Explaining Differences in Terbium & Uranium Electron Configs

In summary: Relativistic effects are more important for the 1s electrons, which are much closer to the nucleus. In summary, the electron configuration of terbium, [Xe] 6s2 4f9, with no electrons in the d subshell and one extra in the f subshell, is due to a combination of factors such as the energy splitting between f- and d-orbitals and relativistic effects. This is different from the electron configuration of uranium, [Rn] 7s2 5f3 6d1, which has one electron in the d subshell and none extra in the f subshell, due to differences in the energy splitting and the impact of relativistic effects. L
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Why is terbium's electron configuration [Xe] 6s2 4f9, with no electrons in the d subshell and one extra in the f sub shell, while uranium's electron configuration is [Rn] 7s2 5f3 6d1, with one electron in the d subshell and none extra in the f subshell?

Are lanthanum and actinide d block elements or f block elements? Does this explain the difference between those electron configurations, and which is correct?

Thanks.

https://www.google.com/search?q=ura...LGc0KHVAHBXsQ_B16BAgBEAM#imgrc=Ljey_loSx3UArM

https://www.google.com/search?q=ter...pB50JHf2mDdQQ_B16BAgBEAM#imgrc=wiIp2saSuwaXbM
 
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There are many factors influencing the precise electron configuration of the lanthanides and actinides, but there are some trends. The f-electrons are bad in screening each other from the nuclear charge, hence, the more f-electrons in an atom, the more tightly they are bound. On the other hand, the f-electrons will screen the d-orbitals from nuclear charge. Both effects lead to an increasing energetic splitting of f and d orbitals. Therefore, both the heavier lanthanides and actinides hardly involve the d-orbitals. In actinides the energetic difference between f and d is lower than in lanthanides, hence the earlier actinides show higher oxidation states (and more often d1).
 
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You shouldn't restrict that question to Tb and U.

In lanthanides, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb have [Xe]4fn6s2 type configuration. Meanwhile, La, Ce, Gd, Lu have [Xe]4fn5d16s2 type configuration.
In actinides, Pu, Am, Bk, Cf, Es, Fm, Md, No have [Rn]5fn7s2 type configuration. Meanwhile, Ac, Th, Pa, U, Np, Cm, Lr have [Rn]5fn6dm7s2 type configuration.

I am not sure if you can consider these two series to have different d-orbital configuration solely as "lanthanide vs actinide" problem.

As @DrDu mentioned, there are many factors that contribute to the ultimate electron configuration of these elements, so it's hard to say what is exactly the reason for this trend. As a matter of fact, one reason don't explain all of the trends. I am going to answer the question with the best I know, and what I've heard in conferences. I'm not a professional in electronic energies of neutral lanthanides and actinides, so please take it with a grain of salt.

First, you should treat f0, f7, and f14 elements as exceptions. These have none, half, or fully occupied f-orbitals so they are relatively stable with that f-electron configuration. It is therefore expected that the remaining electrons will occupy other orbitals, namely d- and s-orbitals. This is why La, Gd, Lu, Ac, Cm, and Lr have d-orbitals occupied.

Second, lanthanides and actinides have different f- and d- orbital energy splitting. Lanthanides have larger energy splitting between f- and d-orbitals due to 4f-orbitals being relatively closer to the nucleus than 5d-orbitals (which is what @DrDu mentioned about nuclear charge screening effect, which changes the effective nuclear charge that orbitals feels). This causes actinides to be more susceptible to having d-orbitals occupied than lanthanides.

Third, you should treat lighter and heavier elements of the series differently. Relativistic effects cause larger energy splitting of f- and d-orbitals with larger atomic number. This means that lighter elements of series tend to have smaller f- and d-orbital splitting, hence the Ce in lanthanides and Th, Pa, U, Np in actinides, both being the lighter element of the series, having occupied d-orbitals. You can see that Th, the lightest actinide other than Ac (exception mentioned above) contains not one but two 6d electrons! You can also see that none of the heavier lanthanides or actinides have any electrons in the d-orbitals.
 

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I would not consider f0, f7 and f14 as exceptions. Binding energy increases and size of f orbitals decreases continuously with increasing nuclear charge. So evidently f0 and f14 are extremal. After f7, orbitals have to be occupied with two electrons, which costs more energy.
Also, while relativistic effects certainly are of high importance in actinides, I don't think they have huge impact on d and f electrons, as these don't get very near the nucleus (outer shells + high angular momentum).
 

FAQ: Explaining Differences in Terbium & Uranium Electron Configs

What is the difference between terbium and uranium electron configurations?

The main difference between terbium and uranium electron configurations is the number of electrons in each element's outermost energy level. Terbium has 7 electrons in its outermost energy level, while uranium has 6 electrons. This difference is due to the fact that terbium is a lanthanide element and uranium is an actinide element, which have different electron configurations.

Why do terbium and uranium have different electron configurations?

The electron configurations of elements are determined by the number of protons and neutrons in their atomic nuclei. Terbium has 65 protons and 94 neutrons, while uranium has 92 protons and 146 neutrons. This difference in atomic structure leads to the difference in electron configurations between terbium and uranium.

What are the similarities between terbium and uranium electron configurations?

Both terbium and uranium have partially filled outermost energy levels, which makes them highly reactive. They also both have similar electron configurations in their inner energy levels, as they both belong to the f-block of the periodic table.

How do terbium and uranium electron configurations affect their chemical properties?

The electron configurations of elements play a crucial role in determining their chemical properties. Terbium and uranium have different electron configurations, which results in different chemical properties. For example, terbium is a highly reactive element and is often used in alloys and electronic devices, while uranium is a radioactive element used in nuclear power and weapons.

Can the electron configurations of terbium and uranium change?

Yes, the electron configurations of elements can change under certain conditions. For example, when terbium is ionized, it loses its outermost electrons and its electron configuration changes. Similarly, when uranium undergoes nuclear reactions, its electron configuration can also change due to the loss or gain of electrons.

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