Answer: Ba, Sr, Ra, Ca, Mg, Be.
Step-by-step explanation:
The ionization energy is the energy required to remove an electron from an atom or a positive ion. It is the energy required to overcome the electrostatic attraction between the electron and the positively charged nucleus. The ionization energy decreases as you move down a group (vertical column) in the periodic table. It also decreases as you move from left to right across a period (horizontal row) in the periodic table.