As we face , increases ACROSS the Period from LEFT to RIGHT, BUT DECREASES down a Group, a column of the the Periodic Table.
So why? Well, we will first define electronegativity of an atom as its ability to polarize electron towards itself in a molecule. The polarization is a contest between (i) nuclear charge, and (ii) shielding by other electrons. Incomplete electronic shells shield the
As a consequence of the former, atoms to the right of the Periodic Table (save for the Noble Gases, which do not normally form compounds) should be highly electronegative. And it is no surprise that oxygen, and fluorine are the most electronegative . Their , for instance their hydrides and oxides, typically show the results of charge polarization: