An element in which all the electrons are paired is a diamagnetic substance. It is repelled in a magnetic field and weighs less. Experimental Measurement - the covalent radius is the center to center distance between 2 adjacent atoms. There are many difficulties associated with obtaining these numbers, but in spite of the difficulties, scientists have assembled a set of approximate atomic radii from interatomic distances. Atomic radius, in general, decreases as we move from left to right in a row of the periodic table.
Atomic radius increases from top to bottom in a family or group.
These 2 trends are the result of 3 influences on size. The number of the principal energy levels which are occupied, or the highest principal energy level which is occupied. As n increases, the size increases, shells with a larger value of n extend further from the nucleus and the covalent radius increases. This accounts for the increase from top to bottom in a group. The nuclear charge is very important.