Chapter 9

Question 1

valence-sheel electron-pair repulsion (VSEPR) theory

Answer 1

• e- have negative charges and repel each other
  
  • VSEPR applies this principle by incorporating the assumption that pairs of val e- are arranged about central atoms in ways that minimize repulsions between the pairs
    
    • to predict molecular shape using VSEPR we need electron-pair geometry and molecular geometry

Question 2

electron-pair geometry

Answer 2

• defines the relative positions in 3D space of all the bonding pairs and lone pairs of valence electrons on the central atom

Question 3

molecular geometry

Answer 3

• defines the relative positions in 3D space of the atoms in a molecule

Question 4

important facts ab lone pairs and bonding pairs

Answer 4

• repulsion between lone pairs and bonding pairs is greater than the repulsion between bonding pairs
• repulsion caused by a lone pair is greater than that cause by a double bond
• repulsion caused by a double bond is greater than that caused by a single bond
• two lone pairs of electrons on a central atom exert a greater repulsive force on the atom’s bonding pairs than does one lone pair

Question 5

bond dipole

Answer 5

• separation of electrical charge created when atoms with different electronegativites form a covalent bond

Question 6

dipole moment(μ)

Answer 6

• the measure of the degree to which a molecule aligns itself in a strong electric field
  
  • a quantitative expression of the polarity of a molecule
  • μ measures the overall separation of positive and negative charge in the molecules of a substances and is determined by measuring the degree to which the molecules align with a strong electric field
    
    • the more polar the molecules are, the more strongly they align with the field

Question 7

valence bond theory

Answer 7

• a quantum-mechanis based theory of bonding incorporating the assumption that covalent bonds form when half-filled orbitals on different atoms overlap or occupy the same region in space
  
  • incorporates assumptions that
    
    • a) a chemical bond btwn two atoms results from overlap of the atoms’ atomic orbitals
    • b) the greater the overlap, the stronger and more stable the bond

Question 8

overlap

Answer 8

• describes bonds arising from two orbitals on different atoms that occupy the same region of space
  
  • shared e- in a chemical bond are located between the nuclei of two atoms and are attracted to both
    
    • this attraction leads to lower potential energy and greater chemical stability than if the atoms were completely independent of one another

Question 9

sigma(σ)bond

Answer 9

• a covalent bond in which the region of highest electron density lies between the two atoms along the bond axis.

Question 10

hybridization

Answer 10

• in valence bond theory, the mixing of atomic orbitals to generate new sets of orbitals that are then available to form covalent bonds with other atoms
  
  • when atomic orbitals of different shapes and energies are mixed to form hybrid atomic orbitals

Question 11

hybrid atomic orbital

Answer 11

• in valence bond theory, one of a set of equivalent orbitals about an atom created when specific atomic orbitals are mixed
  
  • covalent bonds then result from:
    
    • overlap of a hybrid orbital on one atom with an unhybridized orbital on another atom
    • from overlap of a hybrid orbital on one atom with an unhybridized orbital on another atom
    • from overlap of two hybrid orbitals on two atoms

Question 12

pi(π) bond

Answer 12

• a covalent bond in which electron density is the greatest around-not along-the bonding axis

Question 13

molecular recognition

Answer 13

• the process by which molecules interact with other molecules in living tissues to produce a biological effect
  
  • this interaction usually doesn’t involve covalent bond formation
    
    • instead, these noncovalent interactions require that the biologically active molecules and the receptors that respond to them fit tightly together, which means that they must have complementary 3D shapes
  • EX: tomatoes ripening: tomatoes ripen faster in paper or plastic bags b/c they give off ethylene gas as they ripen and this gas accelerates the ripening process when trapped in the bag

Question 14

ketone

Answer 14

• organic molecule containing a carbonyl group bonded to a carbon atom on each side of the carbonyl carbon
  
  • a larger molecule

Question 15

molecular orbital (MO) theory

Answer 15

• a bonding theory based on the mixing of atomic orbitals of similar shapes and energies to form molecular orbitals that extend across two or more atoms
  
  • based on the formation of molecular orbitals

Question 16

molecular orbital

Answer 16

• a region of characteristic shape and energy where electrons in a molecule are delocalized over two ore more atoms in a molecule
  
  • represent discrete energy states in molecules, just as atomic orbitals represent allowed energy states in single atoms
  • as with atomic orbitals, e- fill the lowest-energy MOs first
  • MOs are not linked to single atoms(like in atomic orbitals) but rather belong to all the atoms in amolecule
• molecular orbitals are formed by combining atomic orbitals from each of the atoms in the molecule

Question 17

valence bond theory vs. MO theory

Answer 17

• valence bond theory assumes formation of hybrid atomic orbitals, while MO theory assumes formation of molecular orbitals
  
