Bonding and structure Flashcards
(63 cards)
1
Q
What is an ionic bond?
A
The strong electrostatic attraction between 2 oppositely charged ions.
2
Q
What 2 things affect the strength of an ionic bond?
A
Ionic charges
Ionic radii
3
Q
How does the ionic charge affect the strength of an ionic bond?
A
- The greater the charge of an ion, the stronger the ionic bond, therefore, the higher the mpt/bpt.
- E.g. CaO has a much higher mpt than NaF
4
Q
How does the ionic radii affect the strength of an ionic bond?
A
- Smaller ions can pack closer together than larger ions.
- Electrostatic attraction gets weaker with distance, so small, closely packed ions have stronger ionic bonding than larger ions, which sit further apart.
- So ionic compounds with small, closely packed ions have higher mpt/bpt than ionic compounds made of large ions.
- E.g. NaF has a greater bpt than CsF.
5
Q
What are 2 trends in ionic radii you must know about?
A
- Ionic radius increases as you go down a group.
- Ionic radius of a set of isoelectronic ions decreases as the atomic number increases.
6
Q
Explain why ionic radius increases as you go down a group.
A
- Charge doesn’t change
- Ionic radius increases as the atomic number increases
- Due to extra shells being added
7
Q
Explain why Ionic radius of a set of isoelectronic ions decreases as the atomic number increases.
A
- Number of electrons doesn’t change, but the number of protons increases
- This means that the electrons are attracted to the nucleus more strongly, pulling them in a little, so ionic radius decreases
8
Q
What do dot and cross diagrams look like for ionic compounds?
A
- Separate atoms with all their shells
- A square bracket round each atom, with the charge on the outside of the bracket.
- Dots for 1 species and crosses for another
9
Q
What shape do ionic compounds form? Give an example
A
- Giant ionic lattice
- Each ion is electrostatically attracted in all directions
- NaCl lattice is cube shaped
10
Q
What are some physical properties of ionic compounds that supports the theory of ionic bonding?
A
- High mpt - tells you that they’re held by a strong attraction
- Often soluble in water but not in non-polar solvents - tells you that the particles are charged. Ions are pulled apart by polar molecules, but not non-polar molecules.
- Ionic compounds don’t conduct electricity when solid, only when molten or dissolved. Supports the idea that there are ions fixed in position by strong ionic bonds in a solid, but are free to move as a liquid/in solution.
- They can’t be shaped, due to the repulsion between the same ions being very strong, so ionic compounds are brittle - supports the lattice model.
11
Q
What is a covalent bond?
A
The strong electrostatic attraction between 2 +ve nuclei and the shared pair of electrons between them.
12
Q
What atoms do covalent bonds form between?
A
2 non-metals
13
Q
What atoms do ionic bonds form between?
A
A metal and a non-metal
14
Q
What does a dot and cross diagram for covalently bonded compounds look like?
A
- Atoms are drawn with their outer shells overlapping.
- Shared electrons are drawn within this overlapping area
- Usually central atom ends up with 8 electrons.
15
Q
What is the bond length?
A
The distance between the 2 nuclei
16
Q
What is the relationship between bond enthalpy and bond length?
A
The shorter the bond length, the greater the bond enthalpy
17
Q
Explain how bond length relates to the electron density between the 2 nuclei in a covalent compound?
A
The higher the electron density between the nuclei (the more electrons in the bond), the stronger the attraction between the atoms, the higher the bond enthalpy and the shorter the bond length.
18
Q
Compare a C=C and C-C bond in terms of their electron density, bond length and bond enthalpy?
A
- A C=C bond has a greater bond enthalpy and is shorter than a C-C bond.
- 4 electrons are shared in C=C and only 2 in C-C, so the electron density between the 2 carbon atoms is greater and the bond is shorter.
- C≡C has an even higher bond enthalpy and is shorter than C=C, 6 electrons are shared here.
19
Q
What is a dative covalent bond?
A
Where 1 atom donates both electrons to a bond.
20
Q
How is the dative covalent bond shown in an ammonium ion?
A
Either as 2 circles/crosses, or an arrow from the ‘donor’ atom (nitrogen) to the hydrogen.
21
Q
How does AlCl3 form Al2Cl6?
A
- AlCl3 is a stable covalent compound where the central atom has only 6 electrons.
- 2 AlCl3 molecules can combine to form Al2Cl6.
- 1 Cl in each of the 2 AlCl3 molecules donates a lone pair to the Al on the other molecule, forming 2 dative covalent bonds.
- This gives Al a full outer shell.
22
Q
How are the shapes of molecules and their bond angles determined?
A
- By their number of EP’s, including BP’s and LP’s.
- EP’s repel to maximise separation
- LP’s repel more than BP’s
23
Q
What are the 3 main covalent compounds with 4 EP’s in their outer shell and what are their bond angles?
