Chemistry

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ATOMIC STRUCTURE

  • An atom is neutral because it has an equal number of electrons and protons. The positive charges balance out the negative charges. 
  • Elements with the same atomic number but different mass numbers are isotopes for example: Carbon14.
  • A stable atom's number of protons/electrons is equal to the atomic number, the number of neutrons equals the mass number - the atomic number

Periodic table:

  • GROUPS (number of electron shells) - arranged horizontally
  • PERIODS (number of electrons in outer shell) - arranged vertically
  • 1st shell has 2 electrons maximum, 2nd shell has 8 electrons maximum and 3rd shell has 8 electrons maximum

History of atomic theory:

  • early theory created by John Dalton
  • theory confirmed with evidence by JJ Thompson, Rutherford and Bohr, their new predictions of what an atom would look like where confirmed later
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IONIC BONDING

  • atoms with 8 electons in their outer shell have a stable electronic structure
  • if an atom is unstable, it can be made stable by transferring electrons so both atoms have a stable structure
  • metal atoms lose electrons to become stable, they form positive ions 
  • non-metals atoms gain electrons to become stable, they form negative ions
  • then the positive ion and negative ion attract each other to become a stable molecule

DOT AND CROSS DIAGRAMS can show ionic bonding but they only show the atoms in the outer shell, crosses represent electrons that have been transferred and dots represent the electrons already in the shell

Bonding of SODIUM OXIDE:

  • sodium atom has 1 electron in outer shell whereas the oxygen atom needs 2 electrons to be stable
  • therefore 2 sodium atoms are needed to make 1 oxygen atom stable
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IONIC BONDING 2

Conducting electricity:

  • in a giant ionic lattice, positive ions have a strong electrostatic attraction to negative ions
  • examples of this are: sodium chloride and magnesium oxide

Properties of the 'giant ionic lattice':

  • high melting point - bonds are difficult to break
  • don't conduct electricity when they are solid - as ions can not move
  • conduct electricity when in molten or solution form - due to ions being able to move

Sodium chloride vs. magnesium oxide:

  • magnesium oxide is made up of Mg2+ and O2-, whereas sodium chloride is made of Na+ and Cl-, so magnesium oxide has a higher melting point because there are stronger electrostatic attractions
  • each magnesium atom donates 2 electrons to the oxygen atom which makes a stronger bond than transferring just 1 electron
  • magnesium ions are tiny so can get closer to oxygen - this makes the bond strong
  • magnesium oxide wins - it has the highest melting point
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THE PERIODIC TABLE AND COVALENT BONDING

  • covalent bonding is when an atom has to share a electron pairs to be stable (for example chlorine has 7 electrons in it's outer shell, so 2 chlorine atoms join together and share electrons, this forms Cl2
  • due to them sharing electrons, they have intermolecular forces
  • if a substance has a simple molecular structure, the intermolecular forces are weak and the substance has a low melting point due to the ease in breaking it apart
  • as there are no free electrons in certain covalent bonding because they are all used up to be stable, the molecules do not conduct electricity

History of the periodic table:

  • Newlands put 56 elements into groups and realised that every 8th element behaved similarly, this was not accepted until years later when evidence was gathered
  • Mendeleev arranged the elements in order on a table where he noticed periodic changes in properties, he saw gaps in his pattern and predicted that new elements would be found
  • later investigations confirmed his idea of periodicity and his predictions on the discovery of missing elements
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THE GROUP 1 ELEMENTS (alkali metals)

  • group 1 elements have similar properties: react vigorously with water, the equation is:
  • group 1 element + water ---> metal hydroxide (alkali) + hydrogen
  • Example: 2Li + 2H2O ---> 2LiOH + H2

reactivity of group 1 elements increases going down the group, whereas melting point and boiling point increases going up the group

  • atoms in group 1 have similar properties because they have 1 electron in their outer shell
  • they can lose 1 electron to become a positive ion
  • the easier it is for an atom to lose one electron, the more reactive it is
  • the more shells it has, the easier it is to lose an electron because the electrons become further away from the positive nucleas, so more shells = easier to lose electron = more reactive
  • if electrons are lost it is called oxidation

Flame tests:

  • flame test wires moistened with dilute hydrochloric acid
  • flame test wire dipped into the solid chemical
  • flame test wire put on a blue bunsen flame
  • colour recorded (lithium = red, sodium = yellow, potassium = lilac)
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THE GROUP 7 ELEMENTS (halogens)

