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Edexcel Chemistry > 2. Bonding and structure ( COVALENT) > Flashcards

2. Bonding and structure ( COVALENT) Flashcards

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1
Q

What is covalent bonding

A

a chemical bond formed from the electrostatic attraction between the shared pair of electrons and the two nuclei

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2
Q

What 2 structures does covalent bonding form

A

simple covalent molecule
giant covalent structure

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3
Q

Give properties of giant covalent molecules due to their strong covalent bonds

A

High melting boiling points
Hard
Insoluable

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4
Q

Why do simple covalent molecules have low melting and boiling points

A

While they have strong intramolecular forces, there are only weak forces between molecules.

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5
Q

What is graphene

A

A single layer of graphite

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6
Q

What shape does diamond form

A

Tetrahedral

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7
Q

What is dative covalent bonding

A

When one atom donates 2 electrons to an atom or an ion to form a bond
– > One of the bonding atoms supplies both the shared electrons to the covalent bond

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8
Q

State the type of bond that joins the two AlCl3 molecules together

A

Dative covalent bonding

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9
Q

Explain how ammonia and boron trifluoride react together to form NH3BF3 which contains a dative
covalent bond

A

NH3 shares its lone pair of electrons to BF3 as it is electron deficient and only has 6 valence electrons

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10
Q

How do you represent dative covalent bonding in diagrams

A

Using arrows - arrow points from the atom donating the lone pair to the atom accepting it.

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11
Q

What are valence electrons

A

electrons in the outer shell of an atom

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12
Q

what are lone pairs

A

a pair of electrons not used in the covalent bonding

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13
Q

Which elements can expand their octets

A

Elements after group 3 - utilise their empty subshells to promote an electron

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14
Q

What are electron deficient molecules ( and so don’t follow octet rule) - give an example

A

Molecules with too few electrons in outer shell so dont have 8
eg: BF3 - B can only give 3 electrons

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15
Q

What are sigma bonds

A

Formed from the head on overlap of 2 atomic orbitals:
s orbital + s orbital
s orbital + p orbital
p orbital + p orbital

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16
Q

What is a pi bond

A

Formed from the side overlap of a p orbitals

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17
Q

What bonds does a double bond contain

A

Both sigma bond and pi bond

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18
Q

What does the shape of a molecule depend on

A

Number of bonding pairs and lone pairs

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19
Q

Which repel more: lone pairs or bonding pairs

A

Lone pairs

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20
Q

By how many degrees do lone pairs reduce the angle of the bonding pairs

A

For every lone pair present, bond angles get reduced by 2.5 degrees
–> lone pairs repel more than bonding pairs so they push bond pairs closer together. Therfore if you have a lone pair the angle goes from 109.5 - 2.5 = 107. If you have 2 bonding pairs 109.5 - 5 = 104.5

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21
Q

Give the 6 steps to work out the shape of a covalent molecule or molecular ion by determining the number of lone pairs and bonding pairs

A

1) Work out number of outer shell electrons in the central atom (atom that other atoms are bonded to)
2) Work out how many electrons that bonding electrons bring from molecular formula ( eg in NH3, there are 3)
3) Add answers from 1 and 2 together
4) Add/ remove electrons for ions ( ignore this step if not)
5) Divide by 2 to get number bonding pairs
6) Compare to molecular formula to get number of lone pairs

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22
Q

2BP 0LP

A

Linear, 180

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23
Q

2BP 2LP

A

Bent (non linear), 104.5

24
Q

3BP 0LP

A

Trigonal planar, 120

25
Q

3BP 1 LP

A

Trigonal pyramidal, 107

26
Q

4BP 0LP

A

Tetrahedral, 109.5

27
Q

4BP 2LP

A

Square planar, 90
(idk if this one needed)

28
Q

5BP OLP

A

Trigonal bipyramidal, 90,120

29
Q

6BP, 0LP

A

Octahedral, 90

30
Q

Give example of linear molecules

A

Carbon dioxide (CO2)

31
Q

Give example of bent molecules

A

Water (H20)

32
Q

Give examples of tetrahedral molecules

A

Ammonium ion (NH4+)

