Tentative material to be covered for Exam 2 (Wednesday, October 27) Chapter 17 Many-Electron Atoms and Chemical Bonding 17.1 17.2 17.3 17.4 17.5 17.6 Many-Electron Atoms and the Periodic Table Experimental Measures of Orbital Energies Sizes of Atoms and Ions Properties of the Chemical Bond Ionic and Covalent Bonds Oxidation States and Chemical Bonding (will not be covered on exam) Chapter 18 Molecular Orbitals, Spectroscopy, and Chemical Bonding 18.1 18.2 18.3 18.4 18.5 Diatomic Molecules Polyatomic Molecules The Conjugation of Bonds and Resonance Structures The Interaction of Light with Molecules Atmospheric Chemistry and Air Pollution New material on course web site: Practice exams for Chapters 17 and 18 are on line Answers to Chapter 16 will be distributed by tomorrow morning The slide show for Exam 2 has been upgraded. For those having trouble viewing it, please go to room 211 Havemeyer where the slide show is available from 9-5 A new Powerpoint slide show on material in Chapter 18 has been added Multielectron atoms: Every electron in a multielectron atom is assigned four quantum numbers (n, l, ml and ms) that uniquely define its chemical and physical properties. From the use of quantum numbers, we can envision every electron of a multielectron atom in terms of a characteristic energy (En), size (r), shape (l), orientation (ml) and spin (ms). The highest energy (valence) electrons are of greatest chemical interest The electron configurations of multielectron atoms Row Configuration Shorthand First: 1s2 2[He] Second: 2s22p6 10[Ne] Third: 3s23p6 18[Ar] Fourth: 4s23d104p6 36[Kr] Fifth: 5s24d105p6 54[Xe] Perodic Law by mass: The properties of the atoms of the elements vary periodically with the atomic weights of the elements. All chemical and physical properties of the elements depend on their atomic weights and vary periodically with atomic weight. Periodic Law by electronic structure: The ground state electron configuration of the atoms of elements vary periodically with the atomic number Z. The chemical and physical properties of the elements depend on the electron configurations of the atoms and vary periodically with atomic number. Bohr Atom as a model for the periodic law: En = -(Z2/n2)Ry rn = (n2/Z)a0 = = energy of electrons radius of a Bohr orbit Structure of atoms: Quantum numbers of electrons Electron configurations Core electrons and valence electrons Periodic properties of atoms: Energy required to remove and add an electron Size of atoms The atomic electron configurations of first five rows of the periodic table Row Configuration First: Second: Third: Fourth: Fifth: 1s2 2s22p6 3s23p6 4s23d104p6 5s24d105p6 Shorthand 2[He] 10[Ne] 18[Ar] 36[Kr] 54[Xe] The atomic electron configurations of first five rows of the periodic table give the elements their signature characteristics of metals and non-metals Row Configuration Shorthand First: Second: Third: Fourth: Fifth: 1s2 2s22p6 3s23p6 4s23d104p6 5s24d105p6 2[He] 10[Ne] 18[Ar] 36[Kr] 54[Xe] The Periodic Table built up by electron configurations: the ground state electron configurations of the valence electrons of the elements Shells, subshells and orbitals Shell: a collection of orbitals with the same value of n Example: the orbitals 3s, 3p, 3d comprise a shell with n = 3 Subshell: a collection of orbitals with the same value of n and l Example: for the n = 3 shell there are three subshells, the 3s subshell, the 3p subshell and the 3d subshell Orbital: the individual components of a shell or subshell Example: the px, py and pz orbitals are the components of any p (l = 1) subshell More about Shells, subshell and orbitals Each shell of principal quantum number n contains n subshells n = 1, only one subshell (s) n = 2, two subshells (s, p) n = 3, three subshells (s, p, d) Each subshell of quantum number l contains (2l + 1) orbitals l = 0, (2x0 + 1) = 1 orbital (s) l = 1, (2x1 + 1) = 3 orbitals (px, py, pZ) l = 2, (2x2 + 1) = 5 orbitals (dxy, dyz, dxz, dx2 - y2, dz2) The number of orbitals for a given n is n2 (solutions to wave equation) For n = 1, one orbital; for n = 2, four orbitals, for n = 3, nine orbitals The number of electrons that can fill a given shell = 2n2 For each orbital, there can be a maximum of 2 occupying electrons Building up the Periodic Table For the second row of the periodic table, the valence electrons are electrons in the s and p orbitals: valence electrons = snpm (n less than or equal to 2 and m less than or equal to 8) Atom 3Li 4Be 5B 6C 7N 8O 9F 10Ne Configuration Core/Valence electrons [Ne]2s () [Ne] 2s2 () [Ne] 2s22p1 () [Ne] 2s22p2 () [Ne] 2s22p3 () [Ne] 2s22p4 () [Ne] 2s22p5 () [Ne] 2s22p6 () Magnetic Properties Paramagnetic Closed shell (diamagnetic) Paramagnetic Paramagnetic Paramagnetic Paramagnetic Paramagnetic Closed shell (diamagnetic) How do electronic configurations connect with valence electrons and Lewis structures? Correlation of valence electron and Lewis structures: 2p indicates an unpaired electron in a 2p orbital N 2 [He]2s22px2py2pz O 2 [He]2s22px22py2pz F 2 Ne 2 [He]2s22px22py22pz [He]2s22px22py22pz2 