larger atomic size
Electronegativities generally increase from left to right across a period. This is due to an increase in nuclear charge because of the greater number of protons in the nucleus. Alkali metals have the lowest electronegativities, while halogens have the highest. Because most noble gases do not form compounds, they are generally not assigned electronegativity values. Note that there is little variation among the transition metals. Electronegativities generally decrease from top to bottom within a group due to the larger atomic size.