Ionization energy is the energy required to remove an electron from a neutral atom or molecule in the gas phase.
It is measured in KJ/mol.
The Ionization energy tends to increase as you move from left to right across a period in the periodic table, since the nuclear charge increases while the number of energy levels remains the same. This means that it becomes more difficult to remove an electron from the atom.
The Ionization energy tends to decrease as you move down a group in the periodic table, since the distance between the electrons and the nucleus increases. This makes it easier to remove an electron from the atom.
