Rank from highest to lowest ionization energy. To rank items as equivalent, overlap them.

Answer:
Ranked from highest to lowest
Oxygen - 13.6181 kJ/mol
Silicon - 8.1517 kJ/mol
Nickel - 7.6398 kJ/mol
Manganese - 7.434 kJ/mol
Barium - 5.2117 kJ/mol