Carbon is located in Group 4 of the periodic table of elements (Group 14 in the IUPAC system), carbon has a valence electron configuration of 2s22p2; likewise, all the members of Group 4, sometimes known as the "carbon family", have configurations of ns2np2, where n stands for the number of the period or row that the element occupies on the table. There are two elements carbon, and silicon that are noted for their ability to form long strings of atoms and seemingly endless varieties of molecules. Interestingly silicon is directly below carbon on the periodic table. Silicon, found in virtually all types of rocks except the calcium carbonates, is to the inorganic world what carbon is to the organic. Yet silicon atoms are about one and a half times as large as those of carbon; thus not even silicon can compete with carbon's ability to form a seemingly limitless array of molecules in various shapes and sizes, and having various chemical properties.
Carbon is further distinguished by its high value of electronegativity, the relative ability of an atom to attract valence electrons. Electronegativity increases with an increase in group number, and decreases with an increase in period number. In other words, the elements with the highest electronegativity values lie in the upper right-hand corner of the periodic table. Actually, this statement requires one significant qualification: the extreme right-hand side of the periodic table is occupied by elements with negligible electronegativity values. These are the noble gases, which have eight valence electrons each. Eight, as it turns out, is the "magic number" for chemical bonding: most elements follow what is known as the octet rule, meaning that when one element bonds to another, the two atoms do so to have eight valence electrons for historical, symmetry and stability related reasons.
If the two atoms have an electric charge and thus are ions, they form strong ionic bonds. Ionic bonding occurs when a metal bonds with a nonmetal. The other principal type of bond is a covalent bond, in which two uncharged atoms make sharing of electros to have eight valence electrons. If the electronegativity values of the two elements involved are equal, the shared electrons belong to both of them equally; but if one element has a higher electronegativity value than its partner then the electrons will be more drawn to that element.
While discussing electronegativity and the periodic table, we should ignore the noble gases, which are the chemical equivalent of snobs due to which the term "noble," has been used for them meaning that they are set apart analogous to the then nobles of the society. To the left of the noble gases are the halogens, a wildly gregarious bunch, none more so than the element that occupies the top of the column, fluorine. With an electronegativity value of 4.0, fluorine is the most reactive of all elements, and the only one capable of bonding even to a few of the noble gases.
So question arises ‘why is it so that fluorine which is capable of forming multitudinous bonds is not as chemically significant as carbon’? while there may be a number of answers, a simple one is that because fluorine is too strong, and tends to "overwhelm" other elements, precluding the possibility of forming long chains, it is less chemically significant than carbon. Carbon, on the other hand, has an electronegativity value of 2.5, which places it well behind fluorine. Yet it is still at sixth place (in a tie with iodine and sulfur) on the periodic table, behind only fluorine; oxygen (3.5); nitrogen and chlorine (3.0); and bromine (2.8). In addition, with four valence electrons, carbon is ideally suited to find other elements (or other carbon atoms) for forming covalent bonds according to the octet rule.
Normally, an element does not necessarily have the ability to bond with as many other elements as it has valence electrons, but carbon, with its four valence electrons, happens to be tetravalent, or capable of bonding to four other atoms at once. Additionally, carbon is capable of forming not only a single bond, but also a double bond, or even a triple bond, with other elements including itself. Suppose a carbon atom bonds to two oxygen atoms to form carbon dioxide. Let us imagine that these three atoms are side by side, with the oxygen in the middle. We know that the carbon has four valence electrons, that the oxygens have six, and that the goal is for each atom to have eight valence electrons, some of which it will share covalently.
Two of the valence electrons from the carbon bond with two valence electrons each from the oxygen atoms on either side. This means that the carbon is doubly bonded to each of the oxygen atoms. Therefore, the two oxygens each have four other unbonded valence electrons, which might bond to another atom. It is theoretically possible, also, for the carbon to form a triple bond with one of the oxygens by sharing three of its valence electrons. It would then have one electron free to share with the other oxygen.
Dr.Badruddin Khan teaches in the University of Kashmir,Srinagar, India.