Electrons distribution of oxygen atoms in ozone While dealing with the Lewis structure for ozone, most of the explanations show the electron distribution around the 3 oxygen atoms with a single and double bond. 

But if you count the number of valence electrons of the middle oxygen atom, it will add to 5 valence electrons only, but the number of valence electrons of an oxygen atom is 6. 

Similarly, the oxygen atom with a single bond in ozone, is being shown with 6 valence electrons plus one single bond, which accounts for 7 valence electrons, which does not tally with the expected number of 6 valence electrons. 

Can someone please explain why this discrepancy has been permitted? Or what did we miss to consider?
The ozone molecule is described with two resonant structures(as in figure) in which all the oxygen atoms respect the octet rule. The oxygen in the middle has a positive charge because has only 5 valence electron  and the oxygen with the single bond has a negative charge because it has 7 valence electrons. 
 In the real molecule both the bonds have the same length so there isn't one single bond and one double bond. Also both the terminal oxygen atoms have the same partial negative charge instead of being one neutral and one negative(-1). 
The picture below is more realistic of the situation: 



In the electrostatic potential map you can see that the two lateral oxygen are equivalent: