Define Ionization energy or Ionization potential of an atom.

Ionization energy, also known as ionization potential, refers to the minimum amount of energy required to remove an electron from a neutral atom in its ground state, thereby forming a positively charged ion. It is typically measured in units of electron volts (eV) or kilojoules per mole (kJ/mol).

When an atom undergoes ionization, an electron is detached from its atomic orbit and moves to a higher energy level or completely separates from the atom. The energy required to overcome the attractive forces between the electron and the nucleus is known as the ionization energy. The electron that is removed during ionization is often referred to as a "valence electron," as it is the electron in the outermost energy level.

Ionization energy is an essential property of atoms and is influenced by several factors, including the atomic structure, nuclear charge, electron shielding, and distance between the electron and the nucleus. Generally, ionization energy increases as you move from left to right across a period in the periodic table due to the increasing nuclear charge and the decreasing atomic radius. Conversely, ionization energy tends to decrease as you move down a group in the periodic table due to the increased shielding effect and the larger atomic size.

Ionization energy plays a significant role in chemical reactions and determines an atom's ability to form ions and participate in bonding. Elements with low ionization energies, such as alkali metals, readily lose electrons to form positive ions, while elements with high ionization energies, such as noble gases, are less likely to lose electrons and tend to form stable, non-reactive ions.