2. Which of the following is the reason that ionization energy generally increases from left to right across the first period of the periodic table? (A) As the atomic number increases, the electrons are harder to remove.

(B) Ionization energy increases as the atomic number increases in a period.

(C) As the atomic number increases, more electrons are
added to the atoms, thus increasing the electron-electron repulsions.

(D) As the atomic number increases, more protons in the
nucleus cause an increase in the effective nuclear charge,
making it harder to remove the electrons.