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3.2: Shielding

  • Page ID
    111732

  • Introduction

    Coulomb's Law is from classical physics; it tells us that particles with opposite electrostatic charge are attracted to each other; and the larger the charge on either particle or the closer the distance between them, the stronger the attraction. Coulomb's law explains why atomic size decreases as the charge on the nucleus increases, but it can't explain the nuances and variations in size as we go across the periodic table. Coulomb's Law also explains why electrons in different shells (n), at different distances from the nucleus, have different energies. But on its own, Coulomb's law doesn't quite explain why electron subshells within a shell (like 2s vs 2p) would have different energies. To explain these things, we need to consider how both electron shielding and penetration result in variations in effective nuclear charge (Z*) that depend on shell and subshell.

    Effective Nuclear Charge (Z*)

    Screen Shot 2019-12-02 at 3.05.13 PM.png

    Figure \(\PageIndex{1}\). In a lithium atom, the nuclear charge (Z) is +3. 1s electrons experience an effective nuclear charge (Z*) of +2.69, and 2s electrons experience an Z* of +1.28. (CC-BY-NC-SA; Kathryn Haas)

     

    Coulombs' law works well for predicting the energy of an electron in a hydrogen atom (because H has only one electron). It also works for hydrogen-like atoms: any nucleus with exactly one electron (a He+ ion, for example, has one electron). However, Coulomb's law is insufficient for predicting the energies of electrons in multi-electron atoms and ions.

    Electrons within a multi-electron atom interact with the nucleus and with all other electrons. Each electron in a multi-electron atom experiences both attraction to the nucleus and repulsion from interactions with other electrons. The presence of multiple electrons decreases the nuclear attraction to some extent. Each electron in a multi-electron atom experiences a different magnitude of (and attraction to) the nuclear charge depending on what specific shell and subshell the electron occupies. The amount of positive nuclear charge experienced by any individual electron is the effective nuclear charge (Z*).

    For example, in lithium (Li), none of the three electrons "feel" the full +3 charge from the nucleus (see Figure \(\PageIndex{1}\)). Rather, each electron "feels" a Z* that is less than the actual Z and that depends on the electron's orbital. The actual nuclear charge in Li is \(Z=+3\); the 1s electrons experience a \(Z^* =+2.69\), and the 2s electron experiences a \(Z^* = +1.28\). In general, core electrons (or the electrons closest to the nucleus), "feel" a Z* that is close to, but less than, Z. On the other hand, outer valence electrons experience a Z* that is much less than Z.

    In summary:

    • Core electrons: \(Z^* \lessapprox Z\)
    • Valence electrons: \(Z^* \ll Z\)

    Shielding:

    Shielding is the reduction of true nuclear charge (Z) to the effective nuclear charge (Z*) by other electrons in a multi-electron atom or ion. Shielding occurs in all atoms and ions that have more than one electron. H is the only atom in which shielding does not occur.

    Explanation of shielding: Electrons in a multi-electron atom interact with the nucleus and all other electrons in the atom. To describe shielding, we can use a simplified model of the atom: we will choose an electron-of-interest in a multi-electron atom and treat all the "other" electrons as a group of spherically-distributed negative charge. Classical electrostatics allows us to treat spherical distribution of charge as a point of charge at the center of the distribution. Thus, we consider all the "other" electrons in our atom as a point of negative charge in the center of the atom. While the positive charge of the nucleus provides an attractive force toward our electron, the negative charge distribution at the center of the atom would provide a repulsive force. The attractive and repulsive forces would partially cancel each other; but since there are less "other" electrons than there are protons in our atom, the nuclear charge is never completely canceled. The "other" electrons partially block, or shield, part of the nuclear charge so that our electron-of-interest experiences a partially-reduced nuclear charge, the Z*.

    In reality, there is not a one-for-one "canceling" of the nuclear charge by each electron. Partly due to penetration, no single electron can completely shield a full unit of positive charge. Core electrons shield valence electrons, but valence electrons have little effect on the Z* of core electrons. The ability to shield, and be shielded by, other electrons strongly depends the electron orbital's average distance from the nucleus and its penetration; thus shielding depends on both shell (\(n\)) and subshell (\(l\)).

