In this section you will find background information on molecules and the properties of bonded atoms.

The concept of chemical bonding lies at the very core of Chemistry; it is what enables about one hundred elements to form the more than fifteen million distinct chemical substances that make up our physical world. We need to define what we are talking about: Exactly what is a chemical bond? And what observable properties can we use to distingish one kind of bond from another?

What is a chemical bond?
Chemical bonding occurs when one or more electrons are simultaneously attracted to two nuclei. The forces that hold bonded atoms together are basically just the same kinds of electrostatic attractions that bind the electrons of an atom to its positively-charged nucleus. This is the most important fact about chemical bonding that you should know, but it is not of itself a workable theory of bonding because it does not describe the conditions under which bonding occurs, nor does it make useful predictions about the properties of the bonded atoms.

What is a molecule?
A molecule is an aggregate of atoms that possesses distinctive observable properties. And what are those properties that characterize a particular kind of molecule and distinguish it from others? Just as real estate is judged by "location, location, location", the identity of a chemical species is defined by its structure. In its most fundamental sense, the structure of a molecule is specified by the identity of its constituent atoms and the sequence in which they are joined together, that is, by the bonding connectivity. This, in turn, defines the bonding geometry¡X the spatial relationship between the bonded atoms.

How we depict chemical structures
Chemical species are traditionally represented by structural formulas such as the one for ascorbic acid (vitamin C) which we show here. The lines, of course, represent the "chemical bonds" of the molecule. More importantly, the structural formula of a molecule defines its connectivity. Three-dimensional models reveal much more about a molecule's structure. The ball-and-stick and space-filling renditions are widely employed.

Ordinary structural formula, showing connectivity only.	Ball-and-stick model, showing the "chemical bonds" and bonding geometry, but with the individual atoms unrealisticly separated.	Space-filling model, showing relative sizes of the atoms and general shape of the molecule, but not all atoms visible.

Observable properties of bonded atom pairs



1. Potential energy curves The energy of a system of two atoms depends on the distance between them. At large distances the energy is zero, meaning "no interaction". At distances of several atomic diameters attractive forces dominate, whereas at very close approaches the force is repulsive, causing the energy to rise. The attractive and repulsive effects are balanced at the minimum point in the curve. Plots that illustrate this relationship are known as Morse curves, and they are quite useful in defining certain properties of a chemical bond. The internuclear distance at which the potential energy minimum occurs defines the bond length. This is more correctly known as the equilibrium bond length, because thermal motion causes the two atoms to vibrate about this distance. In general, the stronger the bond, the smaller will be the bond length.

2. Bond energies
The bond energy is the amount of work that must be done to pull two atoms completely apart; in other words, it is the same as the depth of the "well" in the potential energy curve shown above. This is almost, but not quite the same as the bond dissociation energy actually required to break the chemical bond; the difference is the very small zero-point energy.

3. Bond lengths and angles
The length of a chemical bond the distance between the centers of the two bonded atoms (the internuclear distance.) Bond lengths have traditionally been expressed in Angstrom units, but picometers are now preferred (1A = 10-8 cm = 100 pm.) Bond lengths are typically in the range 1-2 A or 100-200 pm. A bond angle is the angle formed by three atoms bonded together. The complete structure of a molecule requires a specification of the coordinates of each of its atoms in three-dimensional space. This data can then be used by computer programs to construct visualizations of the molecule with bond distances and the bond angles. Molecular geometry (molecular structure) is the three-dimensional arrangement of the atoms that constitute a molecule, which can be specified in terms of bond lengths, bond angles.

Source:
Stephen Lower, chem1 virtual textbook, a reference text for General Chemistry, Simon Fraser University.