Ionization energy is the energy required to remove an electron from an atom or ion in its gaseous state. This process is endothermic because energy is absorbed to overcome the attractive force between the positively charged nucleus and the negatively charged electron. When an electron is removed, the atom becomes a positively charged ion, and this requires energy to break the bond between the electron and the nucleus.
In simpler terms, think of it like pulling a magnet away from a metal surface. You need to exert energy to separate them because they are naturally attracted to each other. Similarly, removing an electron from an atom requires energy because the electron is held by the nucleus due to electrostatic forces.
This energy absorption is why ionization energy is considered endothermic. The amount of energy required varies depending on the element and its electron configuration, but the principle remains the same: energy is needed to remove an electron from an atom.