Final answer:
Electronegativity increases as the atomic number increases from 11 to 17 on the periodic table, due to an increase in nuclear charge and a decrease in atomic size from left to right across the period.
Step-by-step explanation:
As the atomic number increases from 11 to 17 on the periodic table, the electronegativity typically increases. This increase in electronegativity is due to an increase in nuclear charge, which enhances the atom's ability to attract bonding electrons. Electronegativity increases across a period because the additional protons in the nucleus exert greater pull on the bonding electrons, despite a gradual increase in the number of inner-shell electrons which slightly shields the outer electrons from the full charge of the nucleus. Conversely, down a group, the size of atoms and their atomic radius increase due to the valence electron shell getting larger and a higher principal quantum number, which means the electronegativity decreases as the valence shell lies further from the nucleus.
Considering this trend, as you move from elements like sodium (atomic number 11) across to chlorine (atomic number 17), the atoms become smaller and the ability to attract electrons becomes stronger. Hence, the electronegativity of chlorine is higher than that of sodium. Chlorine, being a halogen, is highly electronegative with an electronegativity value of 3.16, which is significantly higher than that of sodium. The increase in electronegativity across the period can lead to changes in the distribution of electron density within molecules, impacting the nature of bonds formed between atoms.