chap07_bch1049_07MCWC1b

chap07_bch1049_07MCWC1b - Periodic Properties of the...

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1 Periodic Properties of the Elements Periodic Properties of the Elements (Chapter 7) What’s Ahead: 1. Effective Nuclear Charge Effective Nuclear Charge 2. 2. Trends and Changes: Atomic Size, Ionization Energy, Trends and Changes: Atomic Size, Ionization Energy, Electron Affinity Electron Affinity 3. Periodic Trends Periodic Trends and Properties Periodic Properties of the Elements Effective Nuclear Charge • In a many-electron atom, electrons are both attracted to the nucleus and repelled by other electrons, so the nuclear charge that an electron experiences depends on both factors. • Valence electrons are attracted to the nucleus, but inner (core) electrons will weaken this attraction . • In other words, core electrons ‘shield’ or ‘screen’ the outer electrons from the effect of the nucleus. Simplified estimate of nuclear charge for sodium: Simplified estimate:
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2 Periodic Properties of the Elements Effective nuclear charge, Z eff : Z eff = Z S where Z is the atomic number (number of protons in nucleus) and S is a ‘shielding’ constant , which is often close to the number of inner electrons. Effective Nuclear Charge penetration Actually, situation is more complex; we must consider electron distribution of atomic orbitals: 3 s electron has some probability of appearing inside the [Ne] core electrons (penetration) . Hence the core does not efficiently shield the 3 s electron from nucleus ; Z eff for the 3 s electron in Na is > 1+ (2.5+). Simplified estimate: Periodic Properties of the Elements Effective Nuclear Charge • In a many-electron atom, energies of orbitals with same n value increase with higher values of l , e.g. for carbon (1 s 2 2 s 2 2 p 2 ), energy of 2 p orbital ( l = 1) is higher than 2 s ( l = 0). WHY?
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This note was uploaded on 01/11/2011 for the course BCH 1049 taught by Professor Michaelcwchan during the Spring '06 term at City University of Hong Kong.

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chap07_bch1049_07MCWC1b - Periodic Properties of the...

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