chapter1 - Chapter 1 1.9 I(Ca) vs. I(Zn)? The first...

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Chapter 1 1.9 I (Ca) vs. I (Zn)? The first ionization energies of calcium and zinc are 6.11 and 9.39 eV, respectively (see Appendix 1). Both of these atoms have an electron configuration that ends with 4s 2 : Ca is [Ar]4s 2 and Zn is [Ar]3d 10 4s 2 . An atom of zinc has 30 protons in its nucleus and an atom of calcium has 20, so clearly zinc has a higher nuclear charge than calcium. Remember though, it is effective nuclear charge (Z eff ) that directly affects the ionization energy of an atom. Since I (Zn) > I (Ca), it would seem that Z eff (Zn) > Z eff (Ca). How can you demonstrate that this is as it should be? The actual nuclear charge can always be readily determined by looking at the periodic table and noting the atomic number of an atom. The effective nuclear charge cannot be directly determined, i.e., it requires some interpretation on your part. Read Section 1.6, Penetration and shielding, again. Study the trend for the period 2 p-block elements in Table 1.3. The pattern that emerges is that not only Z but also Z eff rises from boron to neon. Each successive element has one additional proton in its nucleus and one additional electron to balance the charge. However, the additional electron never completely shields the other electrons in the atom. Therefore, Z eff rises from B - Ne. Similarly, Z eff rises through the d block from Sc - Zn, and that is why Z eff (Zn) > Z eff (Ca). 1.10 I (Sr) vs. I (Ba) vs. I (Ra)? The first ionization energies of strontium, barium, and radium are 5.69, 5.21, and 5.28 eV, respectively (see Table 1.6). Normally, atomic radius increases and ionization energy decreases down a group in the periodic table. However, in this case I (Ba) < I (Ra). Study the periodic table, especially the alkaline earths. Notice that Ba is eighteen elements past Sr, but Ra is thirty-two elements past Ba. The difference between the two corresponds to the fourteen 4f elements between Ba and Lu. As explained above in the answer to Exercise 1.9, Z eff rises with each successive element because of incomplete shielding. Therefore, even though radium would be expected to have a larger radius than barium, it has a higher first ionization energy because it has such a large Z eff . 1.11 I
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chapter1 - Chapter 1 1.9 I(Ca) vs. I(Zn)? The first...

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