Bonding And Shapes Of Molecules Flashcards
What are intramolecular bonding forces?
Forces of attraction between atoms within a molecule
What are intermolecular bonding forces?
Forces of attraction between neighbouring molecules
Give examples of intramolecular bonding
- covalent bonding
- polar covalent bonding
- ionic
What is covalent bonding?
Sharing of or more pairs of electrons between two atoms
What is polar covalent bonding
Unequal sharing of one or more pairs of electrons by two atoms
What is ionic bonding?
Electrostatic force of attraction between oppositely charged ions
What are weak Van Der Waals forces?
Temporary dipoles in molecule attracting/repelling temporary dipoles in next molecule
What are permanent dipole-dipole attractions
Permanently polarised molecules attract polarised neighbour, where opposite ends attract
What is hydrogen bonding?
Permanently polarised molecules which have hydrogen and one of the elements N, O or F
What is electronegativity?
It’s the relative attraction an atom has for shared pairs of electrons in a covalent bond
Use electronegativity values to predict the type of bonding in
phosphine
potassium chloride
i) pure covalent
ii) polar covalent
CHART
0 - 0.7 - 1.7
cov - pol cov - ionic
Explain why potassium chloride has a much higher boiling point than phosphine (PH3)
Phosphine is held together by weak Van der Waals forces whereas potassium chloride is held together by ionic bonds
Phosphine is virtually insoluble in water, whereas the corresponding hydride of nitrogen which is in the same group as phosphorus is very soluble in water, how so?
The corresponding hydride is NH3, which contains a lone pair making the whole molecule polar covalent [electronegative differences]
This results in bonding with water and therefore dissolves in water
Phosphine is non polar and has little/no attraction to the water
What is water of crystallisation?
It’s water chemically combined in definite proportions in a crystalline compound
Give two uses of noble gases
Helium - in blimps
Argon - in light bulbs
What is the octet rule?
Atoms on reaction tend to reach an electronic arrangement with eight electrons in their outermost shell
What are the exceptions to the octet rule?
Beryllium and Boron - few electrons in their outer levels so they cannot gain enough electrons to reach eight
D block elements
What is valency?
Number of bonds an atom of an element forms when it reacts
What group of elements have variable Valencies and give an example
Transition elements
Iron - 2,3,6
Copper - 1,2
What do metal atoms become when they lose one or more electrons?
Positively charged - Cation
How is a cation smaller than the corresponding atom?
It has lost electrons and is positively charged and therefore lost a shell
Characteristics of Ionic Substances
Form crystal lattices
High melting and boiling points due to strong forces of attraction
Polar
Soluble in water
Conduct electricity in molten/aqueous state
Give examples of ionic materials in everyday life
Salt tablets - to replace salt lost in sweating
Brine - curing bacon
What factors dictate the extent to which the shared electrons are attracted by different nuclei?
Size of the Atom - smaller atom have a stronger attraction because they can get closer to electrons
Nuclear charge - atoms with bigger nuclear charge will have greater attraction for the shared electrons
Characteristics of Covalent Bonding
Low melting and boiling points - gases at room temperature due to weak intermolecular forces
Do not conduct electricity due to the fact they consist of neutral molecules
Do not dissolve readily in water
Give examples of polar materials in every day life
Water - washing clothes
Glucose - lucozade
Give examples of non - polar materials in every day life
Petrol
Cooking oil
Account for the general trend of electronegativity across a period
Increases
- nuclear charge inc, positive attractive force on electrons
- atomic radius decreases as electrons are pulled in more tightly
- no screening
Account for the general trend of electronegativity down a group
- extra levels means there is extra screening
- atomic radius inc caused by extra energy levels means electrons aren’t as tightly held and no attracted as strongly
Distinguish between a sigma and a pi covalent bond
Sigma - end on overlap of orbitals
Pi - sideway overlap of p orbitals
What does the electron repulsion theory state?
Electron pairs in outer shell of central atom repel each other and end up as far apart as is geometrically possible
Lone pairs have a greater repelling effect than bonding pairs
What are the posible shapes sit molecules of general formula : AB2
Valency = 2
2 bonds
Linear, V-shaped
Account for the difference in shape of NH3 and BF3
Draw out
NH3 - three bond pairs and one lone pair
BF3 - three bond pairs
Describe the bonding in the nitrogen molecule in terms of sigma and pi bonding
Sigma = 1 Pi = 2
What type of intermolecular forces would you expect to find in nitrogen gas? Explain your answer
Van Der Waals, it is non polar
If a molecule contains two bonds, what shapes can it be?
No lone pairs - Linear
Lone pairs - V-shaped
If a molecule contains three bonds, what shapes can it be?
