The key to an atom’s chemical characteristics is its electron configuration. This configuration determines the kinds and number of bonds an atom will form with other atoms. Carbon is a chemical element with symbol C and atomic number 6.
Carbon has 6 electrons, with 2 in the first electron shell and 4 in the second shell; thus, it has 4 valence electrons in a shell that holds 8 electrons. A carbon atom usually completes its valence shell by sharing its 4 electrons with other atoms so that 8 electrons are present to satisfy the octet rule. Each pair of shared electrons constitutes a covalent bond. The methane molecule provides an example: it has the chemical formula CH4. Each of its four hydrogen atoms forms a single covalent bond with the carbon atom by sharing a pair of electrons. This results in a filled outermost shell.
In organic molecules, carbon usually forms single or double covalent bonds. Each carbon atom acts as an intersection point from which a molecule can branch off in as many as four directions. This ability is one facet of carbon’s versatility that makes large, complex molecules possible.
Bond formation with carbon
The most important characteristics of carbon as a basis for the chemistry of life are, that it has four valence bonds, and that the energy required to make or break a bond is at an appropriate level for building molecules, which are stable and reactive. Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily long complex molecules and polymers and the shape of a molecule often determines its function.
When a carbon atom forms four single covalent bonds, the arrangement of its four hybrid orbitals causes the bonds to angle toward the corners of an imaginary tetrahedron. The bond angles in methane (CH4) are 109.5° and they are roughly the same in any group of atoms where carbon has four single bonds. For example, ethane (C2H6) is shaped like two overlapping tetrahedrons. In molecules with more carbons, every grouping of a carbon bonded to four other atoms has a tetrahedral shape. But when two carbon atoms are joined by a double bond, as in ethene (C2H4), the atoms joined to those carbons are in the same plane as the carbons.
Now let’s consider how the rules of covalent bonding apply to carbon atoms with partners other than hydrogen. In the carbon dioxide molecule (CO2), a single carbon atom is joined to two atoms of oxygen by double covalent bonds. The structural formula for CO2 is shown here:
Each line in a structural formula represents a pair of shared electrons. Thus, the two double bonds in CO2 have the same number of shared electrons as four single bonds. The arrangement completes the valence shells of all atoms in the molecule.
Urea, CO(NH2)2, is another organic compound that is found in urine.In this case, one carbon atom participates in both single and double bonds.