Introduction to Chemical bonds

What holds atoms together in chemical compounds?

               Certainly, there must be some force holding them together; the forces that hold atoms together are called chemical bonds. Without chemical bonds, the world would have only free atoms or ions. We could not have the amazing variety of substances. There would be no water and no food. And there would be no life!!. After all, all living things are made up of million of atoms and ions bound together forming molecules. Chemical bonds are the "glue" of every molecule. Although the basic principles of bond formation are always the same, chemical bonds show a variety of forms and strengths. More over, different elements have different bond forming abilities. For example, hydrogen atoms always form chemical bonds, but helium atoms never do.

Image  1: Formation of Hydrogen molecule

Valence electrons

                    As you know electrons move around the nucleus of an atom in different orbitals or various energy levels. These energy levels are numbered from the nucleus to the out side. There is a maximum number of electrons that each energy level can hold. e.g 1s² , 2s² ,  2p², 3s² , 3p⁶, 3d¹⁰………. The number of electrons shown respectively in different energy levels is called the electrons configuration of that particular element. Valance electrons are the outermost electrons of an atom which are important in determining how the atoms reacts chemically with other atoms.

Image 2: Valence electrons of carbon atom

                        Atoms with a complete shell of valence electrons (corresponding to an electron configuration s² p⁶  ) tend to be chemically  insert. The numbers indicate the number of valence electrons for the elements in each vertical column.

Image 3. The periodic table showing valence electrons of elements.

Note; Helium is included in Group 18 because it is a noble gas, but it has only 2 electrons in its complete energy level. 

Table 1 group numbers represent the valence electrons

Groups	Valence Electrons
Group 1 (alkali metal)	1
Group 2 ( alkaline earth metals )	2
Group 13 ( boron group)	3
Group 14 ( carbon group)	4
Group 15 ( nitrogen group)	5
Group 16 ( O group)	6
Group 17 ( halogen group)	7
Group 18 (noble group)	8**

Note; For many years, main group elements have been given group number 1A- 8A or IA - VIIIA. A newer numbering system assigns group numbers of 1-18 going across the periodic table.

The Valence electrons are especially important because these electrons are involved when atoms unite chemically to form compounds.