  • key difference: hybrid atomic orbitals are associated with a particular atom in a molecule, but molecular orbitals are spread out over two or more atoms in a molecule

Question 18

bonding orbitals

Answer 18

• term in MO theory
  
  • some MOs have lobes of high e- density that lie btwn bonded pairs of atoms → bonding orbitals
    
    • serve to hold atoms together in molecules
  • the energies of bonding MOs are lower than the energies of the atomic orbitals that combined to form them, so populating them with e- stabilizes the molecule and contributes to the strength of the bonds holding its atoms together

Question 19

antibonding orbitals

Answer 19

• MOs with lobes of high e- density that are not located btwn the bonded atoms → anitbonding orbitals
  
  • have energies that are higher than the atomic orbitals that combined to form them
  • antibonding orbitals are regions of e- densitry in a molecule that destabilize the molecule b/c they decrease the e- density btwn nuclear centers

Question 20

KEY POINTS AB BONDING AND ANTIBONDING ORBITALS

Answer 20

• *if a molecule were to have the same # of e- in its bonding and antibonding orbitals, it would have never formed
• *the total # of molecular orbitals must match the number of atomic orbitals involved in forming them
  
  • EX: combining two atomic orbitals, one from each of two atoms, produces one low-energy bonding orbital ad one high-energy antibonding orbital
    
    • apply MO theory to H and He to further explore the distinction btwn bonding and antibonding molecular orbitals

Question 21

sigma MO (σ MO)

Answer 21

• a MO in which the greatest e- density is concentrated along an imaginary line drawn through the bonded atom centers(bonding axis)

Question 22

bond order

Answer 22

• Bond order = 1/2(# bonding electrons - # antibonding e-)

Question 23

molecular orbitals of homonuclear diatomic molecules

Answer 23

• Guidelines for contstructing molecular orbital diagrams
  
  • 1) # of molecular orbitals = # of atomic orbitals used to vcreate them
  • 2) atomic orbitals with similar energy and orientation mix more effectively than do those that have different energies and orientations
    
    • EX: s obital mixes better with s than with p
      
      • 1s and 2s have diff energies and sizes, so 1s and 2s are less effective in mixing than two 1s or two 2s
  • 3) better mixing leads to a larger energy difference btwn bonding and antibonding orbitals and thus greater stabilization of the bonding MOs
  • 4) a molecular orbital can accomodate a maximum of two e-; two e- in the same MO have opposite spins
  • 5) e- in ground-state molecular occupy the lowst-energy molecular orbitals available, following the aufbau principle and Hund’s rule

Question 24

pi(π) molecular orbitals

Answer 24

• the molecular orbitals formed by the mixing of atomic orbitals oriented above and below, or in front of and behind, the bonding axis

Question 25

diamagnetic

Answer 25

* describes a substance with **no unpaired e-** that is weakly repelled by a magnetic field

Question 26

paramagnetic

Answer 26

* describes a substance with unpaired e- that is attracted to a magnetic field * the more unpaired e- in a molecule, the greater its paramagnetism * only MO theory accounts for these magnetic behaviors

Question 27

molecular orbitals of heteronuclear diatomic molecules

Question 28

band theory

Answer 28

* an extension of molecular orbital theory that describes bonding in solids * explains the conductivity of copper and other metals by incorporating the assumption that essentially no gap exists btwn the eergy of the occupied lower portion of the valence band and the empty upper portion * therefore, valence e- can move easily from the filled lower portion to the empty upper portion, where they are free to move from one empty orbital to the next and thus flow throughout the solid

Question 29

valence band

Answer 29

* a band of orbitals that are filled or partially filled by valence electrons

Question 30

conduction band

Answer 30

* in metals, an unoccupied band higher in energy than a valence, in which electrons are free to migrate * conduction band is empty and broad enough to overlap the valence band * this overlap means that e- from the vallence band can move to the conduction band, where they are free to migrate from atom to atom(in solid zinc), thereby conducting electricity

Question 31

band gap (E_g)

Answer 31

* the energy gap between the valence and conduction bands * occurs in semimetals(or metalloids)

Question 32

semiconductor

Answer 32

* a semimetal(metalloid) with electrical conductivity btwn that of metals and insulators that can be chemically altered to increase its electrical conductivity * generally, only a few valence-band e- in semimetals have sufficient energy to move to the conduction band, which limits Silicon's ability to conduct electricity * we can enhance the conductivity of solid semimetals by replacing some of its atoms with atoms of an element of similar atomic radius but with a different # of valence e- * this replacement process is called **doping** ad the added element is the **dopant**

Question 33

n-type semiconductor

Answer 33

* semiconductor containing electron-rich dopant atoms that contribute excess electrons

Question 34

p-type semiconductor

Answer 34

* semiconductor containing electron-poor dopant atoms that cause a reduction in the number of electrons, which is equivalent to the presence of positibely charged holes