    Shielding depends on electron penetration

    Coulomb's law shows us that distance of an electron from its nucleus is important in determining the electron's energy (its attraction to the nucleus). The shell number (\(n\)) determines approximately how far an electron is from the nucleus on average. Thus, all orbitals in the same shell (s,p,d) have similar sizes and similar average distance of their electrons from the nucleus. But there is another distance-related factor that plays a critical role in determining orbital energy levels: penetration. Penetration describes the ability of an electron in a given subshell to penetrate into other shells and subshells to get close to the nucleus. Penetration is the extent to which an electron can approach the nucleus. Penetration depends on both the shell (\(n\)) and subshell (\(l\)).

    The penetration of individual orbitals can be visualized using the radial probability functions. For example, Figure \(\PageIndex{2}\) below shows plots of the radial probability function of the 1s, 2s, and 2p orbitals. From these plots, we can see that the 1s orbital is able to approach the closest to the nucleus; thus it is the most penetrating. While the 2s and 2p have most of their probability at a farther distance from the nucleus (compared to 1s), the 2s orbital and the 2p orbital have different extents of penetration. Notice that the 2s orbital is able to penetrate the 1s orbital because of the central 2s lobe. The 2p orbital penetrates somewhat into the 1s, but it cannot approach the nucleus as closely as the 2s orbital can. While the 2s orbital penetrates more than 2p (2s orbital can approach closer to the nucleus), the 2p is slightly closer on average than 2s. The order of Z* in 2s and 2p subshells depends on which factor (average distance or penetration) is more important. In the first two rows of the periodic table, penetration is the dominant factor that results in 2s having a lower energy compared to 2p (see Figure \(\PageIndex{4}\) for values).


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    Figure \(\PageIndex{2}\). Orbital Penetration. A comparison of the radial probability distribution of the 2s and 2p orbitals for various states of the hydrogen atom shows that the 2s orbital penetrates inside the 1s orbital more than the 2p orbital does. Consequently, when an electron is in the small inner lobe of the 2s orbital, it experiences a relatively large value of Z*, which causes the energy of the 2s orbital to be lower than the energy of the 2p orbital.


    An electron orbital's penetration effects its ability to shield other electrons and effects the extent to which it is shielded by other electrons. In general, electron orbitals that have greater penetration experience stronger attraction to the nucleus and less shielding by other electrons; these electrons thus experience a larger (Z*). Electrons in orbitals that have greater penetration also shield other electrons to a greater extent.

    Within the same shell value (n), the penetrating power of an electron follows this trend in subshells (ml):

    s>p>d>f

    Exercises

    Exercises

    1. Compare the 2s and 2p orbitals:
      (a) Which is closer to the nucleus on average?
      (b) Which is more penetrating?
      (c) Which orbital experiences a stronger Z* and is thus lower in energy (consider your experience, but also inspect Figures 1.1.2.3 and 1.1.2.4 from the previous page)? Please explain.
    2. Peruse the Hyperphysics page (click) that shows radial probability functions of several orbitals (click around on various orbitals). Compare the 2p and 3s orbitals:
      (a) Which is farther from the nucleus on average?
      (b) Which is more penetrating?
      (c) Which orbital is lower in energy?
    3. Which atom, Li, or N, has a stronger valence Z*? Explain why.
    4. Explain why 2s and 2p subshells are completely degenerate in a hydrogen atom.
    5. Which atom has a smaller radii: Be or F? Explain.
    6. Which electrons shield others more effectively: 3p or 3d?
    7. Use the clues given in the figures below to label the radial distribution functions shown.

    Screen Shot 2019-11-25 at 5.01.53 PM.png

    8. Examine the plot below. Notice that the probability density plots for the 3s, 3p, and 3d subshells are highlighted.
    (a) For which of these three functions is the highest probability density at the smallest \(r\) value (which closest to the nucleus on average)? Is this the same subshell that penetrates most?
    (b) Use this example to describe how penetration and shielding result in a splitting of subshell energy level in multi-electron atoms.

    9. Explain why the ground state (most energetically favorable) electron configuration of Be is 1s22s2 rather than alternative configurations like 1s22s12p1 or 1s22p2.

    Answer 1

    (a) The 2s orbital is closer to the nucleus on average.

    (b) The 2s orbital is more penetrating than 2p.

