Chemistry 4 (C4) - Specification

A nucleus is made up of protons and neutrons.

• Electron charge -1 and mass of zero
• proton charge +1 and mass of 1
• Neutron charge of 0 and mass of 1

An atom is neutral because it has equal amounts of electrons and protons.

Atoms have a radius of about 10 (-10)  m and a mass of around 10(-23)    g

Isotopes are varies of an element that has the same atomic number but different mass numbers.

The electronic structure tells us how the electrons are arranged in shells.

1st shell - max of 2

2nd + - max of 8

Sodium's atomic number = 11 so there are 11 electrons: 2.8.1

Dalton - all atoms in one element were the same

J.J Thomson - discovered the electron

Rutherford - nucleus

Bohr - electron shells/orbitals

All contributed to the theory of atomic structure:

• the theory changed as new evidence was found
• science explanations are provisional but more convincing when predictions are later confirmed.

Unexpected results (eg Geiger and Marsden's experiment) led to the theory of a nucleur atom.

Atoms with an outer shell of 8 have a stable electronic structure.

• Metals to form positive ions - lose electrons.
• Metals to form negative ions - gain electrons

Ionic bonding is when a metal and non-metal combine by transferring electrons to form positive ions and negative ions which then attract one another.

Sodium chloride solution conducts electricity.

Magnesium oxide and sodium chloride conduct electricity when molten.

The strucutre of Sodium Chloride and Magnesium oxide are giant ionic lattices in which positve ions are strongly attracted to negative ions.

Physical properties of sodium chloride:

• high melting points
• can conduct electricity when molten or in solution because the charged ions are free to move around
• cannot conduct electricity when solid because the ions are held in place and cannot move.

Magnesium oxide has a higher melting point than sodium chloride as the ionic bonds are stronger and need more energy to be broken.

Non-metals combine together by sharing electron pairs and this is called covelent bonding.

Water and carbon dioxide are simple molecules with weak intermolecular forces between molecules.

This means that they have low melting points because bonds are easily broken. They dont conduct electricity because there are no free electrons.

The group number is the same as the number of electrons in the outer shell.

A period to which an element belongs to corresponds to the number of occupied shells in the electronic structure.

Newlands - first scienists to make a table of elements. Law of octaves, where every 8th element behaved the same. Included some compounds when he thought they were elements.

Mendeleev - author of the modern perodic table

Further evidence confirmed Mendeleev's ideas:

• confirmation of his predictions about unknown elements
• how investigations on atomic structure (mass number and electronic structure) agreed with his ideas.

Properties of group 1 elements with water:

• react more vigorously as you go down the group

Predicting the properties of Rubidum or caesium with water - they get more reactive and explosive as you go down the group.

Balanced symbol equation with Group 1 element in water:

2Na + 2H 0       2NaOH + H2

Physical properties of Rubidium or Casesium:

• Density increases as you go down the group
• Melting and boiling points decrease as you go down the group
• Reactivitiy with water increases as you go down the group.

Group 1 metals have similar properties because when they react, they all lose 1 electron. So, a positive ion with a stable electronic structure is made.

Li         Li+     +    e-

The reason why group 1 metals are more reactive with water as you go down the group, is that the outer shell gets further away from the positive attraction of the nucleus. This makes it easier for an atom to lose an electron from its outer shell.

This process is oxidisation because the atom loses an electron.

A flame test is used to identify the presence of lithium, sodium and potassium compounds:

• use of moistened flame test wire
• flame test wire dipped into solid sample
• flame test wire out into blue bunsen flame
• colours of the flames

Lithium = red

Potassium = lilac/purple

Sodium = yellow

Physical appearance of group 7 elements at room temperature:

• chlorine = green gas
• bromine = orange liquid
• iodine = grey solid

Predicting Fluorine of Astastine:

• Melting point and boiling point increase as you go down the group
• Reactive decreases as you go down the group
• Density increases as you go down the group
• Fluorine displaces all other group 7 elements (halogens)

Halogens react vigorously with alkali metals (group 1) to form metal halides:

lithium + chlorine = lithium chloride

2Li + Cl2               2LiCl

• chlorine displaces bromides and iodides
• bromine displaces iodides

potassium bromide + chlorine                potassium chloride + bromine

2KBr                        +    Cl2                         2KCl                  +      Br2

The more reactive element displaces the less reactive element off the salt and turns into an "ide". This less reactive group 7 element takes the "ine".

Group 7 properties have similar properties because in a reaction each atom gains one electron to form a negative ion with a stable electronic structure.

The elements at the top of the group are more reactive than the ones at the bottom because the outer shell is closer to the positive attraction of the nucleus. This makes it easier for an atom to gain an electron.

When a halogen reacts to make a halide:

F2    +    2e-              2F-

These reactions are reduction reactions.

Compounds of transition metals are often coloured:

• copper compounds = blue
• iron (II) compounds = light green
• iron (II) compounds = orange/brown

Transition elements and their compounds are often used as catalysts:

• iron in the Haber process
• nickel in the manufacture of margarine

Thermal decomposition of carbonates of transition metals always produce a metal oxide and carbon dioxide.

FeCO3               FeO +    CO2

CuCO3               CuO    + CO2

MnCO3             MnO     +    CO2

ZnCO3               ZnO    +      CO2

Precipitation is the reaction between sodium hydroxide solution to identify the presence of transition metal ions in solution:

Cu2+ gives a blue precipitate

Fe2+ gives a grey-green precipitate

Fe3+ gives an orange precipitate

Cu2+ + 2OH-         Cu(OH)2

Metals have high melting and boiling points because of the strong forces called metallic bonds. These are the electrostatic attraction between the metal ions and the delocalised electrons. Alot of energy is needed to break these bonds.

Metallic bonding is the strong attraction between a sea of delocalised electrons and close packed positive metal ions.

Metal crytals are made from closely packed positive metal ions in a 'sea' of delocalised electrons. The free movement of the electrons allows the metal to conduct electricity.

Superconductors are metals that have very little or no resistance to the flow of electricity. Metals can become superconductors at low temperatures.

Advantages of superconductors:

• loss free power transmission
• super-fast electronic circuits
• powerful electromagnets

Drawbacks to superconductors:

• only work at temperatures below -200 degrees
• temperatures costly to maintain
• impractical for large-scale uses

It is important to conserve water because it is an important resource for industry as well as being essential for drinking, washing etc.

Water may contain:

• nitrates - from the run off fertilisers
• lead compounds - old pipes in plumbing
• pesticides - from spraying crops near a water supply

Water purification process:

• sedimentation - the wter settles to allow insoluble particles to sink
• filtration - to remove very fine particles
• chlorination - to kill micro-organisms in the water

Some particles cannot be removed from water during purification because they are soluble and are dissolved in the water.

Disadvantages of distillation:

• uses alot of energy
• expensive (equipment)
• currently to costly to distill sea water for the UK

Sulfates can be detected using barium chloride - a white precipiate forms.

sodium nitrate + barium chloride          barium sulfate (white precipitate) + sodium chloride.(Balanced symbol equation)

Silver nitrates are used to detect halide ions:

• chlorides - white
• bromines - cream
• iodides - pale yellow

Sodium chloride + silver nitrate           sikver chloride (white precipitate) + sodium nitrate.

(Balanced symbol equation)