Chemical Bonding and Molecular Structure
Table of contents
- Introduction to Chemical Bonding and Molecular Structure
- Why do atoms combine?
- Types of Chemical Bond
There are about 114 different kinds of elements which constitute all matters. It is well known fact that except for noble gases, no other element exist as independent atoms under ordinary conditions. Most of the atoms exist as molecules which are cluster or aggregates of atoms of same or different elements having a distinct existence and its own characteristic properties. There are also a large number of compounds which do not contain discrete molecules. Instead of molecules they are made up of ions arranged in characteristic three dimensional patterns. Thus, we observe that the atoms combine in different ways and form a variety of molecules each having specific properties.
The existence of a large number of molecules or compounds raises many questions, such as
- Why do atoms combine?
- Why do certain atoms combine whereas others do not?
- What is the nature of the forces which hold the atoms together in molecules?
- Why do atoms have fixed combining capacity?
- Why do molecules possess definite shapes?
We proceed to seek answers to the above important questions, laying stress on the nature of links or bonds between the atoms and the forces which make such linkages or bonds possible.
The attractive force which holds together the constituent particles (atoms, ions or molecules) in a chemical species is known as Chemical Bond.
The tendency of elements to combine with one another is directly related to the valency or valence of the element. The valency is the ability or tendency of elements to combine with one another. The development of various theories of chemical bonding in molecules have closely been related to the understanding of the structure of atom as the valency depends upon the electronic configurations of the atoms. Among the earlier contributions W. Kossel and G. N. Lewis made very important contributions in the development of theories of chemical bonding.
Many theories came forward to explain the formation of chemical bond. In 1916 Kossel and Lewis succeeded independently in giving first successful explanation about the cause of combination between atoms based upon the understanding of electronic configuration of noble gases. It has been observed that atoms of noble gases has little or no tendency to combine with each other or with atoms of different elements. This means that these atoms must be having stable electronic configurations.
The electronic configurations of these elements are given in figure number 2.
From figure number 2, we find that the atoms of all noble gases (with the exception of helium) have eight electrons in their valence shell. Helium on the other hand, has two electrons in its valence shell (first energy shell) because the first energy shell (n =1) cannot have more than two electrons. The electronic configuration of the valence shell for all noble gas except helium can be expressed as ns2np6 (for helium 1s2) and this represents stable configuration and corresponds to maximum stability. Due to the stable configuration, the noble atoms neither have any tendency to gain nor lose electrons and, therefore, their combining capacity or valency is zero. Their inertness is so prominent that they even do not form diatomic molecules and exist as monoatomic gaseous atoms.
All atoms other than noble gases have less than eight electrons in their outermost shells. In other words the outermost shells of these atoms do not have stable configurations. Therefore, they combine with each other or with other atoms to achieve stable noble gas configurations (ns2 np6 or 1s2). To a large extent, the elements undergo electronic re-arrangements to attain stable noble gas configurations. Thus,
“the tendency or urge of atoms of various elements to attain stable configuration of eight electrons in their valence shells, is the cause of chemical combination”
The principle of attaining maximum of eight electrons in the valence shell of atoms, is called Octet Rule.
However, it may be noted that for hydrogen and lithium atoms, the stable configuration means two electrons which corresponds to the nearest noble gas helium.
According to modern view the atoms combine to form chemical bonds to acquire a state of minimum energy. As we know every system in the universe has a natural tendency to attain a state of minimum energy. The state of minimum energy corresponds to state of minimum stability. For Example, water always flows down a hot body radiates energy and cools down. Similarly, it is expected that isolated atoms which have high potential energy enter into chemical combinations to form molecules with lower potential energy. Hence, the atom combine with each other, only if the process leads to lowering of energy. If there is no fall in potential energy of the system, no bonding is possible.
Ionic bond is formed between the atoms by the transfer of one or more valence electrons of one atom to the other atom so as to complete the outermost octets and acquires stable nearest noble gas configuration. For Example: NaCl, CaF2.
Bond formed when there is mutual sharing of electrons between the combining atoms of the same or different elements is called Covalent Bond. For Example: HCl, NH3.
The force of attraction which exist between the metal ion and the valence electron is called Metallic Bond.
Every atom existing in the universe tries to attain stability either by losing excess energy or by sharing electron density to achieve greater stability.
For Example, in water, oxygen shares two unpaired p orbital electrons with unpaired s orbital electron of hydrogen to form O-H single bond and achieve stable configuration similar to neon while hydrogen achieve similar to that of helium. Therefore Chemical bonding occurs by sharing the density of the electrons of each atom both atoms can achieve greater stability.