    (c) You might "know" that the 2s orbital is lower in energy than 2p because 2s fills first. But a close inspection of Figures 1.1.2.3 and 1.1.2.4 from the previous page indicates that while the 2s and 2p elements are degenerate in Ne (element 10), for elements with atomic number 11 and greater 2p has a higher Z* than 2s! This example illustrates that both average distance and penetration are factors in determining Z*, and the factor that is more important may change as we increase in atomic number.

    Answer 2

    (a) The 3s orbital reaches farther away from the nucleus and is on average farther from the nucleus than 2p.

    (b) The 3s orbital is more penetrating than 2p, even though 3s is farther on average!

    (c) The 2p orbital is lower in energy than 3s; this is because 2p is still significantly closer to the nucleus on average and experiences a stronger Z*. (Penetration is not the only consideration!)

    Answer 3

    A nitrogen atom has a stronger effective nuclear charge (Z*) than lithium due to its greater number of protons; even though N also has more electrons that would shield the nuclear charge, each electron only partially shields each proton. This means that atoms with greater atomic number always have greater Z* for any given electron.

    Answer 4

    The hydrogen atom has only one electron; thus there is no shielding to consider. When there are not other electrons to shield the nucleus, penetration and shielding are irrelevant, and subshells within a shell are degenerate.

    Answer 5

    Fluorine has a smaller radii than Beryllium due to F having a greater valence Z* and therefore pulling the valence electrons close to the nucleus and providing a smaller atomic radii.

    Answer 6

    3p shields better than 3d because p orbitals penetrate more than d orbitals within the same shell.

    Answer 7

    Screen Shot 2019-11-25 at 5.04.53 PM.png

    Answer 8

    3d is closest on average, but 3s penetrates most. The three subshells of \(n=3\) differ in their average distance and in their ability to penetrate; these factors result in differences in the Z* experienced by electrons in each orbital. We would expect 3s to be lowest in energy followed by 3p and then 3d.

    Answer 9

    This question is asking why the 2s orbital fills in Be before 2p is occupied. This is a multi-electron atom, therefore core electrons shield the 2s and 2p orbitals to different extents. In Be we expect the 2s orbital to fill before 2p because 2s penetrates more and experiences a higher Z*.

    Slater's rules for estimating Z*

    800px-Effective_Nuclear_Charge.svg.png

     

    Figure \(\PageIndex{3}\). Diagram illustrating effective nuclear charge according to Slater's rules.

    The Z* can be estimated using a number of different methods; probably the best known and most commonly used method is known as Slater's Rules. Slater developed a set of rules to estimate Z* based on how many other electrons exist in the atom and on the orbital location of the electron-of-interest. These two factors are important determinants in shielding, and they are used to calculate a shielding constant (S) used in Slater's formula:

    \[Z*=Z-S\]

    where Z is the actual nuclear charge (the atomic number) and Z* is the effective nuclear charge.

    In the calculation of S, it is assumed that electrons closer to the nucleus than our electron-of-interest cancel some of the nuclear charge; those farther from the nucleus have no effect. To calculate S, all the relevant orbitals in an atom are written out in order of increasing energy, separating them into in "groups". Each change in shell number is a new group; s and p subshells are in the same group but d and f orbitals are their own group. You write out all the orbitals using parentheses until you get to the group of the electron-of-interest, like this:

    (1s)(2s,2p)(3s,3p)(3d)(4s,4p)(4d)(4f)(5s,5p) etc.

    **Critical: The orbitals must be written in order of increasing energy!

    1. Electrons in the same Group(): Each other electron (not counting the electron-of-interest) in the same group () as the chosen electron, contributes 0.35 to S.
      Conceptually, this means electrons in the same group shield each other 35%.
    2. Electrons in Groups() to the left:
      • If the electron-of-interest is in a d or f subshell, every electron in groups () to the left contributes 1.00 to S.
        Conceptually, this means that d and f electrons are shielded 100% by all electrons in the same shell with a smaller value of \(l\), as well as all electrons in lower shells (\(n\)).
      • If the electron-of-interest is in an s or p subshell, all electrons in the next lower shell (n - 1) contribute 0.85 to S. And all the electrons in even lower shells contribute 1.00 to S.
        Conceptually, this means that s and p electrons are shielded 85% by the electrons one shell lower, and 100% by all electrons in shells n - 2 or lower.
    3. 1s electrons: S of a 1s electron is just (S=0.3\), no matter the element.

