Across a period in the periodic table, electronegativity increases due to an increase in the effective nuclear charge (Zeff) experienced by valence electrons. As the atomic number increases along a period, the number of protons in the nucleus increases. However, the number of inner electrons shielding the valence electrons from the nucleus remains relatively constant. This results in a stronger attraction between the valence electrons and the nucleus, leading to a higher electronegativity.
