Chapter 26 - Chemical Bonding

Question 1

What is a molecule? What are they held together by? What are the bonds formed by?

Answer 1

Atoms of many elements that combine. They are held together by attractive forces known as chemical bonds. They are formed by the interaction of valence electrons of the combining atoms.

Question 2

What are intramolecular and intermolecular bonds?

Answer 2

Intramolecular: Hold atoms together such as molecules, and include ionic and covalent bonds. This type of bond is strong.

Intermolecular: Weaker forces between molecules but are important to understanding the physical properties of substances

Question 3

Describe the octet rule:

Answer 3

States an atom tends to bond with other atoms until 8 electrons are in the outer most shell, forming a stable electron configuration to that of Neon
- Exceptions are H and He, Li and Be, B, P and S

Question 4

What is the difference between ionic and covalent bonding?

Answer 4

Ionic: One or more electrons from an atom with smaller ionization energy are transferred to an atom with greater electron affinity, and the resulting ions are held together by electrostatic forces.

Covalent: An electron pair is shared between two atoms
- In many cases the bond is partially covalent and partially ionic (polar covalent bonds)

Question 5

What happens to the atom in ionic bonding that loses/gains electrons? When does this occur?

Answer 5

Loses: Becomes positively charged (cation)

Gains: Becomes negatively charged (anion)

Occurs when the difference in electronegativity is greater than 1.7 on the Pauling scale

Question 6

What are the characteristic physical properties of ionic compounds?

Answer 6

• Form crystal lattices with arrays of positive and negative ions in which the attractive forces between ions of opposite charge are maximized while repulsive forces between ion of like charge are minimized
• High melting and boiling points
• Conduct electricity in (l) and (aq) states not in the (s) state

Question 7

Why do electrons share in covalent bonding??

Answer 7

When atoms have similar electronegativity, energy required to form ions is greater than it is to release upon the formation of an ionic bond, therefore they share electrons in a covalent bond.

Question 8

What are characteristic physical properties of covalent bonds?

Answer 8

• Generally discrete molecular units with weak intermolecular forces
• Low melting point
• Do not conduct electricity in (l) or (aq) states

Question 9

What is an important property of covalent bonds?

Answer 9

They can share more than one pair of electrons

- Two atoms sharing one, two, or three electron pairs are said to be joined by a single, double, or triple bond

Question 10

What is bond order?

Answer 10

The number of shared electron pairs between two atoms

• A single bond = 1 bond order
• A double bond = 2 bond orders
• A triple bond = 3 bond orders

Question 11

What are two key features of covalent bonds?

Answer 11

1) Bond length: The average distance between two nuclei of the atoms involved in the bond
- As the number of pairs increases, the bond length decreases

2) Bond energy: Energy required to separate two bonded atoms
- Strength of a bond increases as the shared electron pairs increase

Question 12

What are bonding electrons? What are non-bonding electrons - what is another name for them?

Answer 12

Shared valence electrons of a covalent bond

Valence electrons not involved in the covalent bond (i.e. lone electron pairs)

Question 13

What is the convenient notation used to represent bonding and non-bonding electrons?

Does it represent the same number of valence electrons from the Lewis structure molecule to an isolated atom and why?

Answer 13

Lewis structure

- No, formal charge and resonance

Question 14

What does a Lewis structure do? What does it have?

Answer 14

A Lewis structure helps to follow valence electrons in a chemical reaction

It has Lewis dot symbols which is a dot for each valence electron in the atom

Question 15

What does formal charge designate in Lewis structures? What is the equation?

Answer 15

Electrons assigned to an atom in Lewis structures do not always equal the number of valence electrons of the free atom - the difference between the two is called formal charge

Formal Charge = V - 1/2 N bonding - N non-bonding
Formal Charge = V - # sticks - # dots

Question 16

Describe a resonance structure:

Answer 16

Some molecules have two or more non-identical Lewis structures that can be drawn; the molecule does not actually exist

Question 17

What are 2 guidelines for Lewis structure?

Answer 17

1) Lewis structure with small or no formal charge is preferred over large formal charges
2) Lews structure with negative formal charges place on more electronegative atoms is more stable than one in which the formal charges are place on less electronegative atoms

Question 18

What are exceptions to the octet rule?

