Is the charge on the molecules always zero ? If not then why do we assume that it is zero while calculating the Oxidation state of a particular atom in that molecule ?
A molecule by definition has no charge . The formal charge of a molecule is it's charge , but if you see there is molecule which is neutral for example which is NaCl then we assume that charge on the molecule is zero and if you see other molecule which like for example take some ionic molecule than oxidation number of the atoms will be calculated with the help of ionic charge which is assigned to it molecules for neutral molecule the oxidation states for all atoms of neutral molecule must add up to zero so that we can find oxidation state of each molecule of given atom .So by the above discussion we can say that the charge of molecule is not always zero in some molecules there is some charges present due to some other reason .
No, we always not assume that charge of molecule to always equal to 0 we assume 0 for that molecules where the cation and anion atoms of molecules have same equal charge for example MgCl is you see that these molecule where the net overall charge of molecule is zero .
for example if we want to calculate the oxidation state of MgCl then the charge on magnesium will be +2 and charge on Cl is -2 therefore the oxidation state of Mg is +2 and for chlorine is -1 for two chlorine molecules there would be -2 .
for example, if we want to calculate the oxidation number of some molecule which is having some charge for example for example of HS0 we know that oxidation state of H=+1 and
O =-2 and we have to find the oxidation states of sulphur , now to calculate the oxidation state of sulphur
, so clearly oxidation state of sulphur is +4 .
hence by above concluision we have prove the given statement