OCR Chemistry 21st Century (new) C4

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The Nucleus:

  • in the middle of an atom
  • contains protons and neutrons
  • has a postive charge because of the protons 
  • almost the whole mass of the atom is concentrated in the nucleus

The Electrons

  • move around the nucleus
  • negatively charged
  • they're tiny, but cover a of area
  • they have virtually no mass
  • they're arranged in shells around the nucleus
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Particle Relative Mass Charge
Proton 1 +1
Neutrons 1 0
Electrons 0.0005 -1

Number of protons = number of electrons 

  • Neutral atoms have no charge
  • Charge on atoms = charge on the protons - but opposite
  • Number of protons = number of electrons in a neutral atom. 
  • If some electrons are added/removed the atom becomes charged and therefore an ion.
  • The number of neutrons aren't fixed but it is usually about the same as the umber of protons.

Each element has a different number of protons
Number of protons decides what element it is. Elements have different properties due to their different atomic structure.

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Chemical Equations


Got to balance as well...
Sodium + Chlorine ---> Sodium Chloride
2Na (s) + Cl2 (g) 2NaCl (s)


s= solid, l=liquid, g=gas, aq= dissolved in water

Balancing equations:
Try one element at a time 
Then experiement! 

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Line Spectrum

  • Some elements emit distinctive colours when heated e.g.

    Lithium --> red flame
    Sodium --> yellow/orange flame
    Potassium ---> lilac flame.

Each element gives a characteristic line spectrum:

  • When heated, the atoms gain energy and release light
    the wavelength emitted can be recorded on a line spectrum.
  • line spectrums can be used to identify elements
  • this is called... SPECTROSCORY

Can also be used to identify new elements
... like caesium and rubidum
.... helium was discovered in the line spectrum of the sun. 

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History of the periodic table

used to be ordered according to relative atomic mass

Doberenier tried to organise elements into Triads
... arranged into groups based on chemical properties

Newland's law of octaves
Critised because:

  • groups contained elements without similar properties e.g. carbon and titanium
  • mixed up metals and non-metals e.g. oxygen and iron
  • didn't leave any gaps for elements that hadn't been discovered yet. 

Dmitri Mendeleev left gaps and predicted new elements: (1896)
Put elements in order of atomic mass (like Newlands)

  • wasn't much evidence to suggest that the elements really did fit together that way 
    --> once new elements were discovered from his table, which was more evidence towards his periodic table 
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The modern periodic table

14 -------------------------> relative atomic mass- tells you the total no of protons and electrons
N               -----------------> symbol
nitrogen     ---------->name
7  -----------> proton number

  • There are 100ish elements, whcih all materials are made of .
  • In the periodic table , the elements are laid out in order of increasing proton number. Arranging the elements like this means there are repeating patterns in the properties of the elements. 
  • Metals are found on the left, and non-metals on the right.
  • Elements with similar properties form columns called groups. The group number also tells you how many electrons there are in the outer shell
  • The rows are called periods. Each new period represents another full shell of electrons.
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Electron Shells


  • Electrons always occupy shells (sometimes called energy levels)
  • The lowest energy levels are always filled first.
  • Only a certain number of electrons are allowed in each shell:
    1st shell: 2, 2nd Shell: 8, 3rd Shell:8
  • Atoms are much happier when they have full electron shells.
  • In most atoms the outer shell is not full and this makes the atom want to react.
  • An element's electron arrangement determines its chemical properties 
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Ionic Bonding

Ions are made when atoms gain or lose electrons

  • when atoms lose or gain atoms they form charged particles called ions
  • Ions can be made from single atoms (the Cl- ion) or groups of atoms( the NO3-).
  • When atoms lose or gain electrons all they are trying to do is fill the outer shell.

A shell with just one electron is well keen to get rid...

All atoms in Group 1 of the periodic table e.g. sodium, potassium have just one atom in their outer shells ---> they have to get rid of it. ---> when they lose an electron, they form positive ions.

