bonding in organic compounds
Why is carbon unique? What accounts for the apparently limitless number of carbon compounds that can be prepared? The answer is that bonds between carbon atoms are stable, allowing chains of carbon atoms to be formed, with each carbon atom of a chain being capable of joining to other atoms such as hydrogen, oxygen, sulfur, nitrogen, and the halogens. Neighboring atoms in the periodic table, such as boron, silicon, sulfur, and phosphorus, can also bond to themselves to form chains in the elemental state, but the resulting compounds are generally quite unstable and highly reactive when atoms of hydrogen or halogen, for example, are attached to them. The elements at the right or left of the periodic table do not form chains at all-their electronattracting or electron-repelling properties are too great.
The forces that hold atoms and groups of atoms together are the electrostatic forces of attraction between positively charged nuclei and negatively charged electrons on different atoms. We usually recognize two kinds of binding. The first is the familiar ionic bond that holds a crystal of sodium chloride together. Each Na@ in the crystal feels a force of attraction to each ClO, the force decreasing as the distance increases. (Repulsion between ions of the same sign of charge is also present, of course, but the stable crystal arrangement has more attraction than repulsion.) Thus, you cannot identify a sodium chloride pair as being a molecule of sodium chloride. Similarly, in an aqueous solution of sodium chloride, each sodium ion and chloride ion move in the resultant electric field of all the other ions in the solution. Sodium chloride, like other salts, can be vaporized at high temperatures. The boiling point of sodium chloride is 1400°.' For sodium chloride vapor, you can at last speak of sodium chloride molecules which, in fact, are pairs of ions, Na@ClO. Enormous energy is required to vaporize the salt because in the vapor state each ion interacts with just one partner instead of many.
The second kind of bonding referred to above results from the simultaneous interaction of a pair of electrons (or, less frequently, just one electron) with two nuclei, and is called the covalent bond. Whereas metallic sodium reacts with chlorine by completely transferring an electron to it to form Na@ and clO, the elements toward the middle of the rows of the periodic table tend to react with each other by sharing electrons.
Transfer of an electron from a sodium atom to a chlorine atom produces two ions, each of which now possesses an octet of electrons. This means of achieving an octet of electrons is not open to an element such as carbon, which has two electrons in a filled inner K shell and four valence electrons in the outer L shell. A quadrinegative ion C4@ with an octet of electrons in the valence shell would have an enormous concentration of charge and be of very high energy, Similarly, the quadripositive ion C4@, which would have a filled K shell like helium, would be equally unstable. Carbon (and to a great extent boron, nitrogen, oxygen, and the halogens) completes its valence-shell octet by sharing electrons with other atoms.
In compounds with shared electron bonds (or covalent bonds) such as methane (CH4,) or tetrafluoromethane (CF4,), carbon has its valence shell filled, as shown in these Lewis structures:
For convenience, these molecules are usually written with each bonding pair of electrons represented by a dash: