Before we examine carbon compounds, we first need to examine the structure and mention some properties of the carbon atom. Carbon has an atomic number of six, meaning it has six protons, and consequently has six electrons. These electrons are distributed with two in the K shell and four in the L shell. In forming compounds, carbon would appear to gain or lose the four electrons in its outer shell. Thus, we have the +4, -4 valences you learned for carbon earlier in this course.
In fact, carbon does not usually exchange electrons with other elements but prefers to share four electrons to complete its L shell. This is the reason that covalent bonding is predominant in organic chemistry.
Carbon atoms have the unique ability to bond to other carbon atoms and form chains which may also have branches.
This is the reason that the molecular size is so great in organic chemistry. Molecular weights in the thousands are not uncommon. Three types of bonds are formed between carbon atoms.
Single Bonds. A single bond is a covalent bond formed by two carbon atoms sharing two electrons. Compounds that contain only single bonds between carbon atoms are called alkanes.
Double Bonds. A double bond consists of two covalent bonds formed by two carbon atoms sharing four electrons as shown below. Compounds that contain at least one carbon-carbon double bond are referred to as alkenes.
Triple Bonds. A triple bond consists of three covalent bonds formed by two carbon atoms sharing six electrons as shown below. Compounds that contain at least one triple bond between carbon atoms are called alkynes.