A
Methane - 0 LP’s, 109.5°
Ammonia - 1LP, 107°
Water - 2LP’s, 104.5°
24
Q
What is the shape and bond angle of a molecule with 2 BP’s round a central atom?
Include an example
A
Linear molecules
180°
BeCl2, CO2
25
What is the shape and bond angle of a molecule with 3 BP's round a central atom?
Include an example
Trigonal planar
120°
BCl3
26
What is the shape and bond angle of a molecule with 2 BP's and 1 LP round a central atom?
Include an example
Bent
119°
SO2
27
What is the shape and bond angle of a molecule with 4 BP's round a central atom?
Include an example
Tetrahedral
109.5°
NH4 +
28
What is the shape and bond angle of a molecule with 3 BP's and 1 LP round a central atom?
Include an example
Trigonal pyramidal
107°
PF3
29
What is the shape and bond angle of a molecule with 2 BP's and 2 LP's round a central atom?
Include an example
Bent
104.5°
H2O
30
What is the shape and bond angle of a molecule with 5 BP's round a central atom?
Include an example
Trigonal bipyramidal
120° and 90° and 180°
PCl5
31
What is the shape and bond angle of a molecule with 4 BP's and 1 LP round a central atom?
Include an example
Seesaw
102° and 87°
SF4
32
What is the shape and bond angle of a molecule with 3 BP's and 2 LP's round a central atom?
Include an example
Distorted T
87.5°
CIF3
33
What is the shape and bond angle of a molecule with 6 BP's round a central atom?
Include an example
Octahedral
90°
SF6
34
What is the shape and bond angle of a molecule with 3 BP's and 2 LP's round a central atom?
Include an example
Square pyramidal
81.9° and 90°
IF5
35
What is the shape and bond angle of a molecule with 4 BP's and 2 LP's round a central atom?
Include an example
Square planar
90°
XeF4
36
Diamond can form a tetrahedral arranged lattice, another compound that can form a 'similar but different lattice arrangement is...
Silicon (IV) dioxide
SiO2
37
What are some properties of giant structures that provide evidence for covalent bonding?
- Very high mpt's - you need to break a lot of very strong before the substance melts, which takes a lot of energy.
- Are often extremely hard - due to very strong bonds all through the arrangement.
- Good thermal conductors - since vibrations travel easily through the stiff lattices.
- Insoluble - covalent bonds mean atoms are more attracted to their neighbours in the lattice than to solvent molecules. The fact they're insoluble in polar solvents shows they don't contain ions.
- Can-t conduct electricity - since there are no charged ions/free electrons that can carry a charge.
38
Explain why graphite can conduct electricity
- Carbon atoms form sheets, with each carbon sharing its outer shell electrons with 3 other carbon atoms.
- This leaves the 4th outer electron in each atom fairly free to move between the sheets, making graphite a conductor.
39
What is graphene?
- A sheet of carbon atoms joined together in hexagons, only 1 atom thick, making a 2 dimensional compound.
- Like graphite, it can conduct electricity as the delocalised electrons are free to move along the sheet.
- Also incredibly strong, transparent, and really light.
40
Describe the structure of metals.
- In metallic lattices, the electrons in the outermost shell are delocalised - they're free to move. Leaving a +ve metal ion.
- The +ve metal ions are electrostatically attracted to the delocalised -ve electrons. Form a lattice of closely packed +ve ions in a sea of delocalised electrons.
41
What do the properties of metals provide evidence for?
- High mpt's due to the strong metallic bonding, with an increase in the number of delocalised electrons per atom increasing the mpt.
- As there are no bonds holding specific ions together, and the layers of +ve metal ions are separated by layers of electrons, metals are malleable and ductile. Layers can be slid over each other without disrupting the attraction.
- The delocalised electrons can pass KE to each other, making metals good thermal conductors.
- They're good electrical conductors because the delocalised electrons are free to move/carry a charge. Any impurities can dramatically reduce electrical conductivity by reducing the number of electrons that are free to move/carry a charge - they transfer to the impurities and form anions.
- They're insoluble, except in liquid metals, due to the strength of the metallic bonds.
42
Electronegativity definition
The ability of an atom to attract the bonding pair of electrons in a covalent bond.
42
How is electronegativity measured and what are some of the most electronegative elements?
Measured using the Pauling scale.
Fluorine is the most electronegative element, oxygen, chlorine and nitrogen are also very electronegative.
43
What are the trends related to electronegativity?
- More electronegative elements have higher nuclear charges and smaller atomic radii.
- So electronegativity increases across periods and up the groups.
44
How can covalent bonds be polarised?
- If both atoms have similar electronegativities, the electrons will sit roughly midway between the 2 nuclei, and the bond will be non-polar.
- The covalent bonds in homonuclear, diatomic gases are non-polar because the electronegativities are equal.
- If the bond is between 2 atoms with different electronegativities, the bonding electrons will be pulled towards the more electronegative atom.
- So electrons are spread unevenly, and there'll be a charge across the bond. So the bond is polar.
