PERIODIC PROPERTIES Chapter 8 "...I have tried to base a system on the magnitudes of the atomic weights of the elements. My first attempt in this respect was the following: I chose the smallest atomic weights and arranged them according to the sizes of their atomic weights. This showed that there existed a periodicity in the properties of these simple substances and that even according to their atomicity [valence] the elements followed one another in the arithmetical sequence of their atomic weights." Dimitri Ivanovich Mcndeleyev (Mendeleev), 1869 "The periodicity in the properties of the elements is connected with the continuing build up and completion of the various electron groups that takes place with increasing atomic number." Niels Henrik David Bohr, 1923 (Nobel Prize in Physics in 1922 "for his services in the investigation of the structure of atoms and of the radiation emanating from them".) Multi-Electron Atoms (8.1-8.5) electron spin Dirac spin quantum number, m s magnetic properties diamagnetic paramagnetic ferromagnetic aufbau (filling up) principle Pauli exclusion principle Hund's Rule 1A Atomic Electron Configurations (8.4) 1 2 H He 1s 1 2A 3A 4A 5A 6A 7A 3 4 5 6 7 8 9 10 Li Be B C N O F Ne 2s 1 11 12 13 14 15 16 17 18 know these exceptions to normal filling order Na Mg Al Si P S Cl Ar 3s 1 3B 4B 5B 6B 7B --------- 8B --------- 1B 2B 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 4s 1 3d 5 4s 1 3d 10 4s 1 37 38 39 4 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 5s 1 4d 4 5s 1 4d 5 5s 1 4d 7 5s 1 4d 8 5s 1 4d 10 4d 10 5s 1 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 6s 1 5d 9 6s 1 5d 10 6s 1 87 88 89 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og 7s 1 6d 10 7s 1 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Lanthanides Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 4f 1 d 1 s 2 4f 7 d 1 s 2 4f 14 d 1 s 2 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Actinides Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 6 d 2 7s 2 5f 2 d 1 s 2 5f 3 d 1 s 2 5f 4 d 1 s 2 5f 7 d 1 s 2 5f 14 d 1 s 2 8A
- 2 - ground state electron configuration octet in previous level FIG I Aufbau (Building Up) Principle (8.3) Hund Pauli EX. 1 For the element lead (Pb) a) Give the full electron configuration of its ground state. b) Give the electron configuration of its ground state using the noble gas core. c) Give the orbital diagram of its valence electrons. d) Predict the most stable ions for Pb.
- 3 - Screening and Effective Nuclear Charge (8.3) FIG II Li Effective Nuclear Charge 2s electron penetrates 1s electron cloud and lowers its energy Li: 1s 2 2s charge that the 2s electron experiences when it is closer or further from the nucleus. shielding (screening) core electrons shield outer electrons from full effect of nuclear charge effective nuclear charge nuclear charge actually experienced by outer electrons due to shielding, decreases with increasing shielding penetration probability of electron being closer to nucleus; order is s < p < d - penetration increases nuclear attraction and decreases shielding so that the energy is lowered Cartoon of Effective Nuclear Charge Due to Screening
- 4 - Periodic Trends (8.6 8.8) FIG III Size of Atoms and Ions (8.6 8.7) trends increased size of atomic orbitals down a group increased effective nuclear charge across a period EX 2. Arrange the following in order of increasing radius and explain your reasoning: Mg 2+, Ar, Br -, Ca 2+ FIG IV First Ionization Energies (IE) of the Elements (8.7): X(g) X + (g) + e - IE energy of orbital from which e - came 1st IE removes highest energy e - (least bound); successive IEs higher in energy distinction between core and valence electrons seen in higher IE reveals shell, subshell structure noble gas configuration good metals effect of e - repulsions EX 3. Arrange the following in order of increasing ionization energy and explain your reasoning: Na, Na +, O, Ne EX 4. The first ionization energy for He is the highest for any element. Which element would have the highest second ionization energy? Explain.
- 5 - FIG V Electron Affinities (EA) of the Elements (8.8) X(g) + e - X - (g) negative values when reaction is exothermic and anion is a more stable species than neutral atom no atom has negative EA for adding a second e - noble gases and most alkaline earths have positive EAs reveals shell, subshell structure noble gas configuration effect of e - repulsions EX 5. Why is the electon affinity of Cl more negative than the electron affinity of S? FIG VI Electronegativities (EN) of the Elements (9.6) ability of an atom in a molecule to attract shared electrons to itself roughly combines IE and EA into one property EX 6. Arrange the following in order of increasing electronegativity and explain your reasoning: H, F, Al, O
- 6 - BONDING BASICS (Review) Chapter 9 "There are therefore Agents in Nature able to make the Particles of Bodies stick together by very strong Attractions.And it is the Business of Experimental Philosophy to find them out." Isaac Newton, 1717 "Two atoms may conform to the rule of eight, or the octet rule, not only by the transfer of electrons from one atom to another, but also by sharing one or more pairs of electrons. These electrons which are held in common by two atoms may be considered to belong to the outer shells of both atoms." Gilbert Newton Lewis, 1916 "We shall say that there is a chemical bond between two atoms or groups of atoms in case that the forces acting between them are such as to lead to the formation of an aggregate with sufficient stability to make it convenient for the chemist to consider it as an independent molecular species." Linus Carl Pauling, 1939 (Nobel Prize in Chemistry in 1954 "for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances" and Nobel Peace Prize in 1962.) CLASSICAL DESCRIPTION OF BONDING (9.2) Chemical bond forces that hold a group of atoms together causing them to function as a unit and lowering the energy of the system Experimental evidence: ionization energy minimum energy to remove an electron in the gas phase electron affinity energy change when an electron is added to a gaseous atom Idealized conceptual models: 1) ionic bond (9.2-9.4) e - transferred 2) covalent bond (9.5) e - shared Coulomb s Law: V = k qq 1qq 2 rr or F = k qq 1qq 2 rr 2 3) polar covalent bond (9.6) partial transfer