Filled shell Building up the third row of the periodic table: From Na to Ar Atom 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar Configuration Magnetic properties Core/Valence electrons [Ne]2s () [Ne] 2s2 () [Ne] 2s22p1 () [Ne] 2s22p2 () [Ne] 2s22p3 () [Ne] 2s22p4 () [Ne] 2s22p5 () [Ne] 2s22p6 () Paramagnetic Closed shell (diamagnetic) Paramagnetic Paramagnetic Paramagnetic Paramagnetic Paramagnetic Closed shell (diamagnetic) The fourth row of the periodic table Atom Configuration 19K 18[Ar]4s 20Ca 18[Ar]4s2 _________________________________ 31Ga 18[Ar] 33As 18[Ar] 32Ge 34Se 35Cl 36Kr 18[Ar] 18[Ar] 18[Ar] 18[Ar] ten d orbitals fill up 3d104s24p1 3d104s24p2 3d104s24p3 3d104s24p4 3d104s24p5 3d104s24p6 What about 21M through 30M? The n + l rule: The ordering of the energies of the orbitals in a multielectron atom increase with the value of n + l. When two orbitals have the same value of n + l the orbital with the lower value of n has the lower energy state. Orbital (n + l) 1s 2s 2p 3s 3p 4s 3d 4p 5s (1 + 0 = 1) (2 + 0 = 2) (2 + 1 = 3) (3 + 0 = 3) (3 + 1 = 4) (4 + 0 = 4) (3 + 2 = 5) (4 + 1 = 5) (5 + 1 = 6) Comment Lower n (2p versus 3s) has lower energy Lower n (3p versus 4s) has lower energy Lower n (3d versus 4p) has lower energy Order of filling for first four rows: 1s2 2s22p6 3s23p4 4s23d104p6 5s2 Following the (n + l) rule, the electron configurations of the transition elements of the fourth row 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 18[Ar]4s23d 18[Ar]4s23d2 18[Ar]4s23d3 18[Ar]4s23d4 instead 18[Ar]4s1()3d5 () 18[Ar]4s23d5 half filled half filled 18[Ar]4s23d6 18[Ar]4s23d7 18[Ar]4s23d8 18[Ar]4s23d9 instead 18[Ar]4s1 ()3d10 () 18[Ar]4s23d10 half filled filled The “surprises” for electron configurations at 24Cr and 29Cu are due to the special stability of half filled subshells and filled subshells. The screening of outer electrons by the core electrons is the basis of Zeff The Ar atom has shells as shown in the profile of electron density as a function of distance from the nucleus This screening is the meaning of the symbol 18[Ar] The 4s electron of K = [Ar]4s. is screened by the 18[Ar] core electrons The spatial distribution of ns electrons as a function of r The spatial distribution of s, p and d electrons as a function of r Effective nuclear charge Effective nuclear charge, Zeff: the net positive charge attracting an electron. An approximation to this net charge is Zeff(effective nuclear charge) = Z(actual nuclear charge) - Zcore(core electrons) The core electrons are in subshell between the electron in question and the nucleus. The core electrons are said to “shield” the outer electrons from the full force of the nucleus. Example: A 3s electron is in an orbital that is closer to the nucleus than a 3 p orbital. Therefore, an electron in a 3s orbital is less shielded from the nucleus than the 3 p orbital. Rule: In many electron atoms, for a given value of n, Zeff decreases with increasing l, because screening decreases with increasing l For a given n: s < p < d < f Since the energy of an orbital depends on Zeff, in a many electron atom, for a given value of n, the energy of a orbital increases with increasing value of l. Using electron configurations to explain Ionization Energies Removal of an electron from a neutral atom: IE1 En = -(Z2/n2) En = -(Z2) if n is fixed (across a row) En = -(n2) if Z is fixed (down a row) IE1 3Li 4Be 5B 6C 7N 8O 9F 10Ne [Ne]2s () [Ne] 2s2 () [Ne] 2s22p1 () [Ne] 2s22p2 () [Ne] 2s22p3 () [Ne] 2s22p4 () [Ne] 2s22p5 () [Ne] 2s22p6 () Using electron configurations to explain Ionization Energies Removal of the second electron from the cation: IE2 IE2 (removal from E+) 3Li 4Be 5B 6C 7N 8O 9F 10Ne [Ne] [Ne] [Ne] [Ne] [Ne] [Ne] [Ne] [Ne] 2s2 () 2s22p1 2s22p2 () 2s22p3 () 2s22p4 () 2s22p5 () 2s22p6 () From the Bohr atom to all atoms: a model for the size of atoms. r = a0(n2/Z) so that for the same value of n r a a0(1/Zeff) When electrons are added to the same shell (same value of n) they are about the same distance from the nucleus as the other electrons in the shell. The electrons in a shell with the same n are spread out and do not shield each other from the positive charge of the nucleus very well. Thus, the effective nuclear charge, Zeff, increases as Z increases across the periodic table. The increasing value of Zeff draws the electrons in closer to the nucleus, and the atom becomes more compact. Conclusion: The atomic radius of an atom decreases as one goes across a period for atoms of the same value of n. This conclusion is apparent from the formula for the radius of a Bohr orbit: Atomic radius: r = (n2/Z)a0 r = kn2 (r increases)if Z is fixed (going down a column) r = k’/Z (r decreases) if n is fixed (going across a row) Electronegativity: a measure of the power of an atom to attract electrons to itself in a bond. Most electronegative atoms: F > O > Cl >N ~ Br > I En = -(Z2/n2) Across row Z increases for similar n (valence electrons see more + as Z increases) Down column Zeff is similar for increasing n and r (valence electrons further away with same Zeff)