INTRODUCTION
Any discussion of chemistry usually begins with the atomic composition of a compound. However the composition alone does not define all of the chemical properties; rather, another key facet is how the atoms interact or bond with one another. The bonds that can form between atoms determine a molecule’s structure, and therefore may help define the properties of molecule. Atoms can bond with one another in several different ways. In this section we shall discuss some of the most common types of bonds.
In the 1900’s, it was observed that noble gas atoms were exceptionally stable and inert – they did not readily engage in chemical reactions. Based on studies, a theory was developed with four basic principles:
- Electrons in the outermost electron shell play a fundamental role in chemical bonding.
- In some cases electrons are transferred from one atom to another creating positively and negatively charged ions, which interact through electrostatic interactions and create ionic bonds.
- In other cases one or more pairs of electrons can be shared between atoms, thus creating covalent bonds.
- Electrons are transferred or shared to ensure that the outermost electronic shell adopts the conformation of a noble gas.
What precisely is meant by an electron shell? For any atom of a given element, the electrons do not exist in infinitely random space around the atom’s center or nucleus, but rather they exist in defined energy levels and shells of specific shape, referred to as orbitals. The orbitals have a finite capacity for electrons. For the purposes of this section, we will primarily refer to the elements in Periods 1 through 3. For elements in Period 1, the maximum number of electrons in the outermost electron shell is two. For the elements in Period 2 and 3, the maximum number of electrons in the outermost electron shell is eight.