    A video explaining how to use Slater's Rules

    Example \(\PageIndex{1}\): Fluorine, Neon, and Sodium

    What is the Z* experienced by the valence electrons in the three isoelectronic species: fluorine anion (F-), neutral neon atom (Ne), and sodium cation (Na+)?

    Solution

    Each species has 10 electrons, and the number of core electrons is 2 (10 total electrons - 8 valence), but the effective nuclear charge varies because each has a different atomic number (Z). The approximate Z* can be found with Slater's Rules. For all of these species, we would calculate the same sigma value:

    Calculating \(S\): (1s)(2s,2p), \(S = 2(0.85) + 7(0.35) = 1.7 + 2.45 = 4.15 \)

    Fluorine anion: \(Z*= 9 - S = 9 - 4.15 = 4.85\)

    Neon atom: \(Z*= 10 - S = 10 - 4.15 = 5.85\)

    Sodium Cation: \(Z*= 11 - S = 11 - 4.15 = 6.85\)

    So, the sodium cation has the greatest effective nuclear charge.

    Exercise \(\PageIndex{1}\)

    Calculate Z* for a 3d-electron in a zinc (Zn) atom.

    Answer

    Write out the relevant orbitals: (1s)(2s,2p)(3s,3p)(3d) (4s)

    Notice that although 4s is fully occupied, we don't include it because in Zn, 4s is higher in energy than 3d. Therefore, 4s is to the right of the d electrons we are considering. The electron-of-interest is in 3d, so the other nine electrons in 3d each contribute 0.35 to the value of S. The other 18 electrons each contribute 1 to the value of S.

    \(S=18(1)+9(0.35)=21.15\)

    \(Z*=30-21.15=8.85\)

    So, although the nuclear charge of Zn is 30, the 3d electrons only experience a \(Z* \approx 8.85\)!

     

    "Best" values for Z*

    Slater's rules are a set of simple rules for predicting \(S\) and Z* based on empirical evidence from quantum mechanical calculations. In other words, the Z* calculated from Slater's rules are approximate values. The values considered to be the most accurate are derived from quantum mechanical calculations directly. You can find these values in a nice chart on the Wikipedia article of Effective Nuclear Charge. The chart is recreated here in Figure \(\PageIndex{3}\) for convenience:

    Screen Shot 2019-11-25 at 2.48.40 PM.png

    Figure \(\PageIndex{4}\). This chart shows Z* values calculated by Clementi et. al. in 1963 and 1967 that are consistent with SCF Calculations. This chart was created using data from the Wikipedia article on Effective Nuclear Charge.

    Z* modulates attraction

    When valence electrons experience less nuclear charge than core electrons, different electrons experience different magnitudes of attraction to the nucleus. A modified form of Coulomb's Law is written below, where \(e\) is the charge of an electron, Z* is the effective nuclear charge experienced by that electron, and \(r\) is the radius (distance of the electron from the nucleus).

    \[ F_{eff}=k \dfrac{Z*e^2}{r^2}\]

    This formula suggests that if we can estimate Z*, then we can predict the attractive force experienced by, and the energy of, an electron in a multi-electron atom (ex. Li).

    The attraction of the nucleus to valence electrons determines the atomic or ionic size, ionization energy, electron affinity, and electronegativity. The stronger the attraction, and the stronger Z*, the closer the electrons are pulled toward the nucleus. This in turn results in a smaller size, higher ionization energy, higher electron affinity, and stronger electronegativity.


    Close inspection of Figure \(\PageIndex{3}\) and analysis of Slater's rules indicate that there are some predictable trends in Z*. The data from Figure \(\PageIndex{3}\) is plotted below in Figure \(\PageIndex{4}\) to provide a visual aid to the discussion below.

    clipboard_ea00e5b5dbdff4d4f6582fb27c8fe75e0.png
    Figure \(\PageIndex{5}\): The Z* values for electrons in each subshell of the first 54 elements (H to Xe) are plotted. Each subshell is plotted as a different series (see legend) and the valence shell is highlighted by a solid black line with open circles.

    The Z* for electrons in a given shell and subshell generally increases as atomic number increases; this trend holds true going across the periodic table and down the periodic table. Convince yourself that this is true for any subshell by examining Figure \(\PageIndex{4}\). (CC-BY-NC-SA; Kathryn Haas)

    Do you notice any exceptions to this general trend?

