The polarity of a magnet usually means the direction of a vector which points from the south pole of the magnet toward the north pole. ... In this case the polarity is expressed in a way that indicates a circular direction, such as clockwise, or as a perpendicular vector representing the curl of the internal field.
Since the atomic mass is an average of the masses of all isotopes of that element, you have to consider which isotope is most abundant. In this case, the atomic mass is closest to twelve, so the most abundant isotope of carbon is Carbon 12.