The element with the highest ionization energy is helium (He), while the element with the lowest ionization energy is francium (Fr).
Ionization energy refers to the energy required to remove an electron from an atom or ion in the gaseous state. Helium has the highest ionization energy because it has a completely filled 1s electron shell, which is the most stable electron configuration. Removing an electron from helium would require a significant amount of energy to disrupt this stable configuration.
On the other hand, francium has the lowest ionization energy because it has the largest atomic radius among all the elements. Francium is located at the bottom of the alkali metal group (Group 1) in the periodic table, and as you move down a group, the atomic radius increases. The larger atomic radius of francium means that its outermost electron is farther away from the positively charged nucleus, resulting in a weaker attraction between the electron and the nucleus. As a result, francium requires the least amount of energy to remove an electron, making it the element with the lowest ionization energy.
It's important to note that ionization energy can vary depending on factors such as atomic structure, electron shielding, and the specific electronic configuration of an element. The values mentioned here represent general trends but may not apply in all cases.