    Inspection of Figure \(\PageIndex{4}\) should confirm for you that the Z* increases as Z increases for electrons in any subshell (like the 1s subshell for example, which is plotted above as a red line with square points). You can see this trend as the positive slope in each series. There is one obvious exception in Period 5 in elements 39 (Y) to 41 (Nb; the Z* of 4s actually decreases across these three elements as atomic number increases. There is also an exception between Y and Zr in the 3d subshell, and between Tc and Ru in the 5s subshell.

    For valence electrons:

    It is useful to understand trends in valence Z* because the valence Z* determines atomic/ionic properties and chemical reactivity. The trends in the valence Z* are not simple because as atomic number increases, the valence shell and/or subshell also changes. The valence Z* is indicated in Figure \(\PageIndex{4}\) as a black line with open circles.

    Down the table: As we go down a column of the periodic table, the valence Z* increases. This is a simple trend because type of subshell is consistent and there is an increase only in shell and in atomic number, Z. This trend is best illustrated by inspection of Figure \(\PageIndex{3}\).

    Across the table: the trend depends on shell and subshell, but generally Z* increases across a period.

    Periods 1-3 (s and p only): As we go across the table in periods 1-3, the shell stays constant as Z increases and the subshell changes from s to p. In these periods, there is a gradual increase in valence Z* as we move across any of the first three periods.

    Periods 4 and 5 (s, p, and d): Now we have some more complex trends because valence subshell and shell are changing as we increase in atomic number. Notice that the valence Z* generally increases going across a period as long as subshell isn't changing; the exception is within the 4d subshell (elements 39-44 or Y-Ru). In general, going from \((n)s\) subshell to \((n-1)d\) subshell, there a relatively large increase in valence Z*. And in going from \((n-1\)d\) subshell to \((n)p\) subshell, there is a relatively large decrease in Z*.

    From one period to another: From Figure \(\PageIndex{4}\), we can see that as we increase Z by one proton, going from one period to the next, there is a relatively large decrease in Z* (from Ne to Na, for example). This is because as Z increases by a small interval, the shell number increases, and so the electrons in the valence shell are much farther from the nucleus and are more shielded by all the electrons in the lower shell numbers.

    Exercises

    Exercise \(\PageIndex{2}\)

    1. Compare trends in Z* and atomic size. Explain how and why atomic size depends on Z*.

    2. Compare trends in Z* and ionization energy. Explain how and why ionization energy depends on Z*.

    Answer

    1. On the periodic table, atomic radius generally decreases across the periods (left to right) and increases down the groups. As atomic number increases across the periodic table, nuclear charge (Z) increases and Z* increases. In turn, the atomic radius decreases because the higher nuclear charge (and thus higher Z*) pulls electrons closer to the nucleus. Atomic radius increases down the periodic table because the shell number increases. Despite an increase in Z* going down the periodic table, larger atomic radii results from electrons occupying higher shells.

    2. Ionization energies (IE) are inversely related to atomic radius; IE increases across the periods and decreases down the groups. Since the nucleus holds valence electrons more strongly (due to higher Z*) across the periods, IE increases because valence electrons are harder to remove. Down the periodic table, larger atomic radius causes electrons in valence orbitals to be shielded by core electrons. Recall that shielding reduces the nuclear charge available to electrons in higher orbital levels, resulting in a lower Z*. With more shielding and lower Z*, the valence electrons are held less tightly by the nucleus such that ionization energy decreases (i.e. valence electrons are easier to remove).

     

     

    References

    1. Petrucci, Ralph H., William S. Harwood, F. Geoffrey Herring, and Jeffry D. Madura. General Chemistry: Principles and Modern Applications, Ninth Edition. Pearson Education Inc. Upper Saddle River, New Jersey: 2007.
    2. Raymond Chang. Physical Chemistry for Biological Sciences. Sausalito, California: University Science Books, 2005
    3. R. S. Mulliken, Electronic Structures of Molecules and Valence. II General Considerations, Physical Review, vol. 41, pp. 49-71 (1932)
    4. Anastopoulos, Charis (2008). Particle Or Wave: The Evolution of the Concept of Matter in Modern Physics. Princeton University Press. pp. 236–237. ISBN 0691135126. http://books.google.com/?id=rDEvQZhpltEC&pg=PA236.

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