Down a Column in the Periodic Table, the Ionization Energy

Multiple Choice

Down a column in the periodic table, the ionization energy ________

A) increases because the n quantum number changes.
B) increases because the quantum number changes.
C) decreases because the outer electrons are further from the nucleus.
D) increases because the atomic number (nuclear charge) increases.
E) changes little because the outer electrons are all in the same outer shell.