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Chem 105 Friday 6 Nov 2009 1) Configurations of ions 2) Trends in atom size (atomic radius) 3) Trends in ion size 4) Ionization Potential 11/6/2009 1 Atomic Radius - Measured in picometers (pm) 1 pm = 10-12 m or Angstroms (Å) 1 Å = 100 pm = 10-8 cm - Generally increase going down a group (down a column) and decrease going across a period (L-to-R in a row) 11/6/2009 2 11/6/2009 3 Group 1 Alkali metals Group 8A Noble Gases 11/6/2009 4 One issue that arises with the term “Atomic Radius” is that the numbers may differ depending on how they are obtained! - “covalent radius” = half distance between bonded atoms or - “calculated radius” = distance out to arbitrary electron density based on quantum mechanics calculation (Schrödinger equation) or - “experimental” based on crystal of metal atoms = ½ interatomic distance 11/6/2009 5 Comparing the electron distribution in a H atom vs. H2 molecule The 0.015 e-/Å3 contour The 0.01 e-/Å3 contour We define the “calculated atomic radius” = 11/6/2009 distance from nucleus out to electron density ~ 0.015 e-/Å3 6 Comparing the electron distribution in a H atom vs. H2 molecule H atom radius = 44 pm H atom H covalent radius = 37 pm H2 molecule H-H dist = 74 pm Generally the covalent radius is smaller than the radius of free atom because the electrons in the 11/6/2009 molecule are attracted by two or more nuclei. This shrinks the whole electron cloud a bit. 7 Crystal structure (experimental) of metallic sodium. Na-Na distance = 365 pm; so, Na radius = 365/2 = 183 pm 365 pm 11/6/2009 8 Group 1 Alkali metals Sodium: 184 pm 11/6/2009 9 Going from element-to-element DOWN a group, you add a complete shell of electrons plus the same number of protons in the nucleus. For example, Group 2: Be, Mg, Ca… e- in 1s orbital e- in 2s orbital 4+ - 2e- - Berylium atom 2e- in 3s orbital - 12+ 2e- - 8e- 8e- in 2s,2p orbitals Magnesium atom Although the nuclear charge increases by 8+, adding a complete inner shell of 8 electrons shields the outer shell electrons from the increased positive charge. 11/6/2009 10 However, atoms get SMALLER going ACROSS a row Left-to-Right. In this case, electrons are added to the same shell - on the periphery of the atom, and the # of inner-shell electrons is constant. The outer-shell electrons DO NOT shield each other from the increasing nuclear charge because they are spread out with approximately same average distance from the nucleus. The nuclear charge increases by +1 for each electron added, and this added proton has a much larger effect on all the electrons compared to the effect of the added electron. 12+ 2e- 8e- Magnesium atom radius = 145 pm 11/6/2009 - - - 13+ 2e8e- - Aluminum atom radius = 118 pm 14+ 2e8e- - Silicon atom radius = 111 pm 11 Transition metals decrease, then increase slightly at end of the series (Cu, Zn) 11/6/2009 12 21+ 2e8e8e- - - Scandium atom radius = 144 pm Decreases due to the increasing nuclear charge which is not shielded by outer electrons. 11/6/2009 - - - 26+ 2e8e8e- - - Iron atom radius = 117 pm - - - - 30+ 2e8e8e- - - Zinc atom radius = 125 pm Increase (or constant depending on which measurement). Now there are SO MANY ELECTRONS in outer shell – they expand due to mutual repulsion. (covalent radii, Inorganic Chemistry, Miessler and Tarr, p 42) 13 Sizes of Ions Cations (remember ca + ion) always SMALLER than corresponding atom (you’re removing electrons – usually a whole shell - without changing the nuclear charge) Anions Always LARGER than corresponding atom (you’re adding electrons – to complete a shell usually – without changing the nuclear charge.) 