3.1.3 Bonding

Is a change of state a physical or chemical change, and is it reversible or irreversible?

physical change

reversible

(does not change the chemical properties or chemical makeup of the substances involved)

What are the changes of state?

What is the difference between evaporation and boiling?

evaporation occurs only at the surface and takes place at temperatures below the boiling point
boiling occurs at a specific temperature (boiling point)

Relationship between temperature and energy during state changes

Ionic bonding

• between a metal (cations) and non metals (anions)

• electrons are transferred from the metal to the non metal to achieve full outer shells. this creates charged particles called ions

• oppositely charged ions are arranged in a lattice with strong electrostatic forces of attraction between oppositely charged ions acting in all directions

covalent bonding

• between two non-metals

• (pairs of) electrons are shared to achieve a full outer shells

• a covalent bond is a shared pair of electrons

dative covalent bonding/ coordinate bonding

• both of the electrons in the shared pair are supplied from a single atom

• the atom with a lone pair donates a pair of electrons to an electron deficient atom

• represented by an arrow from the atom providing the lone pair to the atom that is deficient

Metallic bonding

• lattice of positively charged ions surrounded by a ‘sea’ of delocalised electrons

• strong electrostatic force of attraction between positive metal ions and the sea of delocalised electrons

Factors that affect the strength of metallic bonding

1. Number of protons/ strength of nuclear attraction
   → the more protons, the stronger the bond

2. Number of delocalised electrons per atom
   → the more delocalised electrons, the stronger the bond

3. Size of ion
   → the smaller the ion, the stronger the bond

What are the main types of crystal structure?

ionic

metallic

simple molecular

macromolecular

Properties of ionic structures

• high melting and boiling point
  → strong electrostatic forces of attraction require a lot of energy to overcome

• can conduct electricity when molten or in solution
  → ions are free to move and carry charge

Example of ionic structure

Sodium chloride

Properties of metallic structures

• high melting/boiling point
  → strong electrostatic forces of attraction between positive ions and sea of delocalised electrons

• can conduct electricity when solid or molten/dissolved in solution
  → delocalised electrons are free to move and carry charge through the structure

• malleable
  → layers of positive ions are able to slide over each other

Examples of metallic structures (on the spec)

magnesium

sodium

Properties of simple molecular structure

• low melting/boiling point
  → weak intermolecular forces between molecules, need little energy to overcome

• cannot conduct electricity

Examples of small molecules (on the spec)

iodine

ice

Properties of macromolecular/ giant covalent structure

• high melting/boiling point
  → strong covalent bonds need lots of energy to break

• Diamond and sand: cannot conduct electricity

• graphite: can conduct electricity as there are delocalised electrons between layers

Examples of macromolecular structures

diamond

graphite

What is a charge cloud/orbital?

area where you have a large chance of finding an electron
each orbital holds 2 electrons (a pair)

Pairs of electrons/charge clouds _____ each other

repel

How do pairs of electrons in the outer shell of atoms arrange themselves and why?

as far apart as possible to minimise repulsion

Electron pair repulsion theory

• electron pairs repel each other

• lone pairs cause more repulsion than bonding pairs because they are closer to the nucleus

• electron pairs arrange themselves as far apart as possible to minimise repulsion

Repulsion increases in this order

bond pair- bond pair repulsion < bond pair - lone pair repulsion < lone pair - lone pair repulsion

Three ways to show bonds in a diagram

• stick (both atoms and the bond are in the same plane)

• dashed (bond is going away from viewer/ into the paper)

• wedge (bond coming towards viewer/ out of the paper)

Names of shapes with no bonding pairs

• linear

• trigonal planar

• tetrahedral

• trigonal bipyramidal

• octahedral

2 bonding pairs, no lone pairs =

linear

Linear shape + example

BeCl₂

Linear bond angle

180^o

3 bonding pairs, no lone pairs =

trigonal planar

trigonal planar shape + example

BF₃

trigonal planar bond angle

120^o

4 bonding pairs, no lone pairs =

tetrahedral

tetrahedral shape + exaple

CH₄

tetrahedral bond angle

109.5^o

5 bonding pairs, no lone pairs =

trigonal bipyramidal

trigonal bipyramidal shape + example

PCl₅

trigonal bipyramidal bond angle

90^o

120^o

6 bonding pairs, no lone pairs =

octahedral

octahedral shape + example

SF₆

octahedral bond angle

90^o

How does the presence of lone pairs affect bond angles and why?

lone pairs reduce bond angles by 2.5^o

because lone pair repulsion is stronger and therefore pushes the bonding pairs together

in the example, each molecule has 4 total pairs, but different bond angles

How to determine the number of bonding and lone pairs

• bonding pairs: how many atoms are bonded to the central atom

• lone pairs: subtract the total number of electrons in bonding pairs (multiply b.p by two) and subtract from the number of outer e- (group number) of the central atom. then divide by two for the number of pairs

• if there is a + charge, add one to the number of outer electrons

• if there is a - charge, subtract one from the number of outer electrons

2 bonding pairs, one lone pair =
(give the name, shape, bond angle)

bent
120 - 2.5 = 117.5^o

3 bonding pairs, one lone pair (name, shape, angle)

trigonal pyramidal

109.5 - 2.5 = 107^o

4 bonding pairs, one lone pair = (name, shape, angle)

seesaw

90 - 2.5 = approx 87.5^o

120 - 2.5 = approx 117.5^o

5 bonding pairs, one lone pair = (shape, bond angle, name)

square pyramid
90 - 2.5 = 87.5^o

2 bonding pairs, 2 lone pairs = (name, shape, bond angle)

bent
e.g. water

109.5 - 2.5 - 2.5 = 104.5^o

3 bonding pairs, 2 lone pairs = (name, shape, bond angle)

t-shape

90 - 2.5 = approx 87.5

(we do not do 90 - 2.5 - 2.5 because the repulsion is from above AND below which somewhat cancels each other out)

4 bonding pairs, 2 lone pairs = (name, shape, bond angle)

square planar

90^o

(bond angle does not change due to the equal repulsion from opposite sides cancelling each other out- they are pushing by the same amount so bond angle does not decrease)