Bonding - Unit 1, Section 3

Question 1

Ionic bonding - compounds

Answer 1

electrostatic attraction holds positive and negative ions together.

when atoms are held together in a lattice, it is called ionic bonding.

when oppositely charged ions form ionic bonds you get an ionic compoud.

e.g: NaCl, MgO, MgCl2

Question 2

giant ionic lattices

Answer 2

ionic crystals are giant lattices of ions.

a lattice is a regular structure.

e.g: NaCl

Question 3

Behaviours of ionic compounds x3

Answer 3

Electrical conductivity - conduct electricity when molten or dissolved but not when solid. Ions in a liquid are free to move and carry a charge, whereas in a solid they are in a fixed position by the strong ionic bonds.

Melting point - have high melting points as giant ionic lattices are held together by strong electrostatic forces. Takes a lot of energy to overcome the forces so melting points are very high.

Solubility - ionic compounds tend to dissolve in water. Water molecules are polar and pull the ions away from the lattice, causing it to dissolve.

Question 4

Covalent bonding

Answer 4

occurs when 2 or more atoms are bonded together to form a molecule.

covalent bonds can be single, double or triple bonds.

Question 5

Single covalent bond

Answer 5

2 atoms share electrons so they both have got a full outer shell.

Contains a shared pair of electrons.

Both the positive nuclei are attracted electrostatically to the shared electrons.

Question 6

Double covalent bonds

Answer 6

contain 2 shared pairs of electrons

Question 7

Triple covalent bonds

Answer 7

contains 3 shared pairs of electrons

Question 8

Simple covalent compounds

Answer 8

compounds that are made up of lots of individual molecules.

the atoms in the molecules are held together by strong covalent bonds, but the molecules within the simple covalent compound are held together by weaker van der waals forces (intermolecular forces.
it is the intermolecular forces that determine the properties of simple covalent compounds.

have low melting and boiling points, are electrical insulators.

Question 9

giant covalent structures

Answer 9

a type of crystal structure.

have a huge network of covalently bonded atoms.

sometimes called macromolecular structures.

carbon atoms form this type of structure because they can each form 4 strong covalent bonds.

graphite and diamond are 2 examples of giant covalent structures.

Question 10

Graphite

Answer 10

• the carbon atoms are arranged in sheets of flat hexagons covalently bonded with 3 bonds each.
• The 4th outer electron of each carbon atom is delocalised. The sheets of hexagons are bonded together by weak van der Waals forces.
• the weak bonds between the layers in graphite are easily broken, so the sheets can slide over each other. Graphite feels slippery and is used as a lubricant and in pencils.
• the delocalised electrons in graphite are free to move along the sheets so an electric current can flow.
• the layers are quite far apart compared to the length of the covalent bonds, so graphite has a ow density and is used to make strong, lightweight sports equipment.
• because of the strong covalent bonds in the hexagon sheets, graphite has a very high melting point.
• it is insoluble in any solvent as the covalent bonds are too difficult to break.

Question 11

Diamond

Answer 11

• each carbon atom is covalently bonded to 4 other carbon atoms.
• the atoms are arranged in a tetrahedral shape -its crystal lattice structure.
• diamond has a very high melting point due to strong covalent bonds.
• extremely hard due to strong covalent bonds - it is used in diamond-tipped drills.
• vibrations travel easily through the stiff lattice so it’s a good thermal conductor.
• it can’t conduct electricity as there are no delocoalised electrons.
• won’t dissolve in any solvent.

Question 12

dative covalent bonds (co-ordinate bonds)

Answer 12

one of the atoms provides both of the shared electrons

e.g: NH4+ - nitrogen donates a pair of electrons

in diagrams it is shown by an arrow pointing away from the ‘donor’ atom.

co-ordinate bonds form when one of the atoms in the bond has a lone pair of electrons and the other doesn’t have any electrons available to share.

Question 13

charge clouds

Answer 13

an area where you have a high chance of finding an electron

Question 14

how do molecules get their shapes?

Answer 14

depends on the number of pairs of electrons in the outer shell of the central atom.

electrons can be shared (bonding pairs) or unshared (lone pairs)

Question 15

Electron pair repulsion theory

Answer 15

electrons are all negatively charged, so charge clouds repel each other until they are as far apart as possible.

The shape of the cloud affects how much it repels other charge clouds.

