# What is the element with the greatest first ionization energy?

Identify the element with the greatest first ionization energy.
(A) Ce
(B) C
(C) Cl
(D) Ca
(E) Cs

The book says the correct answer is (B) with such an explanation:

First ionization energy is a periodic trend that increases up and to the right on the periodic table. After looking at the position of these elements on the periodic table, carbon is clearly the best answer-choice (B).

However, a chart I found on Google Images (Ionization Energy Chart) says that Cl has the greatest first ionization energy. What is right then?

• my special chart and web sites - please edit your post to cite your sources for this data – NotEvans. Aug 22 '17 at 13:44
• Find better textbook... – Mithoron Aug 22 '17 at 16:57

## 3 Answers

Your book must be wrong. NIST gives these values (in eV) as 11.260300 for C and 12.96763 for Cl. Even not reaching that website, I would say that a halogen would have a higher ionization potential than a group 14 element of a previous period just because there are no reason for the latter to not have a higher value, as for instance N has (half filled energy sublevel is relatively more stable and electron would leave this atom harder).

Source
Kramida, A., Ralchenko, Yu., Reader, J., and NIST ASD Team (2014). NIST Atomic Spectra Database (ver. 5.2), [Online]. Available: https://physics.nist.gov/asd [2017, August 23]. National Institute of Standards and Technology, Gaithersburg, MD.

Take a look at the first chart in https://en.wikipedia.org/wiki/Ionization_energy. Note that the Ionization Energy is not a smooth function of atomic number, and note specifically that IE on each row (of the Periodic Table) is not monotonic (that is, it zigs and zags). The Periodic Table organizes the elements by atomic number and to some extent by orbital occupancy but I know of no property which is always correctly predicted using it. Its advocates say it shows trends. I don't dispute that but I don't find it useful because you can't trust it to give the right prediction anywhere close to "most" of the cases. Using the typical confidence level of 5%, for it to be a reasonably accurate tool, then it should predict rankings for Periodic trends (see:https://en.wikipedia.org/wiki/Periodic_trends) 95% of the time, or 19 times out of 20. You can see from the IE vs At.No. chart that in the first 20 elements it prediction is wrong 4 times When comparing adjacent elements. In other words, it's really not very good at these predictions, but better'n nuttin'.

A simple way to solve this question is to know the pattern of increasing and decreasing ionization energy on the periodic table. Ionization energy is the energy required to remove an electron from a gaseous atom or ion. Basically how it works is that ionization energy increases from bottom to top on a family (vertical) and increases from left to right on a period (horizontal).

Looking at the periodic table, we usually compare fluorine with other elements around it. Carbon is two boxes away while chlorine is one. Usually this is how you would solve ionization energy questions. However, there are some exceptions, and the vertical increase is different that horizontal increase.

Hope this somewhat helps!