The Shapes of Molecules

Transcription

1 Chapter 1 The Shapes of Molecules The Shapes of Molecules 11 Depicting Molecules and Ions with Lewis Structures 12 Valence-Shell Electron-Pair Repulsion (VSEPR) Theory and Molecular Shape 13 Molecular Shape and Molecular Polarity 1

2 Lewis Electron - Dot Symbols or main group elements - The A group number gives the number of valence electrons Place one dot per valence electron on each of the four sides of the element symbol Pair the dots (electrons) until all of the valence electrons are used Example itrogen,, is in Group 5A and therefore has 5 valence electrons The steps in converting a molecular formula into a Lewis structure Molecular formula Step 1 Place atom with lowest E in center Lewis structure The nonmetal atoms can acquire stable noblegas structure by sharing electrons to form an electron-pair bond Atom placement Step 2 Add A-group numbers Sum of valence e - Step 3 Draw single bonds Subtract 2e - for each bond ctet rule the nonmetals achieve a noble-gas structure by sharing eight electrons This generalization is referred to as ctet rule Remaining valence e - Duet rule ydrogen atoms are surrounded by a duet of electrons in molecules Step 4 Lewis structure Give each atom 8e - (2e - for ) 2

3 Molecular formula Atom placement Sum of valence e - Remaining valence e - or 3 5e - 7e - X 3 = 21e - Total 26 e - Lewis structure Single bond a single electron pair is shared between two bonded atoms Double bond two electron pairs are shared between two bonded atoms Triple bond three electron pairs are shared between two bonded atoms SAMPLE PRBLEM Writing Lewis Structures for Molecules with ne Central Atom PRBLEM Write a Lewis structure for CCl 2 2, one of the compounds responsible for the depletion of stratospheric ozone PLA ollow the steps outlined SLUTI Step 1 Carbon has the lowest E and is the central atom The other atoms are placed around it Steps 2-4 C has 4 valence e -, Cland each have 7 The sum is 4 + 4(7) = 32 valence e - Make bonds and fill in remaining valence electrons placing 8e - around each atom Cl Cl C Cl Cl C 3

4 SAMPLE PRBLEM Writing Lewis Structure for Molecules with More than ne Central Atom PRBLEM Write the Lewis structure for methanol (molecular formula C 4 ), an important industrial alcohol that is being used as a gasoline alternative in car engines SLUTI ydrogen can have only one bond so C and must be next to each other with filling in the bonds There are 4(1) = 14 valence e - C has 4 bonds and has 2 has 2 pair of nonbonding e - C Resonance Delocalized Electron-Pair Bonding 3 can be drawn in 2 ways - either structure is actually correct but can be drawn to represent a structure which is a hybrid of the two -a resonance structure B B A C Resonance structures have the same relative atom placement but adifference in the locations of bonding and nonbonding electron pairs is used to indicate that resonance occurs A C 4

5 Resonance (continued) Three criteria for choosing the more important resonance structure Smaller formal charges (either positive or negative) are preferable to larger charges Avoid like charges (+ + or --) on adjacent atoms A more negative formal charge should exist on an atom with a larger E value SAMPLE PRBLEM Writing Resonance Structures PRBLEM Write resonance structures for the nitrate ion, 3- PLA After Steps 1-4, go to 5 and then see if other structures can be drawn in which the electrons can be delocalized over more than two atoms SLUTI itrate has 1(5) + 3(6) + 1 = 24 valence e - does not have an octet; a pair of e - will move in to form a double bond 5

6 ormal Charge Selecting the Best Resonance Structure An atom owns all of its nonbonding electrons and half of its bonding electrons ormal charge is the charge an atom would have if the bonding electrons were shared equally ormal charge of atom = or A # valence e - = 6 # nonbonding e - = 4 # bonding e - = 4 1/2 = 2 ormal charge = # valence e - -(# unshared electrons + 1/2 # shared electrons) A or B B C # valence e - = 6 # nonbonding e - = 2 # bonding e - = 6 X 1/2 = 3 ormal charge = +1 or C # valence e - = 6 # nonbonding e - = 6 # bonding e - = 2 X 1/2 = 1 ormal charge = -1 SAMPLE PRBLEM Writing Lewis Structures for ctet Rule Exceptions PRBLEM Write Lewis structures for (a) 3 P 4 (pick the most likely structure); (b) BCl 2 PLA Draw the Lewis structures for the molecule and determine if there is an element which can be an exception to the octet rule ote that (a) contains P which is a Period-3 element and can have an expanded valence shell SLUTI -1 P +1 (a) 3 P 4 has two resonance forms and formal charges indicate the more important form P more stable lower formal charges (b) BCl 2 will have only 1 Lewis structure Cl B Cl 6

