Why are the trends and exceptions to the trends in ionization energy observed? Check all that apply. -Ionization energy tends to increase down a group because the electrons get farther away from the nucleus. -Ionization energy tends to increase across a period because the nuclear charge increases. -Ionization energy tends to increase across a period because electrons are added to the same main energy level. -The ionization energies of the elements in Group 16 tend to be slightly smaller than the elements in Group 15 because the fourth electron is added to an unfilled p orbital. -The ionization energies of elements in Group 13 tend to be lower than the elements in Group 2 because the full s orbital shields the electron in the p orbital from the nucleus.