Ionization energy 

Ionization energy is required energy to remove an electron from outermost shell of an isolated atom. 

The electron that is more distance from nucleus feels less attraction by nucleus. Hence, to breakdown this less attraction atom required less energy. Hence, Ionization energy will be less.

That means, electron more distance from nucleus have less ionization energy. Atomic radius increases Ionization energy increases. In a periodic table left to right in a period Ionization energy increases. Because, atomic radius decreases from left to right in a periodic table due to gradually increasing of electron number in same energy state. But yet some difference are found,

For example, B placed in right side than Be in a periodic table in 2nd period. B have less ionization energy than Be

As same, Oxygen's ionization energy is less than Nitrogen

Energy required to remove first loosely bounded electron from outermost electronic shell is First Ionization energy. After that required energy to remove 2nd electron is 2nd Ionization energy. And energy required to remove 3rd electron is called 3rd ionization energy. 

2nd and 3rd ionization energy is very large from first Ionization energy., Because when any atom remove a one electron, it convert into positive charge. Highest positive charge attract more stronger to outermost electron in cation than neutral atom. Moreover, ions electronic configuration more closer to noble gas and more stable structure. Hence, 2nd Ionization energy will be larger than 1st ionization energy. 3rd Ionization energy be more larger than 2nd Ionization energy. 

The more the stable electronic configuration, ionization energy will be more higher. For example, Innert gas have highest ionization energy

Exactly, Ionization energy mainly depends of atomic radius. Certain factors influence impacts on atomic radius. For example, shielding effect and different orbital existence. 

Among all other stable elements Cs have lowest ionization energydue to shielding effect. 

Lanthanides ionization energy dramatically increases because of declining atomic radius. This declining of radius is called lanthanides contraction.