At room temperature... chlorine = green gas, bromine = orange liquid, iodine = grey solid

  • group 7 elements have 7 electrons in their outer shell
  • when they react, they each gain 1 electron to form a stable electronic structure
  • the nearer the outer shell is to the nucleus, the easier it is for an atom to gain one electron, and the more reactive it is

Reactions of halogens:

  • halogens react with alkali metals to form a metal halide, for example: potassium + iodine --> potassium iodide
  • symbol equation = 2K + I2 ---> 2KI

Displacement reactions of halogens:

  • if halogens are bubbled through solutions of metal halides, there is either no reaction or a displacement reaction
  • if the halogen is more reactive than the halogen already in a reaction then it will displace it to be a part of the reaction
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TRANSITION ELEMENTS

  • copper compounds are blue, iron(II) compounds are pale green, iron(III) compounds are orange/brown
  • transition elements are often CATALYSTS, such as iron catalyst in the Haber process (ammonia production for fertilisers) and nickel catalyst for magerine (to harden the oils)
  • transition metal carbonate + heat = metal oxide and carbon dioxide (THERMAL DECOMPOSITION)
  • FeCO3 --> FeO + CO2
  • CuCO3 --> CuO + CO2
  • MnCO3 --> MnO + CO2
  • ZnCO3 --> ZnO + CO2
  • transition metal ions + sodium hydroxide solution --> metal hydroxide precipitate
  • Cu2+ ions form blue solid precipitate
  • Fe2+ ions form grey/green solid precipitate
  • Fe3+ ions form orange/brown gelatinous solid precipitate
  • Cu2+ + 2OH-   --> Cu(OH)2
  • Fe2+ + 2OH-   --> Fe(OH)2
  • Fe3+ + 3OH-   --> Fe(OH)3
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METAL STRUCTURE AND PROPERTIES

  • Physical property of metals: high thermal conductivity (present in copper, can be used for saucepan bases)
  • Chemical property of metals: resistance to attack by oxygen and acid (present in gold for jewellery)
  • Other properties: 
  • malleable, lustrous, shiny, ductile, 
  • also aluminium has a low density for use in aircraft and modern cars, 
  • high melting and boiling points due to strong electrostatic attraction between positive metal ions and delocalised electrons - lots of energy needed to break atoms apart
  • conduct electricity - occurs when delocalised electrons in the metal move (copper, silver and gold are good conductors)

Superconductors:

  • mercury is a superconductor at -268.8c because it's resistance drops to 0
  • if a small magnet is placed above superconductor, it levitates
  • if small magnet brought near superconductor, it is repelled
  • benefits: loss-free power transmission, super-fasr electronic circuits, powerful electromagnets
  • drawbacks: only work at low temperatures so use is limited, superconductors that work at 20c still need to be developed
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PURIFYING AND TESTING WATER

3 stages of water purification:

  • sedimentation: chemicals added to make solid particles and bacteria settle out
  • filtration: a layer of sand on gravel filters out remaining fine particles and some types of sand filter out microbes
  • chlorination: chlorine added to kill microbes
  • renewable but not endless, no rain = not full reservoir
  • in the UK, more homes are being built so there's an increase in demand for water
  • takes energy to pump and purify which increases global warming

Sometimes more stages must be added to water purification if pesticides and nitrates are not removed. Sea water must be distilled before it is drinkable due to the vast amount of substances contaminating it, distillation takes lots of energy so is expensive and only used when no other water is available.

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PURIFYING AND TESTING WATER 2

Precipitate reactions to test for water: (uses barium chloride or silver nitrate)

  • barium chloride + magnesium sulfate --> barium sulfate (white precipitate) + magnesium chloride
  • silver nitrate + sodium bromide --> silver bromide (cream precipitate) + sodium nitrate
  • silver nitrate + sodium iodide --> silver iodide (yellow precipitate) + sodium nitrate
  • silver nitrate + sodium chloride --> silver chloride (white precipitate) + sodium nitrate
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PURIFYING AND TESTING WATER 2

Precipitate reactions to test for water: (uses barium chloride or silver nitrate)

  • barium chloride + magnesium sulfate --> barium sulfate (white precipitate) + magnesium chloride
  • silver nitrate + sodium bromide --> silver bromide (cream precipitate) + sodium nitrate
  • silver nitrate + sodium iodide --> silver iodide (yellow precipitate) + sodium nitrate
  • silver nitrate + sodium chloride --> silver chloride (white precipitate) + sodium nitrate
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