Structure and Bonding

Question 1

Define electronegativity

Answer 1

The ability of an atom to attract an electron to itself

Question 2

What is the most common electronegativity scale used?

Answer 2

The Pauling scale

Question 3

What is the general trend of electronegativity?

Answer 3

Increases up the group and across the period

Question 4

Why does electronegativity decrease down the group?

Answer 4

Electrons get further from the nucleus meaning their effective charge is weaker

Question 5

What is the name of the triange which shows the different types of bonding based on electronegativity and change of electronegativity?

Answer 5

The van Arkel-Ketelaar triangle

Question 6

What changes of enthalpy will occur in a Born-Haber cycle of an ionic compound?

Answer 6

Atomisation, ionisation, electron gain, lattice enthalpy

Question 7

Why cannot NaCl₂ be formed?

Answer 7

The strength of the ionic bonds formed cannot compare to the extremely high second ionisation enthalpy value.

Question 8

What is the trend of ionisation energies across period 3? What atoms don’t fit the pattern and why does this pattern occur?

Answer 8

Increase from left to right as number of protons increases moving left to right so effective nuclear charge increases without being shielded. There is a dip at B as the 2p sub shell begins to be filled and 2p doesn’t penetrate 2s very well. The other dip is at O as the 2p sub shell has electrons pairing up which will repel each other.

Question 9

What is the trend of ionisation energies down the groups and why does this trend happen?

Answer 9

I.E. decreases as the radius of the atom increases and Z_eff decreases.

Question 10

Is the trend of ionisation energies down the group even? Explain

Answer 10

No as the d orbitals in the transition metals are less effective at shielding

Question 11

Which 2 elements in period 3 have endothermic first electron gain enthalpies and why?

Answer 11

Beryillium as the new electron enters the 2p shell and nitrogen as the electron pairs up with another 2p electron

Question 12

What are the 2 trends for lattice enthalpy? Why do these trends exist?

Answer 12

Lattice enthalpy increases as charge increases and ion size decreases. This is because lattice enthalpy generally follows coulombs law.

Question 13

Why do 2+ ions tend to have similar lattice enthalpy of formation values to 1+ ions?

Answer 13

The increase in lattice enthalpy is counteracted by the increase in ionisation enthalpy

Question 14

Why do atoms in the same group have a similar lattice enthalpy despite increasing in ion size?

Answer 14

As ion size increases, ionisation energy decreases

Question 15

What is the trend for how easy it is to get a noble gas to bond? Do noble gases form ions? Why?

Answer 15

Increases down the group. Both first electron gain and ionisation enthalpies are positive which decrease down the group so no ions form and only a few can form bonds strong enough to be thermodynamically favourable

Question 16

What are the range of strengths of hydrogen bonds?

Answer 16

5-30kJmol^-1

Question 17

What are the range of strengths of covalent bonds?

Answer 17

140-1000(for multiple bonds)kJmol^-1

Question 18

What are the range of strengths of ionic bonds?

Answer 18

500-5500kJmol^-1

Question 19

How do multiple bonds compare to single bonds?

Answer 19

They are stronger and shorter

Question 20

How does bond strength and size change as the size of atoms increase? Why?

Answer 20

Bonds to larger atoms are weaker as the orbitals get larger so the electron density is more diffuse and cannot overlap as well. As orbitals are more spread out the bond length increases

Question 21

What are the range of bond lengths of single bonds?

Answer 21

90-220pm

Question 22

What are the range of bond enthalpies of single bonds?

Answer 22

250-600kJmol-1

Question 23

How do bonds to electronegative atoms compare to bonds to electropositive atoms? Why?

Answer 23

They are stronger and shorter as the electrons in the bond have a stronger Z_eff value.

Question 24

What is the distance at which the covalent bond is most thermodynamicaly stable called?

Answer 24

The equlibrium bond length

Question 25

Which elements always obey the octet rule?

Answer 25

Period 3, Li to Ne.

Question 26

Why does BF₃ obey the octet rule?

Answer 26

It has mesomeric effects that forms a double bond to a fluorine atom with formal charges.

Question 27

Name 2 exceptions to the octet rule

Answer 27

[CPh₃]⁺ and AlCl₃ which both have 6 electrons on its central atom

Question 28

What is the term for an atom with an expanded octet? Which atoms do this?

Answer 28

Hypervalent, main group atoms down the table

Question 29

What 3 groups of elements does the octet rule never apply to? Why?

Answer 29

Transition metals, lanthanides and actinides as they have d and f orbits.

Question 30

What are the 4 steps to working out Lewis structures?

Answer 30

1. Number of electrons in valence shell (usually group number)
2. Add one electron for a single bond, 2 for a double, 3 for a triple
3. Add one electron for each unit of negative charge and subtract one electron for each unit of positive charge
4. Formal charge=valence electrons-½(electrons in shared bonds)-unshared electrons

Question 31

What are the 6 steps to work out the shape of a molecule?

Answer 31

1. Take the number of valence electrons (group number)
2. Add electrons for the substituents, 1 for single bond, 2 for double bond, 3 for triple bond
3. Add/remove electrons for charge
4. Divide by 2 and take away 1 for each multiple bond to give the number of valence electron pairs (VEP)
5. Deduce the electron pair geometry (EPG) from the number of VEPs
6. Deduce the molecular geometry from the number of bonding and lone pairs

Question 32

What are the shapes of the molecules with no lone pairs and VEPs from 2 to 6?

Answer 32

2-Linear, 3-Trigonal planar, 4-Tetrahedral, 5-Trigonal bypyramidal, 6-Octahedral

Question 33

What are the shapes of the molecules with 1 lone pairs and VEPs from 3 to 6?

Answer 33

3-Bent, 4-Pyramidal, 5-Seesaw, 6-Square based pyramid

Question 34

What are the shapes of the molecules with 2 lone pairs and VEPs from 4 to 6?

Answer 34

4-Bent, 5-T-shaped, 6-Square planar

Question 35

What are the shapes of the molecules with 3 lone pairs and VEPs from 5 to 6?

Answer 35

5-Linear, 6-T-shaped

Question 36

What are the bond angles in CH₄ and NH₃?

Answer 36

CH₄-109.5º NH₃-106.7º

Question 37

What is the effect of electronegative groups on bond angle?

Answer 37

As they draw the electron pair towards itself, it gives the other bonding pairs more room which reduces the H-C-X bond but increases the H-C-H bond (for CH₃Cl)

Question 38

What is the effect of a multiple bond on molecular shape?

Answer 38

They occupy more space than single bonds so the moclecule is distored in a similar way to a lone pair.

Question 39

Why is the T-shaped shape found for a molecule with 5 electron pairs and 2 lone pairs?

Answer 39

To minimise repulsions between electron pairs

Question 40

Why is VSEPR limited at the bottom of the periodic table?

Answer 40

Because lone pairs can occupy the s orbital and be stereochemically inactive which cannot be accounted for by VSEPR

Question 41

Where does the nucleus lie in a sp hybridised orbital?

Answer 41

In the small lobe, NOT on the radial node

Question 42

How is ethanes carbons hybridised?

Answer 42

sp² hybridisation with one p orbit out of the plane of the shape

Question 43

What effect does hybridisation have on bond length?

Answer 43

As the proportion of s increases in hybridisation, the bonds get shorter as the s electrons are held closer to the nucleus

Question 44

How do you work out what hybridisation an atom takes?

Answer 44

The number of single bonds and lone pairs added is the number of orbits, 2-sp, 3-sp², 4-sp³