No lone pairs - Trigonal Planar
Lone pairs - Pyramidal
If a molecule contains four bonds, what shapes can it be?
Lone pairs - Tetrahedral
What bond angle does linear have?
180
What bond angle does v shape planar have?
104.4
What bond angle does trigonal planar have?
120
What bond angle does pyramidal have?
107.3
What bond angle does tetrahedral have?
109.5
Identify in ammonia, the type of intermolecular bonding and intermolecular forces present
Intramolecular - polar covalent bonding
Intermolecular - hydrogen bonds
What is the molecular formula of benzene
C6H6
Account for the difference in the shapes of BF3 and NH3 molecule
Boron - three bond pairs
Nitrogen - three bond pairs and one lone pair
How do you know how many sigma and pi bonds there are?
Every element forms only a single covalent bond. That is sigma bond. If there are 3 bonds, one must be covalent and other 2 pi
Look at valency to see how many bonds
What is bond energy?
Its the average energy required to break a bond and seperate atoms in gaseous state
The boiling points of hydrogen and oxygen are 20.0K and 90.2K. Account for the higher boiling point of oxygen
Stronger intermolecular forces (dipole-dipole)
Which are the non polar bond shapes?
Linear, trigonal planar, and tetrahedral.
Which are the polar bond shapes?
V-shaped and Pyramidal
If the eletronegativy difference is i) 0-.7 ii).7-1.7 iii) 1.7< what kind of bonds does it have?
i) Covalent (non-polar)
ii) Polar covalent
iii) Ionic
Give one property that is affected by the presence of intermolecular bonding?
Melting point
Boiling point
Density
Solubility in water
Would you expect i) the B-Cl bonds and ii) the BCL3 molecules to be polar or non polar? Justify
i) Non-polar -> large e.n difference
ii) Polar -> unequal sharing of electrons between B and Cl is cancelled due to symmetry of the molecule as the centres of positive and negative coincide
Indicate on your dots and crosses bonding diagram for chlorine monofluoride any full or partial charges that you would expect to result from the formation of the bond between chlorine and fluorine?
E.N values : Cl (3.0) and Fl (4.0)
The atom with greater electronegativity - partial negative
The atom with smaller electronegativity - partial positive
In this case, chlorine is partially positibe and fluorine is partially negative
NH3, PH3, AsH3
Which if any of the three hydrides would you expect to have intermolecular hydrogen bonding and justify
NH3
Hydorgen is bonded to Nitrogen - small and very electronegative
NH3, PH3, AsH3
Suggest a reason why ammonia has the highest boiling point of the three hydrides and phosphine’s boiling point is lower than that of arsine
i) Hydrogen bond - strongest of intermolecular forces
ii) Smaller mr
How would you know if a compound is water soluble or not?
Find electronegativity values and see if nonpolar or polar
Account for the difference in bond angle between the two molecules, 107.3 in ammonia and 109.5 in silane
Lone pair in ammonia - greater repulsion power than bond pair and pushes bonds closer to form a smaller bond angle
Give the reason why a molecule with polar bonds can be non-polar
Symmetry of molecule - Centres of positive and negative charges coincide
Why do ionic substances conduct electricity when molten or dissolved in water but not in solid state?
Molten - ions free to move
Solid - not free, fixed
Show that the ammonia molecule has polar covalent bonding
The electronegativity difference between N and H show unequal sharing
Describe the processes involved when ammonia dissolves in water
- Breaking of hydrogen bonds in water
- Forming of hydrogen bonds between ammonia and water
Suggest a reason why the boiling point of ammonia (-33) is significantly lower than that of water (100)
Weaker bonding in ammonia (small e.n difference)
Draw a diagram illustrating hydrogen bonding in ammonia
H H
H N - H N
H H
Use your knowledge of intermolecular forces to explain why methane has a very low boiling point
very weak intermolecular forces
The relative molecular mass of methane is only slightly lower than that of water but the boiling point of water is much higher. Suggest a reason for this
much stronger hydrogen bonds between water molecules
A thin stream of liquid flows from the burette. A stream of water is deflected towards a positively chargd rod whereas a stream of cyclohexane is undeflected. Account for these observations
polarity of water causes attraction to charged rod
non-polarity of cyclohexane means it is not affected by charged rod
Explain what would happen in the case of the stream of water if the positively charged rod were replaced by a negatively charged rod
stream of water still attracted to rod as molecules (dipoles) arrange themselves with
positive pole towards rod
(weakly polar) Would you expect hydrogen sulfide to be soluble in water? Explain your answer
Only slightly - weakly polar
Describe the bonding in the nitrogen molecule in terms of sigma and pi bonding
one sigma and two pi
Why will water always be deflected by any charged rod?
Because it had a dipole moment i.e the centre of positive charge does not coincide with centre of negative charge
