What is defined as the minimum energy needed to remove an atom's outermost electron?

The minimum energy required to remove an atom's outermost electron is known as ionization energy. This concept is crucial in understanding the reactivity of elements, particularly in how easily an atom can lose electrons to form positive ions.

Ionization energy is influenced by factors such as the nuclear charge, the distance of the outermost electron from the nucleus, and the presence of electron shielding from inner electrons. As you move across a period in the periodic table from left to right, ionization energy generally increases because the nuclear charge increases without a corresponding increase in shielding, making it more difficult to remove an electron. Conversely, as you move down a group, ionization energy decreases due to greater distances between the nucleus and the outermost electron, as well as increased shielding.

In contrast, atomic radius refers to the size of the atom and is not directly related to the energy required to remove an electron. Electronegativity measures an atom's ability to attract and bond with electrons in a chemical compound, and while related, it does not define the energy needed to remove an electron. Electron affinity describes the energy change when an electron is added to an atom, indicating how strongly an atom can attract additional electrons, but it is not applicable in this context of removing