Periodic Table Trend Anomalies

Abstract:

Atomic radius is the physical size of an atom while ionization energy is the energy required to completely pull one electron away from an atom. When it comes to the periodic table, there are accepted periodic trends for both atomic radius and ionization energy. However, there are some instances in which certain elements do not follow the predetermined periodic trends. These are areas in which trend anomalies occur. On the attached graph, four such anomalies are circled, but only three will be discussed: anomaly #2, anomaly #3, and anomaly #4. The purpose of this report is to explain what about these specified regions is unpredictable, and give a concise reasoning, in relation to electron configuration, as to why the anomalies occur.

Ionization Energy:

The general trend for ionization energy is that it increases up a group and also increases from left to right. Due to the fact that the elements involved in the anomalies appear consecutively on the periodic table, the “left to right” trend will be given focus. The reasoning for theis trend is dependent upon the Z_eff. The Z_eff increases concurrently with the number of protons in an atoms nucleus. The more protons in a nucleus, the more attraction there is between the individual electrons and the nucleus which in turn means a larger Z_eff.The higher the Z_eff, the closer the electrons are held to the nucleus and therefore, the more energy is required to separate those electrons from the atom. However, in some cases this trend does not apply to certain elements. The following anomalies occur with respect to the trend of ionization energy on the periodic table of elements:

• Anomaly #2: Elements 7 & 8

The second anomaly found on the graph occurs at elements 7 and 8, Nitrogen and Oxygen. On the periodic table Nitrogen is element 7, indicating that is has 7 protons in its nucleus while Oxygen is element 8, indicating that is has 8 protons in its nucleus. Due to the fact that Oxygen has a larger number of protons in its nucleus, it should also have a larger Z_eff. The larger Z_eff means that there should be a greater attraction between the nucleus and the electrons, giving Oxygen a higher ionization energy than Nitrogen. This, however, is not the case. In fact, Nitrogen has a higher ionization energy that Oxygen.

To understand why this occurs, the electron configurations of both elements should be taken into account. Nitrogen has an electron configuration of 1s²2s²2p³ while Oxygen has an electron configuration of 1s²2s²2p⁴. Nitrogen’s p-orbital is exactly half full, with having 3 of a potential 6 electrons present. This configuration is considered to be a more stable one because there is an equal exchange of energies between the electrons of the 2p-orbital. This configuration is also considered to be more stable than the configuration of Oxygen, which has 4 electrons, more than half, in its p-orbital (Boudreaux, 2017). The increased stability of Nitrogen means that it takes more energy to pull electrons from its orbit than it does to pull electrons from the less stable Oxygen (Woodward, 2017).

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This kind of anomaly also occurs at elements 15 and 16, Phosphorous and Sulfur. The two elements are in the same periodic groups as Oxygen and Nitrogen. Like Nitrogen, Phosphorous has an exactly half full p-orbital, with 3 out of 6 possible electrons. Sulfur however, has a p-orbital with 4 electrons. Like Nitrogen, Phosphorous is considered to have a more stable configuration because the energy levels in the p-orbital are distributed evenly, while the energy levels in the p-orbital of Sulfur are not. Due to this increased stability in takes more energy to pull electrons from Phosphorous than it does for Sulfur, just as it does nor Nitrogen and Oxygen, despite the fact that the periodic trend for ionization energy would predict the exact opposite.

• Anomaly #3: Elements 45-50

The third anomaly on the graph occurs from elements 45 to 50; Rhodium, Palladium, Silver, Cadmium, Indium, and Tin.  Based on the periodic trend for ionization energy, the ionization energy should gradually increase as the graph goes from element 45, Rhodium, to element 50, Tin. This should occur because each consecutive element has more protons in its nucleus than the last, meaning a larger Z_eff and by extension, a larger ionization energy. This does not occur though. Instead, starting at Rhodium, the elements follow and up, down, up, down pattern with Rhodium and Tin marking the ending and the re-starting of the regular pattern, respectively.

To better understand why this anomaly occurs the following table should be taken into consideration:

As previously stated these elements form a pattern that goes up, down, up, down with Rhodium marking the end of the previously regular trend and Tin marking the re-start of that trend. The first element that shoots “up” in ionization energy is Palladium. Palladium has a much larger ionization energy than Rhodium. This is because Palladium has a full d-orbital while Rhodium does not. Palladium’s full d-orbital makes it a more stable element, because its valence orbital is satisfied, than Rhodium therefore, it takes more energy to pull electrons from Palladium’s orbit than it does to pull them from Rhodium’s. Silver’s ionization energy it much lower than Palladium’s however, and it is the first of the elements to go “down” in the pattern. While Silver does have a full d-orbital, it also has a half full s-orbital. Due to the fact that there is a half full s-orbital, Silver’s orbital’s are no longer “satisfied”. Palladium, however, still has a full d-orbital, with no electrons in the s-orbital, making it the more stable configuration. Once again, Palladium has the larger ionization energy because it is considered to have a more stable configuration, and it takes more energy to pull electrons from its orbit than it does for Silver.

