14 Covalent Bonding

Question 1

What is a sigma bond?

Answer 1

-Direct head-on/end to-end overlap of atomic orbitals
-results in electron density concentrated between nuclei of bonding atoms

Question 2

What is a pi bond?

Answer 2

• sideways overlap of atomic orbital
• results in electron density above and below the plane of the nuclei of the bonding atoms

Question 3

Formal Charge

Answer 3

• decide which Lewis structure is preferred from several
• Lewis structure with the atoms having FC values closest to 0 is preferred
• Treats covalent bonds as if they were purely covalent with equal e- distribution
• ignores electronegativity
• counts how may e- belong to each atom in Lewis structure and compares this with the # of valence e- in non-bonded atom

Question 4

Octet Rule exceptions

Answer 4

• incomplete octets
• expanded octets

Question 5

Delocalization

Answer 5

• electrons that are shared between all atoms in a molecule/ion opposed to being localized between a pair of atoms

Question 6

Resonance

Answer 6

• 2 or more Lewis structures to represent the same molecule
• structures that cannot be described by a single structure

Question 7

Why do expanded octets exist?

Answer 7

• d orbital available in valence shell of these atoms have energy values relatively close to those of p orbitals
• Example: from 3p to 3d orbitals –> allows additional e- pairs to form

Question 8

Species with 5 e- domains

Answer 8

• triangular bipyramidal shape (all 5 domains are bonding pairs)
• angles of 90, 180 and 120
• shape will change if there are non-bonding e- as they cause greater repulsion
• 1 non-bonding pair – > seesaw
• 2 non-bonding pairs –> T-shaped
• 3 non-bonding pairs –> linear

Question 9

Species with 6 e- domains

Answer 9

• octahedral
• angles of 90
• 1 non-bonding pair –> square pyramidal
• 2 non-bonding pairs –> square planar

Question 10

How to interpret Formal Charge

Answer 10

• low formal charges means less charge transfer has taken place in forming a structure from it’s atoms (stable structure)
• sum of formal charges in molecule for neutral molecules to equal charge of ion
• more stable Lewis structure has have negative values for more electro negative atoms+

Question 11

Oxygen dissociation wavelength

Answer 11

<242nm

Question 12

Ozone dissociation wavelength

Answer 12

<330nm

Question 13

Does it take less energy to break ozone or oxygen and why?

Answer 13

• it takes less energy to break ozone because it has a resonance structure which counts its bonds as 1.5 opposed to the double (2) bond of oxygen

Question 14

Why is ozone important?

Answer 14

• it absorbs radiation in the range of 200-315 which is UV-B and UV-C (veri scawy to living tissue)
• henceforth, ozone protec human being

Question 15

Catalytic destruction of Ozone

Answer 15

• when ozone absorbs UV radiation, it become unstable and allows it to react with other compounds such as Nitrogen oxides (NOx) and chlorofluorocarbons (try saying that 5 times fast) (CFCs)
• both compounds produce highly reactive free radials that catalyze the decomposition of ozone to oxygen

Question 16

Catalytic destruction of Ozone (Nitrogen Oxides)

Answer 16

• Nitrogen dioxide forms from the oxidation of Nitrogen Oxide (also a free radical)

Question 17

Catalytic destruction of Ozone (chloroflurocarbons)

Answer 17

• higher energy UV radiation breaks CFCs down releasing free chlorine atoms (reactive free radicals)
• weaker C-Cl bond breaks in preference to the C-F bond, thus, Cl radicals catalyze the decomposition of ozone

Question 18

Sigma Bond

Answer 18

• overlap along bond axis
• always forms in a single covalent bond
• electron density is concentrated between the nuclei of bonded atoms

Question 19

Pi bond

Answer 19

• overlap sideways
• weaker than sigma bonds
• only form during the overlap of p orbitals –> pi bonds can only form in double or triple bonds
• electron is concentrated above and below bond axis

Question 20

What forms a hybrid orbital?

Answer 20

• mixing of different types of atomic orbitals on the same atom

Question 21

Formation of Covalent Bonds

Answer 21

• often starts with excitation of atoms
• excitation is where and e- is promoted within the atom from the 2s orbital to the vacant 2p orbital

Question 22

Hybridization

Answer 22

• mixing atomic orbitals to form new hybrid atomic orbitals
• there is an unequal atomic orbitals as a result of excitation
• these orbits are mixed together resulting gin hybrid atomic orbitals
• this mix of atomic orbits will balance themselves out due to the imbalance (refer to paint analogy)

Question 23

sp3 hybridization

Answer 23

Example: Carbon (CH4)
- when carbon forms 4 single bonds, it undergoes sp3 hybridization, producing 4 equal orbitals
- forms a tetrahedron (109.5 degrees)
- each hybrid orbital overlaps with the atomic orbital of another atom forming 4 sigma bonds

Question 24

sp2 hybridization

Answer 24

Example: Carbon (C2H4)
- produces 3 equal orbits
- triangular planar (120 degrees)
- forms 3 sigma bonds
-p orbitals form pi bonds

Question 25

sp hybridization

Answer 25

Example: Carbon (C2H2)
- 2 equal orbits
- linear (180 degrees)
- 2 sigma bonds
2 pi bonds form representing 4 lobes of e- density around the atoms

Question 26

Lone pairs in Hybridization

Answer 26

Example: Amonia
- non-bonding pair allows for sp3 hybrid orbital even though it has only 3 bonding pairs

Question 27

Hybridization to predict molecular shape

Answer 27

Tetrahedral = sp3 hybridized
Triangular planar = sp2 hybridized
Linear = sp hybridized