is one short of a full shell. They both combine with a single Hydrogen atom and their normal valences are 1. As a side note, Chlorine can also have valences of 3, 5 and 7. The reasons are well beyond the scope of this introductory essay.

The rules above can be summarised as follows: The normal valency of an atom is equal to the number of outer electrons if that number is four or less. Otherwise, the valency is equal to 8 minus the number of outer electrons.

The atoms with full electron shells (Helium, Neon, Argon) are chemically inert forming few compounds. The atoms don't even interact with each other very much. These elements are gases with very low boiling points.

The atoms with a single outer electron or a single missing electron are all highly reactive. Sodium is more reactive than Magnesium. Chlorine is more reactive that Oxygen. Generally speaking, the closer an atom is to having a full electron shell, the more reactive it is. Atoms with one outer electron are more reactive than those with two outer electrons, etc. Atoms that are one electron short of a full shell are more reactive than those that are two short.

The next two sections will discuss exactly how atoms combine together.

The Lithium has its single outer electron and the Fluorine has seven outer electrons, or a single electron missing in its outer shell.

When these two atoms come together, it provides an easy way for each to get a complete outer shell of electrons. The single electron in the outer shell of the Lithium atom jumps across and joins the seven electrons in the outer shell of the Fluorine atom. Lithium and Fluorine then have a complete outer shell of electrons. This is a more stable arrangement for each atom.

However, the Lithium atom has one too few negatively charged electrons to balance its positively charged protons in its nucleus. The Lithium atom now has a net positive charge.

The Fluorine atom now has one too many electrons. It has a net negative charge.

Atoms that have charges are called ions. We have a positively charged Lithium ion and a negatively charged Fluorine ion. These attract each other because opposite charges attract.

Lithium Flouride consists of a regular stack of Lithium and Fluorine ions held together by strong electrostatic forces. The regularity of the structure can be seen by the fact that the compound is a crystaline solid. The stability by the fact that it has a high melting point. Similar compounds are Sodium Chloride (NaCl, common salt) and Potassium Fluoride (KF). These all involve the transfer of a single electron from one atom to another.

Two electrons can also be transferred as in the compounds Calcium Fluoride (CaF₂) and Magnesium Chloride (MgCl₂). In these compounds, the outer two electrons of the Calcium or Magnesium are transferred to two Fluorine or Chlorine atoms. In compounds like Sodium Sulphide (Na₂S) or Potassium Oxide (K₂O), two atoms of Sodium or Potassium each transfer their single outer electrons to the one atom of Sulphur or Oxygen.

Compounds like these are held together by Electrovalent or Ionic bonds. They tend to be crystaline solids with high melting points. They are usually soluble in water and the solution conducts electricity because the ions can move easily in solution.

Transferring electrons is not the only way that atoms can combine.

The Oxygen is two electrons short of a full outer shell. The Hydrogen atoms need to gain an extra electron to complete their outer shell which can hold two electrons. Transferring electrons would require too much energy in this case. What happens is that the electrons are shared. The Oxygen shares one of its electrons with the first Hydrogen atom and another with the second Hydrogen atom. The Oxygen atom then has eight electrons orbiting it: equivalent to a full outer shell. Each Hydrogen atom shares an electron with the Oxygen atom, thereby having two. The diagram below shows this.

This produces a molecule of water.

Molecules are discrete entities which have strong bonds between the atoms. The molecules themselves only stick together loosely. This type of bond is a called a Covalent Bond.

These types of compounds tend to have low melting points. H₂O (water) is a liquid at room temperature. They tend not to conduct electricity.

Other examples of Covalent compounds are Carbon Dioxide (CO₂ - each Oxygen atom shares two electrons with the Carbon atom) and Ammonia (NH₃ - each Hydrogen atom shares its electron with the Nitrogen atom). Both of these are gases at room temperature. Many Organic compounds are covalent.