Lone-pair charge clouds repel more than bonding pair charge clouds, so bond angles are often reduced because bonding pairs are pushed together by lone-pair repulsion.

lone-pair/lone-pair angles are biggest

lone-pair/bonding-pair angles are second biggest

bonding-pair/bonding-pair angles are smallest

Question 16

drawing shapes of molecules

Answer 16

solid line - shows bonds that aren’t pointing away from or towards you

wedge - shows a bond pointing towards you

broken line - shows a bond pointing away from you

Question 17

finding the number of electron pairs

Answer 17

1. find the central atom
2. work out how many electrons are in the outer shell of the central atom
3. add 1 electron for every atom that the central atom is bonded to
4. add up all the electrons
5. divide by 2 to find the number of electron pairs
6. compare the number of electron pairs to the number of bonds to find the number of lone pairs and the number of bonding pairs on the central atom

Question 18

molecular shapes x11

Answer 18

LINEAR
- 2 bonding pairs
- bond angle = 180*

TRIGONAL PLANAR
- 3 bonding pairs
- bond angle = 120*

TETRAHEDRAL
- 4 bonding pairs - no lone pairs
- bond angle = 109.5*

TRIGONAL PYRAMIDAL
- 3 bonding pairs and 1 lone pair
- bond angle = 107*

NON-LINEAR
-2 bonding pairs and 2 lone pairs
-bond angle = 104.5*

TRIGONAL BIPYRAMIDAL
- 5 bonding pairs
- 3 bonds angles = 120*
- 2 bond angles = 90*

SEESAW
- 4 bonding pairs and 1 lone pair
- bond angles = 102* and 86.5*

T-SHAPE
-3 bonding pairs and 2 lone pairs
-bond angle = 87.5*

OCTAHEDRAL
- 6 bonding pairs
- bond angle = 90*

SQUARE PYRAMIDAL
- 5 bonding pairs and 1 lone pair
- bond angle = 90*

SQUARE PLANAR
- 4 bonding pairs and 2 lone pairs
- bond angle = 90*

Question 19

Polarisation

Answer 19

occurs because of the nature of different atomic nuclei

Question 20

electronegativity

Answer 20

the ability to attract the bonding electrons in a covalent bond

measured on the Pauling scale

a higher number means an element is more electronegative

fluorine is the most electronegative

Question 21

Non-polar bonds

Answer 21

covalent bonds in diatomic gases are non-polar because the atoms have equal electronegativities so the electrons are equally attracted to the nuclei

Question 22

Polar bonds

Answer 22

in a covalent bond between 2 atoms of different electronegativities, the bonding electrons are pulled towards the more electronegative atom. This makes he bond polar.

the greater the difference in electronegativity, the more polar the bond.

the difference n electronegativity between the 2 atoms causes a dipole.

Question 23

what is a dipole?

Answer 23

a difference in charge between the 2 atoms in a covalent bond caused by a shift in electron density in the bond.

Question 24

Polar molecules

Answer 24

molecules that have a permanent dipole.

a molecule has a permanent dipole if charge is distributed unevenly over the whole molecule.

whether or not a molecule is polar depends on whether it has any polar bonds and its overall shape.

Question 25

Simple polar molecules

Answer 25

HCl has one polar bond that means charge is distributed unevenly across the whole molecule so it has a permanent dipole.

Question 26

Complicated molecules - CO2

Answer 26

CO2 has 2 polar bonds that are arranged symmetrically so the dipoles cancel each other out. This means the molecule does not have a permanent dipole and is non-polar.

Question 27

Complicated polar molecules - H20, CHCl3, CH2F2

Answer 27

the polar bonds are arranged so they all point in roughly the same direction but they do not cancel each other out. This means the charge is arranged unevenly across the whole molecule and this results in a permanent dipole and polar molecule.

Question 28

what are intermolecular forces?

Answer 28

forces between molecules

much weaker than covalent, ionic or metallic bonds

3 types = induced dipole-dipole (van der Waals forces), permanent dipole-dipole forces and hydrogen bonding.

Question 29

Van der Waals forces (Induced dipole-dipole)

Answer 29

• cause all atoms and molecules to be attracted to each other

-at any moment, the electrons in an atom are more likely to be at one side. This causes a temporary dipole.

• this dipole can cause another temporary dipole in the opposite direction on a neighbouring atom.
• the 2 dipoles are then attracted to each other.
• the second dipole can then cause another dipole in a third atom.
• it is a domino effects as electrons are constantly moving so dipoles are created and destroyed all the time.
• even though dipoles keep changing, the overall effect is for atoms to be attracted to each other.
• not all van der Waals forces have the same strength - larger molecules have larger electron clouds and therefore stronger forces.

