Why does water have the formula

The water molecule always has a composition in which two hydrogen atoms combine with one oxygen. Why  

We have not increased the number of electrons at all. All we have done is shared them between the two elements, thereby enabling each atom to have a full outer shell. This approach is known as the electron-pair theory. 

He produced a conceptual framework to explain molecular bonding, but in practice it could not explain the shapes of many molecules. 

The concept of the full outer shell is crucial if we wish to understand covalent bonds. 

Nevertheless, even today, we often discuss the bonding of organic compounds in terms of Lewis structures and valence bond theory. 

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