A. distance of the outermost electron(s) from the nucleus
B. size of the positive nuclear charge
C. ability to attract shared electron(s)
D. screening effect of the inner electron(s)

Correct Answer: Option C

C. ability to attract shared electron(s)

Explanation

These 3 factors affect ionization energy:

Size of the positive nuclear charge
Size of atom (the distance of the outermost electron from the nucleus)
Screening effect of inner shell electrons

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