Which of the following effects can be used to rationalize the general trend of increasing ionization energy of gas phase atoms from left to right across a period?
(a) The valence electrons are held further from the nucleus
(b) This is not true since ionization energy decreases from left to right across a period.
(c) Electrons in the same shell do not substantially contribute to shielding each other from the nucleus.
(d) Atomic radii increase from left to right across a period and thus there is less electron-electron repulsion.
(e) The effective nuclear charge felt by the valence electrons increases when moving from left to right across a period.