-6- Seven SIMPLE BONDING CONCEPTS (9.7-9.9) 1) Lewis structures guidelines (9.7 9.9) determine total number of valence electrons from group numbers (GN); most atoms obey octet rule exceptions to octet rule: 2 H, He 4 Be 6 B, Al species with odd number of electrons some species with an element beyond 2nd period draw structure with all bonding pairs and lone pairs using symmetry show all resonance structures (atoms do not move) determine formal charge (FC) on all atoms having any FC = GN number of lone pair electrons 1/2 number of bonding pair electrons valence shell expand (VSE) if atom is beyond 2nd period and 1) has a formal charge separation or 2) octet rule gives insufficient number of bonding electrons; VSE to minimize FC separation while maximizing number of resonance structures 1. NH 3 2. CO 3. C 2 H 2 4. CO 3 2-5. NaOH 6. SO 3 2) resonance (9.8) all resonance structures have same number of bonds and lone pairs 3) formal charge (9.8) usually more electronegative element has neg charge; but CO
- 7 - THE SHAPES OF MOLECULES Chapter 10.1-10.5 "... the size of the valency group... assume that the electron pairs occupy much the same positions whether they are shared or not... determines, the type of spatial arrangement adopted... With a quartet of electron, the molecule is linear (as in Cl-Hg-Cl). With a sextet, the arrangement is planar, and the valency angles 120, giving with a covalency of 3 the plane symmetrical molecule (as in BF 3 ) and where it is 2-covalent, as in SnCl 2 a triangular molecule. With an octet there appear[s]... the tetrahedron..., the 2-covalent being triangular and the covalent pyramidal... The decet when fully shared (5-covalent) gives the trigonal [bi]pyramid... The 2-covalent decet is... linear, as if derived from the trigonal bipyramid by removing all three equatorial groups. The duodecet when fully shared (6- covalent) is octahedral... The 4-covalent form is... square, and so to be derived from the octahedron by removing two trans groups." Nevil V. Sidgwick and Herbert M. Powell, 1940 "The stereochemistry of an atom in any particular molecule depends on the number of pairs of electrons in its valency shell... The general arrangement of the valencies around any atom is determined by the fact that the lone pairs... arrange themselves as far apart as possible... A more detailed and exact description of the shapes of molecules can be given if it is assumed... that a lone pair repels other electron pairs more than a bonding pair of electrons..." Ronald J. Gillespie and Ronald S. Nyholm, 1957 "We were concerned about how to best give students some understanding of the shapes of inorganic molecules... We found that we could explain the shapes of essentially all molecules of the type AX on the basis that the electron pairs in the valence shell of a central atom keep as far apart as possible. Moreover, by making allowances for the differences between bonding and nonbonding electron pairs, we could account for small deviations from the basic idealized shapes..." Ronald J. Gillespie, 1984 4) Valence Shell Electron Pair Repulsion (VSEPR) Theory repulsions: BP/BP < BP/LP < LP/LP steric number (number of electron pairs ) => electronic geometry molecular geometry (shape) => bond angles, distortions electronegativity => bond polarity molecular polarity => dipole moment FIG VIII Various Geometries Predicted by VSEPR FIG VII Electron Pair Geometries
- 8 - linear, bent, and trigonal planar geometries for two and three electron pairs (steric number = 2, 3) FIG IX Molecular Geometries for Four Electron Pairs (Steric Number = 4) 5) valence shell expansion (VSE) FIG X Molecular Geometries for Five Electron Pairs (Steric Number = 5) FIG XI Placement of Lone Pairs of Electrons; Distortions from Perfect VSEPR Geometry
-9- FIG XII Molecular Geometries for Six Electron Pairs (Steric Number = 6) Bond Polarity, Electronegativity, and Dipole Moment (9.6, 10.5) 6) for molecular polarity the molecule must have a dipole moment: 1. have a polar bond 2. vector addition of bond dipoles is nonzero red - negative blue - positive Electrostatic Potential Surfaces FIG XIII Polarity of Some Tetrahedral Molecules
6) electronegativity and atomic size effects - 10 - Many bond angles can be rationalized on the basis of electronegativity or size arguments. In a bond between elements of differing electronegativity, the more electronegative element pulls the bonding pair electrons more strongly to itself. If a central atom is surrounding by atoms of large electronegativity, the bonding electrons are drawn away from the central atom, reducing the repulsive effect of these electrons and leading to smaller bond angles. On the other hand, if the central atom has a large electronegativity, bonding electrons are pulled toward it, increasing electronic repulsions and a larger bond angle results. In comparing a bond containing a larger atom to one containing a smaller atom, the bond with the larger atom naturally has its bonding pair of electrons further removed from it just due to its size. So a molecule with a large central atom would tend to have smaller bond angles than a smaller central atom. when is electronegativity the more important effect when is size the more important effect The effect of electronegativity and size revealed by the periodic table: EX 7. Determine the stronger acid in the following pairs and explain why. a) H 3 PO 3 or H 3 PO 4 b) CH 4 or NH 3 c) H 2 AsO - 2-4 or HAsO 4 d) HIO or HClO e) H 2 Se or H 2 Te EX 8. Which compound has the smallest bond angle in each series? Why? a) PI 3 AsI 3 SbI 3 b) SbI 3 SbBr 3 SbCl 3 Bonding Theory: Valence Bond Lewis structures (based on valence electrons) Valence Shell Electron Pair Repulsion (VSEPR) Theory Bonding based on valence atomic orbitals (coming next!) overlap of orbitals on adjacent atoms hybridize when necessary