33
Q

Give examples of trigonal planar molecules

A

BCl3

34
Q

Give example of trigonal pyramidal molecules

A

Ammonia (NH3)

35
Q

Give example trigonal bipyramid molecules

A

PCl5
–> the one with both 90 and 120 degrees

36
Q

Give example of octahedral molecules

A

SF6

37
Q

Give example of square planar molecules

A

XeF4

38
Q

How to explain each shape of molecule

A

1) state number of each type of electron pair
2) state that electron pairs arrange themselves to point of maximum separation for minimal repulsion
3) if there are no lone pairs state that bonding pairs repel equally
4) if there are lone pairs, state the lone pairs repel more than bonding pairs
5) state shape and angle

39
Q

What is electronegativity

A

The tendency of an atom to attract the bonding pair of electrons to itself within a covalent bond.

40
Q

Give the trend of electronegativity across the period and down a group

A

increases across a period (same shielding but increased nuclear charge) , decreases down a group (increased atomic radius)

41
Q

What is the Pauling scale used for (given in data booklet)

A

measures electronegativity to be compared
- large difference means bond is more ionic as electron pair will be pulled more to one end of the bond
- difference of 0 is purely covalent (this occurs where electronegativity is evenly arranged within a bond)

(to work out difference of electronegativity work out difference between different elements - you do not need to take into account the little numbers)

42
Q

Describe how to test for a polar substance

A

1) hold charged rod close to a jet of liquid flowing from burette - can do this by rubbing a cloth on rod to make it positively charged
2) polar liquids will bend towards the rod as the dipoles arrange themselves to be attracted to the rod
3) greater bend = greater dipole

43
Q

How does a polar covalent bond form

A

forms between two atoms with different electronegativities. Bonding electrons pulled closer to the slightly negative atom.
- this causes a unequal distribution of electrons and a dipole forms

44
Q

When do atoms with polar covalent bonds not produce polar molecules

A

–> When the atoms have equal electronegativities so bonding electrons are directly in the middle of atoms
–> When the molecule is symmetrical so electronegativities point in opposite directions and polar bonds cancel each other out

45
Q

What is the most electronegative element in the periodic table

A

Fluorine
–> more electronegative than oxygen and nitrogen because despite having same number of shells, F has a larger nucleur charge

46
Q

What are London forces and how are they formed

A
  • instantaneous dipole-induced dipole attractions
  • present in all molecular substances and noble gases but not ionic substances
    1) temporary dipole forms as electrons are moving constantly and randomly leading to a fluctuation in electron density
    2) this distorts electron density in neighbouring molecules and induces another temporary dipoles
    3) these two dipoles form an attraction to each other for a tiny amount of time
47
Q

Give the main factor affecting strength of London forces

A

size of atom - more electrons increasing chances of dipoles forming which increases the strength of the London forces between molecules

(also surface area)

48
Q

Explain why do boiling points of halogens increase down group 7

A

London forces - more electrons as atoms get bigger

49
Q

What are permanent dipole-dipole forces

A
  • these are the weak electrostatic forces that occur between polar molecules.
  • stronger than London forces but weaker than hydrogen bonds
  • London forces are also present
50
Q

What is hydrogen bonding

A

the intermolecular force in which a hydrogen atom that is bonded to a highly electronegative atom is attracted to an lone pair of electrons of an electronegative atom in a nearby molecule
- stronger than other 2 types of intermolecular forces

51
Q

Which 3 elements can Hydrogen form hydrogen bonds with

A

Nitrogen
Oxygen
Fluorine

52
Q

What is the bond angle of a hydrogen bond

A

180°

53
Q

What causes the hydrogen bonding to be stronger in a molecule

A

The number of hydrogen bonds it has - eg: Water is able to form hydrogen bonds to 4 other molecules but HF only to 2 other molecules.

54
Q

Give the order of strength of the intermolecular forces, going from weakest to highest

A

London forces - weakest
Permanent dipole - dipole forces
Hydrogen bonding - strongest

55
Q

Explain why ice floats on water

A

In ice the molecules are held further apart by the hydrogen bonds than in liquid water due to 3 dimensional structure. The hydrogen bonds in ice are longer than covalent bonds

Edexcel Chemistry (15 decks)

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