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Unformatted text preview: kapur (rk7259) – Homework 4 – Lyon – (52380) 1 This print-out should have 40 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. This HW assignment is due Tuesday, February 24, by 11PM. 001 10.0 points In the neutral Al atom, what are the effective nuclear charges for a 1 s electron and a valence electron, respectively? 1. +12; +3 2. +3; +3 3. +13; +3 correct 4. +10; +13 5. +12; +12 Explanation: The effective nuclear charge is total protons- total inner shell electrons . Neutral Al has 13 protons and 13 electrons (obtain this number from the periodic table). There are 2 electrons in the n = 1 inner shell, 8 electrons in the n = 2 inner shell, and 3 electrons in the n = 3 valence shell. The 1 s electron is in the n = 1 shell, the shell closest to the nucleus. There are no shells between n = 1 and the nucleus so the effective nuclear charge would be the full charge of the nucleus. Electrons in the same shell do not shield against one another. The effective nuclear charge is 13- 0 = +13 . The valence electron is in the n = 3 shell. There are 2 shells, the n = 1 and n = 2 shells between the valence electron and the nucleus that act to shield the nucleus. The total inner shell electrons from n = 1 and n = 2 is 10. The effective nuclear charge is 13- 10 = +3 . 002 10.0 points Which of the following correctly rationalizes the increase in atomic radii down and to the left on the periodic table, based on what we discussed in class? 1. As you move to the left across a period, decreasing ENC means the outer electrons are less tightly held and can move further from the nucleus. As you move down a group, the electrons occupy orbitals that are further from the nucleus. correct 2. Because the elements are easier to ion- ize, they have a larger electronegativity, and therefore their electron affinity is not suffi- cient to reduce atomic radii. 3. The periodic table was set up to group atoms by size to facilitate comparisons. 4. None of these. Atomic radii increase up and to the right. 5. The elements are simply larger due to more protons and neutrons. 6. Larger elements have an increasing pro- portion of d and f orbitals, which are intrinsi- cally larger than all of the s and p orbitals. Explanation: We used ENC to rationalize the overall pe- riodic trends in class. The nuclei are larger due to more protons and neutrons, but not the atom as a whole – most of an atom’s size is the space taken up by the electrons, the nucleus is tiny in comparison. Orbital size in- creases with the principal quantum number n so that a 7 s orbital would probably be larger than a 4 f orbital – the d and f orbitals are not automatically larger. The periodic table was organized by properties of the macroscopic el- ements, not by the size of the atoms. The other answer choice is just a mindless spewing of chemistry vocabulary....
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This note was uploaded on 01/30/2010 for the course CH 301 taught by Professor Fakhreddine/lyon during the Spring '07 term at University of Texas.
- Spring '07