Does electronegativity increase or decrease when you go a period on the periodic table? Does it increase or decrease when you move down a group on the periodic table?

As we face the Periodic Table, electronegativity 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 density 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 #"nucular charge"# VERY INEFFECTIVELY, and this is physically manifested in the well-known decrease in atomic radii, from left to right across the Period.

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 elements. Their compounds, for instance their hydrides and oxides, typically show the results of charge polarization: #""^(+delta)H-F^(delta-)#, and #""^(-delta)OH_2^(delta+)# are common representations. This polarization manifest in the disproportionately high boiling points of both hydrides, and the electronegativity of the heteroatom is key to an understanding of this phenomenon.