The first ionization potential of the elements B, C, and N (atomic numbers 5, 6, and 7) steadily increases, but that of O is less than that of N. The best interpretation of the lowervalue for O is that:1. the ionization potential of N is a maximum and the values decrease steadily for the elements O, F, and Ne.2. there is more shielding of the nuclear charge in O than in B, C, or N.3. the electron removed from O is farther from the nucleus and therefore less tightly bound than that in N.4. the electron removed from O corresponds to a different value of the quantum number L than that of the electron removed from B, C, or N.5. the half-filled set of p orbitals in N makes it more difficult to remove an electron from N than from O.