Electronegativity tends to decrease moving down a group and tends to increase traversing left to right across periods. Electronegativity tends to decrease when moving down a group because the elements' atomic radii increase, moving any reaction further from the protons in the nucleus. Electronegativity tends to increase moving right across periods because it becomes easier to gain electrons rather than lose them.
There are several exceptions to the general trends of electronegativity in the periodic table. The first is the element hydrogen, which is a group 1 element. Its electronegativity is over twice as high as the next element down, lithium.
Noble gases, however, are an exception to the rule as a group. Electronegativity is largely irrelevant to the noble gases helium, neon, argon, krypton, xenon and radon. Under normal conditions, these elements have no tendency to attract electrons nor any tendency to lose them because they have a full valence shell. They are unreactive, outside of extreme conditions.
Other exceptions to the general trend are the lanthanides and actinides.The complex chemistry of these elements does not fit any general pattern.
With regard to the transition metals, very little variance is seen within groups and periods due to the elements' metallic properties.