After Silver comes Cadmium. Cadmium is the second “up” element in the pattern. While both of the elements, Silver and Cadmium, have full d-orbitals, Cadmium also has a full s-orbital. Due to the fact that Cadmium has both s and d-orbitals full it is considered to have a more stable configuration than Silver, explaining why Cadmium has a much larger ionization energy that Silver does. The next element to go “down” in the pattern is Indium. Cadmium is an extremely stable element because both its s and d-orbitals are full. Indium, however, has only 1 electron in its p-orbital making it a much less stable configuration than that of Cadmium (Barrens, 2007). Due to the fact that Indium is much less stable than Cadmium, it takes less energy to pull electrons from its orbit, giving reason to why Cadmium has a much larger ionization energy than Indium. The last element in the pattern, Tin, marks the re-start of the general ionization energy trend. Even though the electron configuration of Indium and Tin are very similar, Indium only has 1 electron in its p-orbital while Tin has 2. Despite the fact that the elements have similar configurations Tin is still considered to be a more stable element and therefore it has a larger ionization energy than Indium. After Tin, the accepted trend for ionization energy begins again.

In relation to the huge jump in ionization energy between Cadmium and Indium, Zinc and Gallium also demonstrate the same kind of jump. Zinc and Gallium are in the same periodic groups as Cadmium and Indium. Zinc has an electron configuration of 4s²3d¹⁰ while Gallium has an electron configuration of 4s²4p¹. Once again, Zinc`s 4s and 3d-orbitals are full, meaning it has a more stable configuration then Gallium, explaining why it has a high ionization energy. It should also be noted that the big drop in ionization energy occurs when a new subshell starts. The starting of a new subshell decreases the stability of an atoms configuration, making it easier to pull electrons from the orbit of that atom (Wiberg & Wiberg, 2001).

Atomic Radius:

The accepted periodic trend for atomic radius is as follows: atomic radius increases down a group as well as from right to left on the periodic table. Due to the fact that the elements involved in the anomalies appear side by side on the table, focus will be given to the “right to left” trend. Atomic radii decrease from left to right due to the fact that effective nuclear charge, Z_eff, increases from left to right. The Z_eff is the overall pull an electron feels from the nucleus; the greater the attraction between the nucleus and the electrons, the greater the Z_eff. This means that as the number of protons in the nucleus increases, so does the Z_eff because there is a greater attraction between the nucleus and the individual electrons. The greater the pull of the electrons to the nucleus the smaller the atomic radius. This trend, however, is not always followed. The following anomaly occurs with respect to the trend for atomic radius on the periodic table of elements:

• Anomaly #4: Elements 58 to 72 – First Row Inner-transition Metals

This anomaly occurs from element 58 to 71, Cerium to Hafnium. Based on the trend explained above, the atomic radius for these elements should increase from Hafnium to Cerium due to the fact that each element, going backwards, has less protons in its nucleus than the last, therefore, having a smaller Z_eff. This, however, does not occur and instead the graph shows the inner-transition metals to have almost or exactly the same atomic radii.

This anomaly occurs due to what is called Lanthanoid contraction. To understand this anomaly the electron configuration of these elements must be taken into consideration. All of these elements have a 4f-orbital, which makes them unique (NCERT, 2017). Typically, atomic radius tends to decrease when moving from left to right on the periodic table because there is room for more electrons in the existing energy levels.  When more electrons are added to these energy levels atomic radius tends to get smaller because the additional protons attract the electrons more, and pull the outer “shell” of electrons closer to the nucleus. This does not happen with electrons in the f-orbitals though. Instead of electrons being added to the outer shell of the atom, electrons are added to an inner-shell where f-orbital elements are concerned (Wicks, 2015). This causes a shielding effect. The shielding effect occurs when the inner-shell electrons “shield” the outer-shell electrons from the full magnitude of the nuclear charge, or attraction to the nucleus (Bains, 2014). This shielding effect is Lanthanoid contraction.  In elements 58 to 71, Lanthanoid contraction causes the 4f electrons to shield each other from their attraction to the nucleus. Due to the fact that these elements do not feel the full attraction of the nucleus the atomic radius does not increase a large amount. This explains why the inner-transition metals have atomic radii that are very similar, and do not differ very much in magnitude (Encyclopedia, 2011).

References

Bains, Amrit. (2014). Lanthanide Contraction. Retrieved from Chemistry LibreTexts: https://chem.libretexts.org/Core/Inorganic_Chemistry/Descriptive_Chemistry/Elements_Organized_by_Block/4_f-Block_Elements/The_Lanthanides/aLanthanides%3A_Properties_and_Reactions/Lanthanide_Contraction

Barrens, Richard. (2007). Zinc and Gallium Ionization Energy . Retrieved from Students` Technical Activities Body : https://stab-iitb.org/newton-mirror/askasci/chem07/chem07038.htm

Boudreaux, Kevin. (2017). Periodic Trends – Ionization Energy. Retrieved from Angelo State University: https://www.angelo.edu/faculty/kboudrea/periodic/trends_ionization_energy.htm

Britannica Encyclopedia. (2011). Lanthanois Contraction . Retrieved from Britannica Encyclopedia : https://www.britannica.com/science/lanthanoid-contraction

NCERT (National Council for Edication and Training). (2017). The d- and f- block Elements. Retrieved from National Council for Education and Training : http://ncert.nic.in/ncerts/l/lech108.pdf

Wiberg, Egon., & Wiberg, Nils. (2001). Inorganic Chemistry. In E. Wiberg, & N. Wiberg, Inorganic Chemistry (p. 1306). San Diego: Academic Press.

Wicks, Kurt. (2015). Exceptions to the General Trend for Atomic Radius. Retrieved from Chemistry Lecture Notes: http://www.chemistrylecturenotes.com/html/exceptions_to_the_general_tren.html

Woodward, Pat. (2017). Ionization Energy . Retrieved from Ohio State University : http://cbc-wb01x.chemistry.ohio-state.edu/~woodward/ch121/ch7_ie.htm