Which of the following elements would have the highest first ionization energy? A) Boron, B) carbon, C) oxygen, D) fluorine, or E) nitrogen.
The first ionization energy is the energy required to remove an electron from the outer shell of an atom of an element, assuming the atom is in its ground state. The energy required to remove electron goes up as that electron gets closer to a nucleus. But it also goes up if the nucleus has a higher charge. But the presence of electrons between the nucleus and the outer electron also has an effect. The more electrons there are between the nucleus and the electron, the more shielding that electron will experience. So for this question, we’re going to need to take all these into account. Let’s start off by looking at the electron configurations of atoms of each of these elements.
Boron has an atomic number of five. That means an atom of boron has five electrons in total. So the electron configuration of a neutral boron atom is 1s² 2s² 2p¹. So the outer electron is in the 2p subshell. The element carbon has atomic number six. So an atom of carbon has electron configuration 1s² 2s²
2p⁶ [2p²], six electrons with the outer electron in the 2p subshell. An atom of oxygen has eight electrons, with electron configuration 1s² 2s² 2p⁴. Again, the outer electron is in the 2p subshell. An atom of fluorine with nine electrons has electron configuration 1s² 2s² 2p⁵. And an atom of nitrogen has the electron configuration 1s² 2p² 2p³. So all five types of atom have the same valence shell and the same number of core electrons, two electrons in the first electron shell.
Electrons in the same electron shell have a much lower contribution to shielding. So let’s have a look at the charge of the nucleus for each. For that, we can go back to the atomic number Z, which is equivalent to the number of protons in the nucleus. A boron nucleus has five protons. So it’s got a charge of five plus. A carbon nucleus has six. An oxygen nucleus has eight. A fluorine nucleus has nine. And a nitrogen nucleus has seven. So, based purely on the nuclear charge and the number of protons in the nucleus, we would expect the strongest force of attraction between the outer electron of a fluorine atom and its nucleus. But to do this properly, we should really consider the effective nuclear charge experienced by the outer electron. The effective nuclear charge is the charge an electron experiences once you factor in the repulsion of core electrons. So the effective nuclear charge is a better representation of the force you might experience if you’re an electron bound to an atom in the outer shell.
Actually, calculating the effective nuclear charge for a given electron can be quite complicated. So we’re going to use an approximation where we say the effective nuclear charge is equal to the nuclear charge minus the number of core electrons. For all our candidates, the number of core electrons is two, the number of electrons in the first electron shell. So the effective nuclear charge experienced by the 2p electron in an atom of boron is roughly three. For carbon, it’s around four. For oxygen, about six, for fluorine about seven, and for nitrogen about five. On this basis, we expect the element with the highest first ionization energy to be fluorine.
There is a trick to getting to the answer more quickly. And that’s to use the periodic table. All our candidate elements are in the second period of the periodic table. Moving left to right across a period, generally, we see an increase in the ionization energies. Fluorine is furthest to the right in this case. So we expect it to have the highest first ionization energy. So our answer is definitely fluorine. But just in case you’re interested, here are the numbers. The first ionization energy for boron is 800.6 kilojoules per mole. That’s about two and a half times the energy required to break single carbon carbon bonds. The first ionization energy of carbon is 1086.5 kilojoules per mole. We can see that as we’re increasing the effective nuclear charge for the outer electron, the ionization energy is increasing.
We see a similar behaviour moving from carbon to oxygen, where oxygen’s first ionization energy is 1313.9 kilojoules per mole. At the top of the pack, we have fluorine, with a first ionization energy of 1681.0 kilojoules per mole, roughly five carbon carbon single bonds worth. The value for nitrogen is 1402.3 kilojoules per mole, clearly less than that of fluorine. But interestingly, the value for nitrogen is actually greater than the value for oxygen even though our calculated effective nuclear charge is greater for oxygen than for nitrogen. This just goes to show that our model isn’t perfect. And we may need to adjust the way we calculate effective nuclear charge if we’re to accurately predict these properties.
However, for our purposes, we can be confident that, of the five elements given, the one with the highest first ionization energy is fluorine.