Welcome to the forums. You have purchased a 9 speed chain but it should work properly with your 7 speed sprockets. There was very little different in the thickness of the spocket itself itself between 7 and 9 speeds, with 9 speed sprockets being only 0.07mm thinner. The one caveat in all this is the amount of wear on the sprockets. If some of the sprockets are highly worn, the new chain may not mesh properly with them.