7 The orientation of polar molecules in an electric field Polarity is called bond polarity or chemical polarity Polarity describes how equally bonding electrons are shared between atoms egative end of Polar is a result of an unsymmetrical electron distribution (, 2 ) Dipole is the bond or molecule which contains positive or negative pole onpolar is a symmetrical distribution of electrons which leads to the a bond or molecule without positive or negative pole(c 2, B 3 ) Electric field Electric field Positive pole of the electric field If the electric field is off, the polar molecule,, orientated randomly If the electric field is on, the polar molecule,, aligns their positive ends toward negative poles of the electric field SAMPLE PRBLEM Predicting the Polarity of Molecules PRBLEM rom electronegativity(e) values and their periodic trends, predict whether each of the following molecules is polar and show the direction of bond dipoles and the overall molecular dipole when applicable (a) Ammonia, 3 (b) Boron trifluoride, B 3 PLA Draw the shape, find the E values and combine the concepts to determine the polarity SLUTI (a) 3 E = 3 E = 21 bond dipoles molecular dipole The dipoles reinforce each other, so the overall molecule is definitely polar 7

8 SAMPLE PRBLEM continued Predicting the Polarity of Molecules (b) B 3 has 24 valence e - and all electrons around the B will be involved in bonds The shape is AX 3, trigonal planar B 12 (E 4) is more electronegative than B (E 2) and all of the dipoles will be directed from B to Because all are at the same angle and of the same magnitude, the molecule is nonpolar The Central Themes of VB Theory Basic Principle A covalent bond forms when the orbitals of two atoms overlap and the overlap region, which is between the nuclei, is occupied by a pair of electrons The two wave functions are in phase so the amplitude increases between the nuclei Themes A set of overlapping orbitalshas a maximum of two electrons that must have opposite spins The greater the orbital overlap, the stronger (more stable) the bond The valence atomic orbitalsin a molecule are different from those in isolated atoms There is a hybridization of atomic orbitalsto form molecular orbitals 8

9 ybrid rbitals Key Points The number of hybrid orbitals obtained equals the number of atomic orbitalsmixed The type of hybrid orbitals obtained varies with the types of atomic orbitalsmixed Types of ybrid rbitals sp sp 2 sp 3 sp 3 d sp 3 d 2 VSEPR --Valence Shell Electron Pair Pair Repulsion Theory Each group of valence electrons around a central atom is located as far away as possible from the others in order to maximize repulsions These repulsions maximize the space that each object attached to the central atom occupies The result is five electron-group arrangements of minimum energy seen in a large majority of molecules and polyatomic ions The electron-groups are defining the object arrangement,but the molecular shape is defined by the relative positions of the atomic nuclei Because valence electrons can be bonding or nonbonding, the same electron-group arrangement can give rise to different molecular shapes A -central atom X -surrounding atom E -nonbonding valence electron-group AX m E n integers 9

10 igure 12 Electron-group repulsions and the five basic molecular shapes linear trigonalplanar tetrahedral trigonalbipyramidal octahedral The sp hybrid orbitals in gaseous BeCl 2 atomic orbitals hybrid orbitals orbital box diagrams 1

11 The sp 2 hybrid orbitals in B 3 The sp 3 hybrid orbitals in C 4 11

12 The sp 3 hybrid orbitals in 3 The sp 3 hybrid orbitals in 2 12

13 The sp 3 d hybrid orbitals in PCl 5 The sp 3 d 2 hybrid orbitals in S 6 13

14 SAMPLE PRBLEM Postulating ybrid rbitals in a Molecule PRBLEM Use partial orbital diagrams to describe mixing of the atomic orbitals of the central atom leads to hybrid orbitalsin each of the following (a) Methanol, C 3 (b) Sulfur tetrafluoride, S 4 PLA Use the Lewis structures to ascertain the arrangement of groups and shape of each molecule Postulate the hybrid orbitals Use partial orbital box diagrams to indicate the hybrid for the central atoms SLUTI (a) C 3 C The groups around C are arranged as a tetrahedron also has a tetrahedral arrangement with 2 nonbonding e - pairs SAMPLE PRBLEM Postulating ybrid rbitals in a Molecule continued 2p sp 3 2p sp 3 2s single C atom hybridized C atom 2s single atom hybridized atom (b) S 4 has a seesaw shape with 4 bonding and 1 nonbonding e - pairs S 3d 3d 3p 3s S atom sp 3 d hybridized S atom 14

15 Lewis electron -dot symbols for elements in Periods 2 and 3 15