Final answer:
The atomic radius is the least consistent periodic trend, with some exceptions like the oxygen radius being greater than the nitrogen radius due to electron-electron repulsions.
Step-by-step explanation:
The periodic trend that is the least consistent is the atomic radius. The trend of atomic radius generally decreases from left to right across a period. However, there are exceptions to this trend, such as the oxygen radius being slightly greater than the nitrogen radius. This inconsistency can be attributed to variations in electron-electron repulsions that can cause the atom's size to increase, offsetting the expected decrease due to increased nuclear charge.
Other periodic trends, such as ionization energy and electronegativity, tend to be more consistent. Ionization energy usually increases from left to right across a period and decreases from top to bottom within a group. Electronegativity generally increases from the lower left to the upper right of the periodic table, with fluorine being the most electronegative element.