Answer 18

Atoms found in or beyond the third period can have more than 8 valence electrons since they may occupy the d-orbital which can be assigned more than 4 bonds in Lewis structures

Question 19

What does covalent bonding depend on?

Answer 19

The relative electronegativities of the atoms sharing the electron pairs

Question 20

What are types of covalent bonds?

Answer 20

1) Polar Covalent
- Occurs between atoms with small differences in electronegativities (0.4-1.7 Pauling units)
- Bonding pair is not shared equally, pulled toward element with higher electronegativity
- Higher electronegative atom has a partial negative charge, and the less electronegative atom requires a partial positive charge
- Gives molecule ionic character
- A molecule with separation of positive/negative charge is called “polar”

2) Non-Polar
- Atoms have same electronegatities
- Bonding electron pair is shared equally
- Occurs in diatomic molecules

3) Coordinate
- Shared electron pair comes form the lone pair of one of the atoms
- Bond forms and becomes indistinguishable from other covalent bonds - making is useless unless keeping track of valence electrons and formal charges
- Found in Lewis acid-base compounds (Acids accept electron pair and bases donate electron pair to form covalent bonds)

Question 21

How is molecular geometry predicted? What is the theory - why?

Answer 21

Valence shell electron-pair repulsion (VSEPR) uses Lewis structures to predict the molecular geometry of covalently bound molecules

Theory: 3D arrangement of atoms around central atom is determined by repulsion between bonding/non-bondng pair around central atom to arrange themselves as far apart as possible to minimize repulsion

Question 22

State the non-bonding pairs, geometric arrangement, shape, angles, and an example for the following valence electron arrangements:

• 2 electron pairs
• 3 electron pairs
• 4 electron pairs (hint 3 of them)
• 5 electron pairs
• 6 electron pairs

Answer 22

Page 299

Question 23

What does polarity of a molecule depend on?

Answer 23

Depends on the polarity of the constituent bonds and shape of the molecule

Question 24

Is a non-polar bond always non-polar?

Answer 24

Yes

Question 25

Is a polar bond always polar? Explain.

Answer 25

No

• Depends on the orientation of the molecule
• If the molecule has a net dipole movement, it is polar
• Take the vector sum of each bond to determine, if they cancel out = non-polar

Question 26

What is an anti-bonding and bonding orbital? What forms?

Answer 26

Bonding: Overlap of two atomic orbitals; if the sign of the two atomic orbitals are the same (pi bond)

Anti-bonding: Overlap of two atomic orbitals; if the sign of the two atomic orbitals are different… overlap head to head (sigma bond forms)

Question 27

Name and describe the 4 types of intermolecular forces?

Answer 27

1) Dipole-ion interactions

2) Hydrogen bonding
- Strong form of dipole-dipole
- When H bound to F, O, N the H carries little electron density of covalent bond
- Positive H atom then interacts with negative charge on electronegative atoms nearby the molecule
- Interaction between partially negative and positive charge on nearby molecule is the H-bond
- Unusually high BP and MP

3) Dipole-dipole interactions
- Polar molecules tend to orient themselves such that positive region is close to negative region of another molecule which is energetically favourable because an attractive dipole force is formed between two molecules
- Solid and liquid phases, negligible in gas phase
- Higher BP and MP

4) London dispersion forces
- Bonding electrons are shared equally in covalent bonds but at any time they will be randomly located in the orbital
- Permits unequal sharing causing rapid polarization/counter-polarization of electron clouds
- Dipoles interact with electron cloud inducing more formation of dipoles (LDF)
- Weaker, do not extend long distances, strength depends on how easily electrons moves:
- Large molecules with far electrons from nucleus are easy to polarize and have greater dispersion forces
- Without this force, noble gases would not liquify at any temperature

Question 28

What is the increasing order of intermolecular forces?

Answer 28

DI > HB > DD > LDF

Question 29

What are intermolecular forces used for? How?

Answer 29

Determining melting and boiling points

• Held together tightly = higher MP and BP
• Held together loosely = lower MP and BP

Question 30

Describe the 3 types of carbon-carbon bonding:

Answer 30

1) Single
- Longest bond length
- Lowest bond energy = simplest to break

2) Double
- Average bond length
- Average bond energy = average to break

3) Triple
- Shortest bond length
- Highest bond energy = difficult to break