Problem%20Set%205%20301%20S11%20Sparks%20Key - Problem Set...

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Problem Set 5 Dr. Sparks, CH 301 Periodic Trends 1. Briefly describe the reasons for and effects of the following periodic trends across a row and down a column: effective nuclear charge (Z eff ), atomic radii, ionic radii, ionization energy, electron affinity, and electronegativity. Across a period, the number of electrons and protons increase but the number of shielding electrons (electrons in lower shells) remains the same, resulting in an increasing effective nuclear charge (ENC or Z eff ) . Down a group the electrons occupy shells that are farther from the nucleus and these electrons are more shielded, so ENC decreases. Atomic radius decreases from left to right across a period because the ENC increases pulling electrons closer to the nucleus. Atomic radius increases down a group because of the increasing size of orbitals with higher principal quantum numbers, and decreasing ENC. Ionic radius decreases from left to right across a period. Elements experience a higher ENC and electrons will be more tightly bound (closer to the nucleus). In general, cations are smaller than anions because they have lost electrons. Hence, they have usually lost a whole outer energy level and they have more protons than electrons, allowing the protons to more effectively pull the electrons closer in towards the nucleus. Ionic radius increases as you go down a group because they have more energy levels. Ionization energy generally increases from left to right across a period due to increasing ENC. There are some exceptions where atoms with half-filled shells are more stable than atoms with more electrons, such as nitrogen, 1s 2 2s 2 2p 3 , versus oxygen, 1s 2 2s 2 2p 4 . Ionization energy decreases down a group because electrons further away from the nucleus are more shielded. Electron affinity generally increases (becomes more negative) across a period from left to right due to increasing ENC, making it more attractive for an electron to join the atom. An element in Group VIIA becomes more stable by gaining an electron and having a filled valence shell than does an element in Group 1A. Down a group, electron affinity generally decreases. Any additional electrons will be very shielded and not very attracted to the nucleus. Electronegativity generally increases from left to right across a period. The atoms in groups on the right side of the periodic table attract electrons to themselves in order to have a stable filled valence orbital, like the noble gases. Down a group, ENC decreases, so atoms are less likely to attract electrons. Thus, electronegativity decreases down a group. 2. Arrange the following in terms of increasing effective nuclear charge
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This note was uploaded on 11/05/2011 for the course CH 301 taught by Professor Fakhreddine/lyon during the Fall '07 term at University of Texas at Austin.

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Problem%20Set%205%20301%20S11%20Sparks%20Key - Problem Set...

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