Final answer:
Electronegativity increases from left to right across a period due to increased nuclear charge and decreases from top to bottom within a group owing to an increase in atomic size. Metals display low electronegativity whereas halogens exhibit some of the highest values, except for noble gases which often do not form compounds.
Step-by-step explanation:
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. When discussing how electronegativity varies within a period on the periodic table, we notice that electronegativity generally increases as we move from left to right across a period. This trend is primarily due to the increase in nuclear charge, which allows the nucleus to pull the bonding electron pair closer to itself. Metals, particularly alkali metals, have low electronegativity values, while halogens exhibit some of the highest values on the periodic table. Although most noble gases do not form compounds and hence do not have electronegativity values, the transition metals show little variation in their electronegativities.
On the other hand, within a group, electronegativity typically decreases from top to bottom. This decrease corresponds to an increase in atomic size, which means the electron being added is further from the nucleus and more shielded by the inner electrons, making the atom less effective at attracting bonding electrons.
It is important to differentiate electronegativity from electron affinity, as the latter is specifically the energy change when an electron is added to a neutral atom, while electronegativity involves the atom in a molecule. The trends in electronegativity observed across the periodic table reflect the balance between the increasing nuclear charge and the atomic radius, which affects the atom's power to attract bonding electrons.