Chemical Bond

Question 1

What is electronegativity? (2)

Answer 1

• The ability of an atom in a molecule to attract electrons to itself.
• On the periodic table, electronegativity increases as you go…
– …from left to right across a row.
– …from the bottom to the top of a column.

Question 2

The most stable (most likely) structure has: (2)

Answer 2

• the lowest formal charge on each atom

* the most negative formal charge on the most electronegative atom.

Question 3

There are three classes of exceptions to the octet rule:

Answer 3

1. molecules with an odd number of electrons,
2. molecules in which one atom has less than an octet, and
3. molecules in which one atom has more than an octet.

Question 4

• In predicting molecular shape, a molecule adopts the 3D-dimensional geometry that minimizes the ______ between the valence electrons.
• The arrangement that minimizes repulsions depends on the number of electron _____.

Answer 4

repulsion

domains

Question 5

How is the shape of a molecule determined?

Answer 5

• The shape of a molecule is determined by its bond angles i.e. the angles made by the lines joining the nuclei of the atoms in a molecule.

Question 6

How is the shape and size of a molecule determined?

Answer 6

• The bond angles and bond lengths accurately define the shape and size of a molecule.

Question 7

Name the electron domain geometry & bond angles of a structure with 2 electron domains:

Answer 7

Question 8

Name the electron domain geometry & bond angles of a structure with 3 electron domains:

Answer 8

Question 9

Name the electron domain geometry & bond angles of a structure with 4 electron domains:

Answer 9

Question 10

Name the electron domain geometry & bond angles of a structure with 5 electron domains:

Answer 10

Question 11

Name the electron domain geometry & bond angles of a structure with 6 electron domains:

Answer 11

Question 12

Name the molecular of a structure with 2 electron domains:

Answer 12

Question 13

Name the molecular of a structure with 3 electron domains:

Answer 13

Question 14

Name the molecular of a structure with 4 electron domains:

Answer 14

Question 15

Name the molecular of a structure with 5 electron domains:

Answer 15

Question 16

What is the effect of electron domains on bond angles?

Answer 16

Electron domains for non-bonding electron pairs exert greater repulsive forces on adjacent electron domains and thus tend to compress the bond angles.

Question 17

Name the molecular of a structure with 6 electron domains?

Answer 17

Question 18

What are the levels of hybridisation? (6)

Answer 18

Question 19

Pi (π) Bond:

Answer 19

Pi (π) Bond: this results from the overlap between two p orbitals orientated perpendicularly to the internuclear axis.

Question 20

Why are pi bonds weaker than sigma bonds?

Answer 20

Because of the sideways overlap of the p orbitals, total overlap in a pi-bond tends to be less than in a sigma-bond; pi-bonds are weaker than sigma-bonds.

Question 21

What is bond enthalpy?

Answer 21

The strength of a bond is measured by determining how much energy is required to break the bond.

Question 22

• A double bond consists of one __ -bond and one __-bond.

* A triple bond consists of one ___-bond and two ___-bonds.

Answer 22

sigma; pi

sigma; pi

Question 23

• The unhybridized p orbitals on carbon are involved in ___-bonding.
• ___-bonds require that portions of a molecule be planar therefore they can introduce rigidity into molecules.

Answer 23

pi

pi

Question 24

Which atoms can form pi bonds?

Answer 24

• bonds can form only if unhybridized p orbitals are present on the bonded atoms
• only atoms having sp or sp2 hybridization can be involved in pi bonding

Question 25

• Characteristics of Molecular orbitals:

> Like atomic orbitals. (3)

Answer 25

 Each contains a maximum of two electrons with opposite spins.
 Each has a definite energy.
 Electron density distribution can be visualized with contour diagrams.

Question 26

> Bonding MO: Lower-energy MO – why? (2)

Answer 26

> Concentrates electron density between the nuclei, so the bonding MO holds the atoms together in a covalent bond.
Results from the AO wave functions enhancing each other in the bonding region.

Question 27

Antibonding MO: Higher energy MO – why? (2)

Answer 27

> The greatest electron density is on opposite sides of the nuclei. This MO excludes electrons from the region in which a bond must form.
Results from the AO wave functions cancelling each other in the bonding region.

Question 28

The stability of a covalent bond is related to its bond order:

Answer 28

Bond order = ½ (bonding electrons – antibonding electrons)

Question 29

– Bond order = _ for a single bond. 
– Bond order = _ for a double bond. 
– Bond order = _ for a triple bond. 
– Fractional bond orders are possible  some molecules 
have an odd no. of electrons. 
–  Bond order = _ ----> NO BOND FORMS

Answer 29

1
2
3
0

Question 30

• As bond order increases:

Answer 30

• As bond order increases: bond length decreases and bond energy increases.

Question 31

Two types of magnetic behaviour are: (i) Paramagnetism (3)

Answer 31

– Unpaired electrons in the molecule
– Strong attraction between magnetic field and molecule
– The more unpaired electrons, the stronger the attraction

Question 32

Two types of magnetic behaviour are: (ii) Diamagnetism (2)

Answer 32

– no unpaired electrons in molecule

– weak repulsion between magnetic field and molecule

Question 33

True or false

Diamagnetism is a much weaker effect than Paramagnetism

Answer 33

True

Question 34

How do you know if bond is delocalized?

Answer 34

The lone pairs next π bonds are delocalized because they are in the p orbital of an sp2 hybridized atom. If the lone pairs can participate in forming resonance contributors – they are delocalized, if the lone pairs cannot participate in resonance, they are localized.