Here are some key factors that contribute to low ionization energy:

* Large Atomic Radius: The further the outermost electron is from the nucleus, the weaker the attraction, making it easier to remove.

* Low Nuclear Charge (number of protons): Fewer protons in the nucleus means a weaker attraction to the outer electron.

* More Electron Shells: Having more electron shells increases the distance between the nucleus and the outer electron, reducing attraction.

Examples of elements with low ionization energy:

* Alkali metals (Group 1): Lithium (Li), Sodium (Na), Potassium (K), etc. They have only one valence electron that is relatively easy to remove.

* Alkaline earth metals (Group 2): Beryllium (Be), Magnesium (Mg), Calcium (Ca), etc. They have two valence electrons, but the outer one is still relatively far from the nucleus.

Important Note: The ionization energy generally decreases as you go down a group and increases as you move across a period on the periodic table.