Question

exceptional cases in periodic table

Answer 1

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Here is your answer
Here are some of the exceptional cases in a periodic table:
Electronegativity

This is the easiest of the exceptions. We have said that electronegativity increases to the right. This is true except that the trend does NOT include the noble gases. That means that fluorine has the highest electronegativity, not Ne or He. This is because the definition of electronegativity contains the phrases "in a covalent bond." Since the noble gases don't bond, they don't have electronegativities.

It should be noted that a few of the noble gases have been forced to bond (at high temperture and high pressure with fluorine). You may find tables that, therefore, list values for the electronegativity of Xe and Kr, but these are not terribly important values to know.

Ionization Energy

If you were to graph Ionization Energy v. Atomic number for the elements in the second period (row) of the table,you would expect something that looks like this: (cant attach a pic so just sending the note:)
 Note that Boron has a lower Ionization energy than Beryllium and that oxygen's ionization energy is lower than nitrogen's.