Answer: The energy required to remove an electron from a gaseous atom is called ionization energy.
Explanation:
Ionization energy is defined as the energy required to remove the most loosely bound electron from a neutral gaseous atom.
When we move across a period from left to right then there occurs a decrease in atomic size of the atoms. Therefore, ionization energy increases along a period but decreases along a group.
Smaller is the size of an atom more will be the force of attraction between its protons and electrons. Hence, more amount of energy is required to remove an electron.
Thus, we can conclude that the energy required to remove an electron from a gaseous atom is called ionization energy.