-shape impacts forces - long, straight molecules can lie closer together than branched ones - the coser together the 2 molecules are, the stronger the forces.

Question 30

permanent dipole-dipole forces

Answer 30

in a substance made up of molecules that have permanent dipoles, there will be weak electrostatic forces of attraction between the opposite charges on neighbouring molecules. These are called permanent dipole-dipole forces.

Question 31

Hydrogen bonding

Answer 31

• the strongest intermolecular force.
• only happens when hydrogen is covalently bonded to fluorine, nitrogen or oxygen.
• fluorine, nitrogen and oxygen are very electronegative so they draw the bonding electrons away from the hydrogen atom.
• the bond is very polarised and hydrogen has a high charge density as it is so small, that the hydrogen atoms form weak bonds with lone pairs of electrons on the fluorine, nitrogen or oxygen atoms of any other molecule.
• molecule which have hydrogen bonding are usually organic, including OH and NH groups.

Question 32

effects of hydrogen bonding

Answer 32

• substances with hydrogen bonding have higher melting and boiling points because extra energy is required to break the hydrogen bonds.
• as water turns into ice, more hydrogen bonds are formed and are arranged into a regular lattice structure.
• ice is less dense than water because the distance between H20 molecules is greater in ice than in water. This is unusual as most substance are more dense as solids than they are as liquids.

Question 33

behaviour of simple covalent compounds

Answer 33

• have strong covalent bonds within molecules but weak forces between molecules

Electrical conductivity - don’t conduct electricity because there are no free ions or electrons to carry the charge.

Melting point - have low melting points because the weak forces between molecules are easily broken.

Solubility - some dissolve in water depending on how polarised the molecules are.

Question 34

trends in melting and boiling points - Group 7

Answer 34

• the main factor affecting melting and boiling points is the strength of the van der waals forces, unless the molecule can form hydrogen bonds.
• as you go down group 7, the polarity of molecules decreases, so the strength of the permanent dipole-dipole interactions decreases.
• as you go down group 7, the number of electrons in the molecules increases so the strength of the van der waals forces increases.
• the boiling points increase as you go down because the increasing strength of the van der waals has a greater effect than the decreasing permanent dipole-dipole interactions.

Question 35

metallic bonding

Answer 35

• the positive metal ions are attracted to the delocalised negative electrons. They form a giant lattice of closely packed positive ions in a sea of delocalised electrons.

Question 36

properties of metals

Answer 36

Melting point - have high melting points because of the strong electrostatic attraction between positive metal ions and the delocalised electrons. The number of delocaised electrons per atom affects the melting point. E.g: the more there are, the stronger the bonding will be and the higher the melting point. The size of the metal ion and the lattice structure also effect the melting point.

Ability to be shaped - as there are no bonds holding specific ions together, metal ions can slide over each other when the structure is pulled, so metals are malleable and ductile.

Conductivity - delocalised electrons can pass kinetic energy to each other making metals good thermal conductors. The delocalised electrons can also move and carry a charge so metals are good electrical conductors.

Solubility - metals are insoluble, except in liquid metals, because of the strength of the metallic bonds.

Question 37

melting and boiling covalent substances

Answer 37

• in simple covalent substances, the covalent bonds don’t break during melting and boiling. To melt or boil them, you only have to overcome the weak intermolecular forces that hold the molecules together.This is why they have relatively low melting/boiling points.

To melt or boil a giant covalent substance, you need to break the covalent bonds holding the atoms together. This is why they have very high melting/boiling points.

Question 38

Physical properties of materials

Answer 38

Melting/Boiling points - determined by the strength of attraction between its particles.

Electrical conductivity - a substance can only conduct electricity if it contains charged particles that are free to move. e.g: delocalised electrons.

Solubility - how soluble a substance is depends on the type of particles that it contains.

Question 39

summary of ionic bonding properties

Answer 39

• high melting/boiling point
• solid under standard conditions
• only conducts electricity when molten or dissolved
• soluble in water

Question 40

summary of simple covalent bonding properties

Answer 40

-low melting/boiling points
- usually gas or liquid under standard conditions
- can’t conduct electricity
- an be soluble in water depending on how polar the molecules are.

Question 41

summary of giant covalent bonding properties

Answer 41

-high melting/boiling points
- solid under standard conditions
-can’t conduct electricity (expect graphite)
- insoluble in water

Question 42

summary of metallic bonding properties

Answer 42

• high melting/boiling points
  -solid under standard conditions
  -can conduct electricity
• insoluble in water