Ch. 3: Bonding

Question 1

what makes carbon so special?

Answer 1

1. carbon has unique bonding properties
2. it is tetravalent (it can form bonds with up to four other atoms, allowing for the massive versatility required to form the foundation of biomolecules and life itself)
3. as it is located near the center of the periodic table, it can form bonds with many different elements because of its moderate electronegativity
4. because carbon atoms are fairly small, the bonds that they form are strong and stable

Question 2

defn: ionic vs. covalent bonds

Answer 2

ionic = electrons are transferred from one atom to another and the resulting ions are held together by electrostatic interactions

covalent = electrons are shared between atoms

Question 3

defn + range: principal quantum number (n)

Answer 3

corresponds to the energy level of a given electron in an atom and is essentially a measure of size

the smaller the number, the closer the shell is to the nucleus and the lower its energy

MCAT tests on 1-7

Question 4

defn + range: azimuthal quantum number, l

Answer 4

describes the subshells that can be within each electron shell

range: 0 - n-1 for a given energy shell

Question 5

what do the l values 0, 1, 2, and 3 correspond to?

Answer 5

the s, p, d, and f subshells

Question 6

defn + range: magnetic quantum number, ml

Answer 6

describes the orbitals within each subshell

range: - l to l

Question 7

char: s-orbital

Answer 7

spherical and symmetrical, centered around the nucleus

Question 8

char: p-orbital

Answer 8

composed of 2 lobes located symmetrically about the nucleus and contains a node (an area where the probability of finding an electro is zero) at the nucleus

you can picture it as a dumbbell that can be positioned along the x, y, or z axis

Question 9

char: d-orbital

Answer 9

composed of 4 symmetrical lobes and contains 2 nodes

four of the d-orbitals are clover-shaped and the fifth looks like a donut wrapped around the center of a p-orbital

Question 10

defn + range: spin quantum number, ms

Answer 10

each orbital can hold two electrons, which are distinguished by this quantum number

the only values are -0.5 and 0.5

Question 11

defn: molecular orbitals

Answer 11

when two atomic orbitals combine, this is what they form

Question 12

how are molecular orbitals obtained mathematically?

Answer 12

by adding or subtracting the wave functions of the atomic orbitals

Question 13

defn: bonding orbital vs antibonding orbital

Answer 13

BONDING: produced if the signs of the wave functions are the same; lower-energy, more stable

ANTIBONDING: produced if the signs of the wave functions are different; higher-energy, less stable

Question 14

defn: sigma bond

Answer 14

results when a molecular orbital is formed by head-to-head or tail-to-tail overlap

Question 15

defn: pi bond

Answer 15

forms when two p-orbitals line up in a parallel (side-by-side) fashion and their electron clouds overlap

this is a bonding molecular orbital

Question 16

are all single bonds sigma bonds or pi bonds?

Answer 16

sigma bonds

Question 17

how does a double bond form from sigma and pi bonds? what about a triple bond?

Answer 17

double bond: one pi bond on top of an existing pi bond

triple bond: a sigma bond and 2 pi bonds

Question 18

can a pi bond exist independently of a sigma bond?

Answer 18

no, only after the formation of a sigma bond will the p-orbitals of adjacent carbons be parallel and in position to form the pi bond

Question 19

why are shorter bonds stronger than longer bonds?

Answer 19

they hold atoms more closely together and require more energy to break

Question 20

double bonds are stronger than single bonds, but are individual pi bonds stronger than sigma bonds?

Answer 20

no, individual pi bonds are weaker than sigma bonds

thus, it is possible to break only one of the bonds in a double bond, leaving a single bond intact

Question 21

what is the difference in freedom of motion between single and multiple bonds?

Answer 21

single bonds: allow free rotation of atoms around the bond axis

double and triple bonds: hinder rotation, and in effect, lock the atoms in position

Question 22

the more bonds that are formed, the shorter or longer the bond length?

Answer 22

the more bonds = the shorter the bond length

Question 23

func: hybridization

Answer 23

a way of making all of the bonds to a central atom equivalent to each other

Question 24

how are hybrid orbitals formed?

Answer 24

formed by mixing different types of orbitals

Question 25

char (3): sp3 hybridization

Answer 25

1. use advanced math to merge 3 p-orbitals and 1 s-orbital, which forms 4 identical sp3 orbitals with new, hybridized shapes
2. all 4 of these orbitals point toward the vertices of a tetrahedron to minimize repulsion, which explains the tetrahedral geometry
3. there are no unhybridized p-orbitals to form pi bonds

Question 26

how is sp3 hybridization accomplished?

Answer 26

by promoting one of the 2s electrons into the 2pz orbital

this produces 4 valence orbitals, each with one electron

Question 27

how would you answer an MCAT question that tests how much “s character” a certain hybrid orbital has, for example?

Answer 27

we simply need to determine the type of hybridization and use the name to solve

in sp3, we have 1s and 23p, so the bond has 25% s and 75% p character

Question 28

what is the hallmark shape and hybridization of carbon-containing compounds?

is this the only possibility for carbon?

Answer 28

sp3, tetrahedral

no!

Question 29

char (5): sp2 hybridization

Answer 29

1. 1 s-orbital is mixed with 2 p-orbitals to make 3 sp2-hybridized orbitals
2. seen in alkenes
3. the third p-orbital of each carbon is left unhybridized, this is the orbital that participates in the pi bond
4. the 3 sp2 orbitals are oriented 120deg apart, allowing for maximum separation
5. trigonal planar geometry

Question 30

what do the hybrid orbitals in sp2 hybridization do?

Answer 30

for example, in ethene

2 of the sp2 hybridized orbitals will participate in C-H bonds and the other hybrid orbital will line up with the pi bond to form the sigma component of the C=C double bond

Question 31

char (4): sp hybridization

Answer 31

1. seen in triple bonds
2. 2 of the p-orbitals form pi bonds, the third p-orbital will combine with the s-orbital to form two sp-orbitals
3. these orbitals are oriented 180deg apart, which explains the linear structure of molecules containing sp-hybridized carbons
4. the two pi bonds can be between the carbon and one other atom (forming a triple bond) or between the carbon and two different atoms (forming 2 double bonds in a row)

Question 32

when does resonance delocalization of electrons occur?

Answer 32

in molecules that have conjugated bonds

Question 33

process (2): conjugation

Answer 33

1. requires alternating single and multiple bonds because this pattern aligns a number of unhybridized p-orbitals down the backbone of the molecule
2. pi electrons can then delocalize through the p-orbital system, adding to stability of the molecule

Question 34

explain why resonance structures aren’t in equilibrium?

Answer 34

the electron density is distributed throughout and the true form is a hybrid of the resonance structures

unless the stability of the various resonance forms differ, then the true electron density will favor the most stable form

Question 35

what are four reasons why particular resonance structures might be favored?

Answer 35

1. they lack formal charges
2. they form full octets on highly electronegative atoms
3. there is stabilization of positive and negative charges through induction
4. aromaticity