A nearly full shell is well kell to get that extra electron...
Group 7 e.g. chlorine---> they became negative ions

Once reacted, these elements are held together through ionic bonds
Oppositvely charges ions are strongly attractched to each other. 

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Ionic Bonding (2)

Ionic Compunds form a regular lattice

  • Solid ionic compounds like sodium chloride are made up of giant lattice of ions. Each lattice forms a single crystal
  • When ionic compounds became molten or are dissolved in water. they can conduct electricity because the ions are able to move.
  • Actually, the fact that molten compounds of metal and non-metals can conduct electricity is used as evidence that they're made up of ions.
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Ions and Formulas

The charges of in an ionic compound add up to zero.

  • Some metals (like iron, copper and tin) can form ions with different charges. The number in brackets after the name tells you the size of the positive charge on the iron e.g.
    iron (II) ion has a charge of 2+, so it's Fe^2+
  • The charge must always add up to zero in COMPOUNDS

Easy ones:
Li+  + F-  ------> LiF

Hard ones:
Ca^2+   +  Cl-   ---> CaCl2          Fe^3+   + SO4^2-   ----------> Fe2(SO4)3

You can also work out  the charges on the ions:
ex. Find the charge on the lithium ion in LiBr if the charge on the bromide is 1-
lithium ion must be 1+ 

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Group 1- The Alkali Metals

  • include lithium, sodium and potassium
  • they all have one other shell electron. This makes them very reactive and also have similar properties. 
  • Alkali metals are shiny when freshly cut, but react quickly with the oxygen in the air and tanish.

As you go down the group 1, the alkali metals:

  • become more reactive
  • higher density
  • lower melting point 
  • higher boiling point

Reaction with Cold Water produced hydrogen gas
When Li, Na and K are put in H2O, they react rigorously ---> move around surface, fizzing.
Make an alkali solution
Makes a  hydroxide
Reaction with chloride produced salts (colourless and crystalline) 

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Group 7- Halogens

  • include chlorine, bromine and iodine
  • all have seven outer electrons. This makes them very reactive, gives them similar properties and means they make similar compounds
  • They  form DIATONIC MOLECULES which are pairs of atoms.

As you go down, they become:

  •  less reactive --> additional electrons are attracted less strongly
  • high melting point 
  • higher boliling points

Are coloured vapours

Fluorine (g) 
Chlorine (g) Bromine(l) Iodine(g)  (as solid) 

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Halogens- Reactions

  • Reactions with Alkali metals
    The halogens react with alkali metals like lithium to form salts called metal halides e.g. sodium chloride. The reactions become less vigorous as you go dwon the group
  • Reactions with iron
    Form colourless solids called iron halides.
  • Displacement reactions
    ... where a more reactive element displaces a less reactive element to form a component.
    Chlorine is more reactive than iodine. So chlorine reacts with potassium iodide solution to form potassium chloride, and the iodine is left in solution. 
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Lab safety

Common hazards:

  • Oxidising- provides oxygen which allows some materials to burn e.g. liquid oxygen
  • Highly flammable- catches fire easily e.g. petrol
  • Toxic- can cause death either by swallowing, breathing in or absorption through the skin, e.g. hydrogen cyanide
  • Harmful- like toxic but not quite as dangerous e.g. copper sulfate
  • Explosive- can explode e.g. some peroxides
  • Corrosive- attacks and destroys living tissues including eyes and skins e.g. concentrated sulfuric acid
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Safety Precautions

Alkali Metals:

  • really reactive and can combust spontanetously
  •  + water vapour = BOOM! -- fire extinguisher
  • can never be touched by bare hands
  • all apparatus must be dry to insure water doesn't come into contact
  • alkali solutions they form are corrosive and my cause blistering


  • harmful
  • Fluorine cannot be used inside a lab
  • liquid bromine is corrosive + no contact with skin
  • Halogens have poisonous vapours --> must be used in fume cupboards.
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Just what I needed ^_^ Thank you!


All good apart from the relative atomic mass tells you the proton and neutron number not the proton and the electron number 

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