11/6/2009 14 12+ 2e- - 12+ 2e8e- - 8e- Magnesium atom radius = 145 pm Magnesium 2+ ion radius = 72 pm Huge shrinkage - you’re stripping away the whole outer shell - 17+ 2e8e- - - - Chlorine atom radius = 99 pm 11/6/2009 - - 17+ 2e8e- - - Chloride ion (Cl-) radius = 181 pm Outer shell expands (a lot) because you’re adding an e- without adding a proton in the nucleus. 15 Atom and Common Anion Size Comparison These 3 anions have 10 e- radii in picometers pm 11/6/2009 16 Cations radii in picometers pm 11/6/2009 17 Place the following atoms in order of increasing atomic radii: K, Mg, Ca, Rb... K < Mg < Ca < Rb K < Mg < Rb < Ca Mg < Ca < K < Rb K < Rb < Mg < Ca Mg < K < Ca < Rb N = 75 21% 9% a R < C < K b M g < R K < < g M a M g < < Ca g M < K < R < K b R < C < g 11/6/2009 < M Rb Ca K K C a C < R < a Mg b 1% b 1% b 1. 2. 3. 4. 5. 67% Excellent work, chem students! JK 18 (OWL question on ion sizes) 11/6/2009 19 11/6/2009 20 Ionization energy (= ionization potential) definition A (g) ---> A+ (g) + 1eA+ (g) ---> A2+ (g) + 1e- 11/6/2009 ∆E = 1st ionization energy (kJ/mol) ∆E = 2nd ionization energy (kJ/mol) 21 First Ionization Energies H(g) H+(g) + e- ΔH = +1312.0 kJ (For comparison, the thermite reaction gives off way less energy per mole of iron oxide consumed. Fe2O3(s) + 2 Al(s) Al2O3(s) + 2 Fe(l) He(g) He+(g) + e- ΔH = -851.5 kJ) ΔH = +2372.3 kJ The first ionization energy for helium is about twice the ionization energy for hydrogen because each electron in helium feels the attractive force of two protons, instead of one. Far less energy is required to remove an electron from a lithium atom, which has three protons in its nucleus. Li(g) Li+(g) + e- 11/6/2009 ΔH = +572.3 kJ 22 The 1st ionization energy (A --> A+ + e-) decreases going down a group. He Ne Group 8A Ar Kr Xe Li 11/6/2009 Na K Rb Cs Rn Fr 23 The 1st ionization energy (A --> A+ + e-) decreases going down a group. Li Na K Rb Cs Fr Group 1A 11/6/2009 24 Electron in 2s orbital Li Na + Electron in 3s orbital Electron in 4s orbital - + As you go down a group, the outermost electron(s) are further from nucleus, and are easier to remove. - This is the same order as the chemical reactivity of these metals as reducing agents. (“Donate electrons”) K > Na > Li K + 11/6/2009 Another way to put this is that, as you go to larger atoms in the same group, the effective nuclear charge decreases due to shielding by inner electrons. 25 The 1st ionization energy (A --> A+ + e-) generally increases going (L-to-R) across a row. He Ne Ar Kr You’re adding a proton in nucleus and electron around the periphery of atom. Electrons in the same shell do not shield each other from the nuclear charge – too spread out. Xe Li 11/6/2009 Na K Rb Cs Rn Fr 26 In Period 2, B,C, and N (and O, F, and Ne even more so) all have slightly lower ionization potentials than expected. He Ne l ai t n e t o P n o it az i n o I ts 1 F N H Be C O Mg B Li 0 1 2 3 Na 4 5 6 7 8 9 10 11 12 13 Atomic Number -> 11/6/2009 27 B and O have anomolously low 1st ionization potentials, which means an outer electron is unexpectedly easy to remove. Boron, carbon, and nitrogen outermost electrons go into a p-orbital, which is less stable than an s-orbital (). Oxygen, fluorine and neon outermost electrons go into a two-electron p-orbital () . These are further destabilized by electron-electron repulsion within the orbital. 11/6/2009 28 The End 11/6/2009 29