H & He - O & Ne and co-valent bond

Now we will learn another method by which atoms of elements combine to form molecules of a new substance. Come, let us understand this with the help of an example given below.

Making of an Hydrogen atom
 

We know that the Hydrogen Atom has only one Electron which orbits in the first shell. Observe that the closest inert gas to hydrogen according to the Periodic table is helium. We have learnt earlier that the two electrons of helium totally fill up the first shell. (Remember, that the first shell of an atom has a capacity of a maximum of two electrons). So the two atoms of hydrogen, each having one electron, share these two electrons in such a way that both electrons stay in between both atoms and are attracted by both positively charged nuclei.

Due to this sharing of electrons, there are two electrons around each hydrogen nucleus and their configuration resembles that of the helium atom. So the hydrogen element is chemically more stable as a molecule H₂, as compared to the atom H. That is why, on earth, under normal temperature and pressure conditions, the element hydrogen is mostly found in the form of H₂, a molecule. The bond that is formed between the atoms by sharing of electrons is known as a Covalent Bond.

Formation of an Hydrogen Molecule: an atom shares an electron with another atom. the formation is a molecule, which is more stable than an atom.
 

The electron configuration around each molecule of Hydrogen is like that of the Helium atom. 

Other examples of elements formed by covalent bonding are O₂, Cl₂, NH3 etc.

Making of an Oxygen Molecule
 

Every atom of oxygen shares two electrons with another atom, which forms the oxygen molecule. The outermost shell of this molecule has the same number of electrons a Neon. Neon has eight electrons in the outermost shell. Now we can say, when a molecule of oxygen is formed, its outermost shell gets eight electrons and its electronic configuration resembles that of neon.

Formation of an Oxygen molecule: sharing of two electrons by each atom with each other.

Electronic configuration of the Oxygen molecule is like that of the Neon atom.
 

Note: Sometimes, electrons in the outermost shell of an atom are shown by big dots and sometimes by crosses. Whenever we make a new molecule, we should be clear about which electron is from which atom and how many electrons are shared by which atoms. What matters for you is the number of electrons.