Tentative material to be covered for Exam 2
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 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
Quantum mechanics provides an intellectual structure for describing all of the properties of atoms and molecules. For atoms quantum mechanics the concept of orbitals (wavefunctions) provides a description of the energies, the sizes of atoms and the basis for bonding of atoms and the construction of the periodic table. The orbitals for the H atom, which are known precisely, are used as starting approximation for building up the electron configuration of multielectron atoms. Every electron in an atom is assigned four quantum numbers (n, l, ml and ms) that uniquely define its spatial distrbution and spin state. Thus, we can envision every electrons in terms of a characteristic energy, size, shape, orientation and spin.
Properties of electrons in atoms Quantum numbers of electrons Electron configurations Core electrons Valence electrons Energy required to remove an electron Energy required to add an electron Size of atoms
Building up the ground state configuration of atoms Every atom possesses the SAME set of available orbitals Every electron of an atom MUST be in one of these orbitals: 1 s, 2 p, 3 s, 3 p, 3 d, etc. The energy and size of these orbitals depend on the atom (Z), but the shape and orientation is space of any orbitals of the same l are the same for all atoms. The energy ranking of the orbitals for the representative elements is generally: 1 s < 2 p < 3 s < 3 p. From this point on the next lowest energy orbital may be 4 s or 3 d, depending on the number of electrons in the neutral atom.
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 therefore vary periodically with atomic weight. The ground state electron configuration of the atoms of elements vary periodically with the atomic number Z. All chemical and physical properties of the elements that depend on electron configurations vary periodically with atomic number.
Ground state electron configuration: Z electrons (Z = atomic number of the atom) are placed seriatim into the orbitals according to the following guidelines. Aufbau principle: electrons go into lowest energy orbitals first. Pauli principle: No more than two electrons in any one orbital. Filled orbitals have spins paired. Hund’s rule: When there are orbitals of equal energy in a subshell to fill, the electrons first go into different orbitals with parallel spins one at a time.
Valence electrons, Lewis structures and electronic configurations The valence electrons are electrons in the s and p orbitals: valence electrons = snpm Atom 3 Li 4 Be 5 B 6 C 7 N 8 O 9 F 10 Ne Configuration [He]2 s [He] 2 s 22 p 1 [He] 2 s 22 p 2 [He] 2 s 22 p 3 [He] 2 s 22 p 4 [He] 2 s 22 p 5 [He] 2 s 22 p 6 Comment Paramagnetic Closed shell (diamagnetic) Paramagnetic Paramagnetic Closed shell (diamagnetic)
Correlation of valence electron and Lewis structures
Building up the third row of the periodic table: From Na to Ar Atom Configuration Comment 11 Na [Ne]2 s [Ne] 2 s 22 p 1 [Ne] 2 s 22 p 2 [Ne] 2 s 22 p 3 [Ne] 2 s 22 p 4 [Ne] 2 s 22 p 5 [Ne] 2 s 22 p 6 Paramagnetic Closed shell (diamagnetic) Paramagnetic Paramagnetic Closed shell (diamagnetic) 12 Mg 13 Al 14 Si 15 P 16 S 17 Cl 18 Ar
d orbitals From photoelectron spectroscopy, the 3 d subshell for elements 21 through 29 (Sc through Cu) lies well above the 3 d subshell. However, the energy of the 3 d subshell is very close in energy to the 4 s subshell: 3 p << 3 d ~ 4 s 1 s << 2 s < 2 p << 3 s < 3 p < 4 s ~ 3 d Thus is some cases the specifics of orbital configurations place 3 d below 4 s and in other cases th 4 s is below the 3 d.
The fourth row of the periodic table Atom Configuration 19 K 18[Ar]2 s 20 Ca 18[Ar]2 s 2 _________________ 31 Ga 18[Ar] 33 As 18[Ar] 32 Ge 34 Se 35 Cl 36 Kr 18[Ar] 2 s 22 p 1 2 s 22 p 2 2 s 22 p 3 2 s 22 p 4 2 s 22 p 5 2 s 22 p 6 What about 21 M d orbitals fill up through 30 M?
The electron configurations of the transition elements 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Co 28 Ni 29 Cu 30 Zn 18[Ar]4 s 23 d 2 18[Ar]4 s 23 d 3 18[Ar]4 s 23 d 4 18[Ar]4 s 23 d 5 instead 18[Ar]4 s 13 d 5 18[Ar]4 s 23 d 6 18[Ar]4 s 23 d 7 18[Ar]4 s 23 d 8 18[Ar]4 s 23 d 9 18[Ar]4 s 23 d 10 instead 18[Ar]4 s 13 d 10 The “surprises” for 24 Cr and 29 Cu are due to ignored electron-electron repulsions. For 24 Cr the stability of half shells trumps one filled subshell and a partially filled subshell. For 29 Cu the stability of a full d shell and half filled 4 s subshell trumps a partially filled 3 d subshell.
Example: IE drops dramatically from He to Li. Why? He = 1 s 2 versus Li 1 s 22 s. 2 s on average further away from nucleus for same average charge (after screening by 1 s 2).
Compare Be = [He]2 s 2 versus B = [He]2 s 22 p Compare N = [He]2 s 22 px 2 py 2 pz versus O = [He]2 s 22 px 22 py 2 pz
Bond length: the distance between the centers (nucleus) of bonded atoms.
Atomic radius: the atomic radius of a neutral atom generally decreases from left to right across a period (larger Z) and increases down a group (increase in n).
The electron affinity (EA) of an atom is the energy change which occurs when an atom gains an electron. X(g) + e. Xe- (g) Electron affinities of the representative elements: What are the correlations across and down?
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
- Slides: 19