how is the general formua of the oxides in the VII group of mendeleev's periodic table R2O7?? isnt it supposed to be R2O??

The elements of group VII of Mendeleev's periodic table form oxides of the formula R₂O₇. These include the halogens (group VII A) and metals such as manganese and technetium (group VII B). Both manganese and technetium show a valency of 7, because of which the formula of their oxides is R₂O₇. The halogens, however are electronegative in nature and therefore show a valency of one. But since oxygen is more electronegative than halogens (except fluorine which is the most electronegative), therefore in oxides of halogens, the halogen atom will lose all its valence electrons and therefore have an oxidation state of +7 (except fluorine). Thus their valency will be 7 and hence their oxides will have the chemical formula R₂O₇.