This is because it loose it's outer valence electron thus it's one of valence disappear and due to increase of effective charge of nucleus it tightly push it's shell together result to small atomic radius corresponding to that of chlorine as it doesn't lose hence it's effective charge of nucleus towards it's shells is relatively weak compare to that of K+
The line of sight must be perpendicular to measuring scale to avoid parallax error. Formation of bubbles inside the cylinder should be completely avoided.