- In a polar bond, the difference in electronegativity between both atoms causes a dipole.
45
Give an example of a case where polar bonds don't make polar molecules?
If the polar bonds point in opposite directions, they cancel each other out, so the molecule is non-polar overall. E.g. CO2
46
What are 2 types of intermolecular forces?
- London forces
- Permanent dipole-permanent dipole bonds
- Hydrogen bonding
47
How are London forces formed?
- Electrons in charge clouds are always moving really quickly.
- At 1 moment, the electrons in an atom are likely to be more to 1 side than the other. At this moment, it would have an temporary/instantaneous dipole.
- This dipole can induce another temporary dipole on a neighboring atom. Both dipoles are attracted to each other.
- The 2nd dipole can induce another dipole in a 3rd atom.
- Dipoles are constantly created/destroyed, overall effect is for the atoms to be attracted to each other.
48
How are iodine molecules held in a lattice?
- I atoms are held together in pairs by strong covalent bonds to form I2 molecules.
- The molecules are then held together in a molecular lattice arranged by weak London forces.
- Known as a simple molecular structure
49
How does the strength of London forces affect MPT's and BPT's?
- Larger molecules with larger electron clouds have stronger London forces.
- Molecules with greater surface areas also have stronger London forces because they have a bigger exposed electron cloud.
- Stronger intermolecular forces = higher MPT/BPT
50
Explain how intermolecular forces relate to organic molecules?
- Longer carbon chains have stronger london forces, due to their being more molecular surface contact.
- Branched chain alkanes can't pack closely together, and their molecular surface contact is small compared to straight chain alkanes of similar molecular mass.
51
What is hydrogen bonding?
- It's the strongest intermolecular force
- Only happens when hydrogen is covalently bonded to F, N or O
- F, N and O are very elctronegative, so they draw the bonding electrons away from the H atom
- The bond is so polarised, and H has such a high charge density because it's so small, that the H forms weak bonds with LP's on the F, N or O atoms of other molecules.
52
What are 3 examples of substances with hydrogen bonding?
Water
Ammonia
Hydrogen Fluoride
53
Which organic molecules contain H bonds?
Alcohols
Amines
54
How do H bonds explain why ice floats on water?
- Ice is another simple molecular structure
- In ice, the water molecules are arranged so that there's the max number of H bonds - the lattice structure formed in this way 'wastes' lots of space
- As the ice melts, some of the H bonds are broken and the lattice breaks down, allowing molecules to 'fill' the spaces
- This means ice is a lot less dense than water, so floats on water
55
Why are alcohols less volatile than similar alkanes?
- All alcohols contain a polar hydroxyl group with a δ- on the oxygen and δ+ on the hydrogen. This helps alcohols form H bonds.
- H bonding gives alcohols low volatilities (higher BPT's) compared to non polar compounds, e.g. alkanes, of similar sizes and similar number of electrons.
56
What needs to happen for 1 substance to dissolve in another?
- Bonds in the substance have to break
- Bonds in the solvent have to break
- New bonds have to form between the substance and solvent
- Usually a substance will only dissolve if the strength of the new bonds formed is about the same as/greater than, the strength of the bonds that are broken
57
What are 2 types of solvent?
- Polar: Made of polar molecules, such as water., which are bonded to each other by H bonds. Not all polar solvents can form H bonds, propanone can only form London forces and permanent dipole-permanent dipole bonds.
- Non-polar: E.g. hexane, can only form London forces.
58
How are ionic substances dissolved in water?
- When an ionic substance is mixed with water, the ions in the ionic substance are attracted to the oppositely charged ends of the water molecules.
- The ions are pulled away from the ionic lattice by the water molecules, which surround the ions. Process is called hydration.
59
Why can't some ionic substances dissolve in water?
The bonding between their ions is too strong. E.g. Al2O3 is insoluble in water because the bonds between the ions are stronger than the bonds they'd form with the water molecules.
60
How do alcohols dissolve in water?
- The polar O-H bond in an alcohol is attracted to the polar O-H bonds in water
- H bonds form between the LP's on the δ- O atoms and the δ+ H atoms.
- The carbon chain part of the alcohol isn't attracted to water, so the more carbons there are, the less soluble the alcohol will be.
61
What is an example of a molecule with polar bonds that doesn't dissolve in water?
- Halogenoalkanes contain polar bonds but their dipoles aren't strong enough to form H bonds with water.
- The H bonding between water molecules is stronger than the bonds that would be formed with halogenoalkanes, so halogenoalkanes don't dissolve.
- But halogenoalkanes can form permanent dipole-permanent dipole bonds, so can dissolve in polar solvents that also form these bonds.
62
What do non-polar substances dissolve in?
- Non-polar substances, such as ethene, have london forces between their molecules.
- They form similar bonds with non-polar solvents such as hexane, so dissolve in them.
- Water molecules are attracted to each other more strongly than they are to non-polar molecules such as iodine, so non-polar substances don't easily dissolve in water.
