Structure and Bonding Flashcards
Define electronegativity
The ability of an atom to attract an electron to itself
What is the most common electronegativity scale used?
The Pauling scale
What is the general trend of electronegativity?
Increases up the group and across the period
Why does electronegativity decrease down the group?
Electrons get further from the nucleus meaning their effective charge is weaker
What is the name of the triange which shows the different types of bonding based on electronegativity and change of electronegativity?
The van Arkel-Ketelaar triangle
What changes of enthalpy will occur in a Born-Haber cycle of an ionic compound?
Atomisation, ionisation, electron gain, lattice enthalpy
Why cannot NaCl2 be formed?
The strength of the ionic bonds formed cannot compare to the extremely high second ionisation enthalpy value.
What is the trend of ionisation energies across period 3? What atoms don’t fit the pattern and why does this pattern occur?
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.
What is the trend of ionisation energies down the groups and why does this trend happen?
I.E. decreases as the radius of the atom increases and Zeff decreases.
Is the trend of ionisation energies down the group even? Explain
No as the d orbitals in the transition metals are less effective at shielding
Which 2 elements in period 3 have endothermic first electron gain enthalpies and why?
Beryillium as the new electron enters the 2p shell and nitrogen as the electron pairs up with another 2p electron
What are the 2 trends for lattice enthalpy? Why do these trends exist?
Lattice enthalpy increases as charge increases and ion size decreases. This is because lattice enthalpy generally follows coulombs law.
Why do 2+ ions tend to have similar lattice enthalpy of formation values to 1+ ions?
The increase in lattice enthalpy is counteracted by the increase in ionisation enthalpy
Why do atoms in the same group have a similar lattice enthalpy despite increasing in ion size?
As ion size increases, ionisation energy decreases
What is the trend for how easy it is to get a noble gas to bond? Do noble gases form ions? Why?
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
What are the range of strengths of hydrogen bonds?
5-30kJmol-1
What are the range of strengths of covalent bonds?
140-1000(for multiple bonds)kJmol-1
What are the range of strengths of ionic bonds?
500-5500kJmol-1
How do multiple bonds compare to single bonds?
They are stronger and shorter
How does bond strength and size change as the size of atoms increase? Why?
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
What are the range of bond lengths of single bonds?
90-220pm
What are the range of bond enthalpies of single bonds?
250-600kJmol-1
How do bonds to electronegative atoms compare to bonds to electropositive atoms? Why?
They are stronger and shorter as the electrons in the bond have a stronger Zeff value.
What is the distance at which the covalent bond is most thermodynamicaly stable called?
The equlibrium bond length
Which elements always obey the octet rule?
Period 3, Li to Ne.
Why does BF3 obey the octet rule?
It has mesomeric effects that forms a double bond to a fluorine atom with formal charges.
Name 2 exceptions to the octet rule
[CPh3]+ and AlCl3 which both have 6 electrons on its central atom
What is the term for an atom with an expanded octet? Which atoms do this?
Hypervalent, main group atoms down the table
What 3 groups of elements does the octet rule never apply to? Why?
Transition metals, lanthanides and actinides as they have d and f orbits.
What are the 4 steps to working out Lewis structures?
- Number of electrons in valence shell (usually group number)
- Add one electron for a single bond, 2 for a double, 3 for a triple
- Add one electron for each unit of negative charge and subtract one electron for each unit of positive charge
- Formal charge=valence electrons-½(electrons in shared bonds)-unshared electrons
What are the 6 steps to work out the shape of a molecule?
- Take the number of valence electrons (group number)
- Add electrons for the substituents, 1 for single bond, 2 for double bond, 3 for triple bond
- Add/remove electrons for charge
- Divide by 2 and take away 1 for each multiple bond to give the number of valence electron pairs (VEP)
- Deduce the electron pair geometry (EPG) from the number of VEPs
- Deduce the molecular geometry from the number of bonding and lone pairs
What are the shapes of the molecules with no lone pairs and VEPs from 2 to 6?
2-Linear, 3-Trigonal planar, 4-Tetrahedral, 5-Trigonal bypyramidal, 6-Octahedral
What are the shapes of the molecules with 1 lone pairs and VEPs from 3 to 6?
3-Bent, 4-Pyramidal, 5-Seesaw, 6-Square based pyramid
What are the shapes of the molecules with 2 lone pairs and VEPs from 4 to 6?
4-Bent, 5-T-shaped, 6-Square planar
What are the shapes of the molecules with 3 lone pairs and VEPs from 5 to 6?
5-Linear, 6-T-shaped
What are the bond angles in CH4 and NH3?
CH4-109.5º NH3-106.7º
What is the effect of electronegative groups on bond angle?
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)
What is the effect of a multiple bond on molecular shape?
They occupy more space than single bonds so the moclecule is distored in a similar way to a lone pair.
Why is the T-shaped shape found for a molecule with 5 electron pairs and 2 lone pairs?
To minimise repulsions between electron pairs
Why is VSEPR limited at the bottom of the periodic table?
Because lone pairs can occupy the s orbital and be stereochemically inactive which cannot be accounted for by VSEPR
Where does the nucleus lie in a sp hybridised orbital?
In the small lobe, NOT on the radial node
How is ethanes carbons hybridised?
sp2 hybridisation with one p orbit out of the plane of the shape
What effect does hybridisation have on bond length?
As the proportion of s increases in hybridisation, the bonds get shorter as the s electrons are held closer to the nucleus
How do you work out what hybridisation an atom takes?
The number of single bonds and lone pairs added is the number of orbits, 2-sp, 3-sp2, 4-sp3
