1 - Atomic Structure and the Periodic Table

• • Elements: A substance that is made of only one sort of atom 
• • Mixtures: consists of two or more different substances, not chemically joined together - to separate these you can use a physical separation.
  • Molecules: has any elements combined even if they're the same
• Compounds: contains two or more different elements chemically combined in fixed proportioned 

  • Compounds usually have different properties to the elements they are made from
  • To separate a compound- you need to use a chemical reaction
• Filtration: 

  • Used to separate and insoluble from a liquid (insoluble: a solid will not dissolve in a liquid)

  1. Pour mixture down a filter funnel with filter paper, the liquid passes through the tiny pores in the filter paper
  2. However the solid material cannot pass through so its trapped
  3. In the end you have the liquid separated from the solid 
• Crystallisation: : 

  • To separate the soluble solids from a liquid,
  • (aq)= aqueous this means that solid is dissolved in the liquid 

  1. Leave the solution in a beaker for a few days then the water will evaporate and it will leave behind solid crystals- Crystals begin to grow as solids come out of solution due to decreasing solubility
  2. To make it faster- you can heat the solution, but some chemicals will break down by heat 
• Paper Chromatography

  1. Take chromatography paper and draw a  pencil line at the bottom then put a dot of your first colour onto the pencil line and next to that put a dot of another colour- can do this for several colours
  2. Then you dip it in a solvent (a liquid that will dissolve substance)- The solvent makes it way up the paper and it dissolves the ink in the two coloured dots, they’re now carried up the paper as well 
  3. Then once its done, eg: red stays the same- colour A is a pure colour, and green- colour B would show yellow and blue, this means it’s a mixture of two colours 

  
• Paper chromatography

  • Paper is the stationary phase: it does not move, Solvent is the mobile phase: it does move 
  • Pure compound produces a simple spot in all solvents
  • The compounds in a mixture may separate into different

   spots depending on the solvent 

  • Depending on the solvent the placing of the change in dots 

  maybe different 
• • Paper chromatography works bc different substances have different solubilities, a more soluble substance travels further up the paper than a less soluble one
  • You draw your starting line in pencil as if it was in pen ink, it would move up the paper with the solvent 
• Using paper chromatography to identify an unknown substance 

  • Same process a usual, except with an unknown chemical, it would move and tell you whether it is a pure or mixture  
  • The Solvent moves up, Measure distance moved by the unknown chemical- from the pencil line to centre of the spot, and then measure the distance from the solvent

  Use those numbers to calculate the RF value 
• Paper chromatography RF value

  • Rf = Distance moved by substance / distance moved by solvent (no unit) - Rf value in a database will tell the chemical
  • However: many substances may have the same Rf value, which means we have to use a different solvent ALSO, if the substance has never been analysed there will not be an Rf value 
• Plum Pudding Model:

  • Suggested that atoms are a ball of positive charge with 

  negative electrons embedded in it 

  • Scientists wanted to know if this was true, so they carried 

  The alpha scattering experiment

  
• Alpha Scattering Experiment 

  1. First they take a piece of gold foil, they used gold is bc you can Hammer gold out into very thin foil, just a few atoms thick
  2. Then scientists fired tiny particles at the gold- called the alpha particles - they have a positive charged
  3. The first thing they noticed is that most of the alpha particles fired straight though the gold foil without changing direction
  4. Sometimes though they were deflected- this means they changed direction as it passes through the gold foil and sometimes the particles jumped straight back off the foi
• Summary Alpha scattering experiment

  • Most of the alpha particles went through the gold atmos; therefore atoms are mainly empty space; this means the plum pudding method is wrong
  • Some particles deflected; Therefore the centre of an atom must have a positive charge. Alpha particles that come close are repelled and change direction 
  • Some alpha particles bounced back; This must mean the centre of an atom must contain a great deal of mass- now call this the nucleus 

  
• • Neild Bohr: found that electrons orbit the nucleus at specific distances, his work agreed with the result of experiment by other scientists 
  • We now call the “orbits” energy levels or shells 
  • the positive charge in the nucleus is due to tiny positive  particles called protons
• • James Chadwick found that the nucleus also contains neutral 

  particles called the neutrons w

  • Atoms have no overall charge- the number of electrons is the same as the number of protons 
  • Radius of an atom= 0.1nm (1x10-10 m), radius of nucleus is 1x10-14m (only an approximate)
•  Charges & Masses

  • Relative charge: the charge of one particle compared to another particle 

  A) +1

  B) 1

  C) 0

  D) 1

  E) -1

  F) very small

  
• • Isotopes: atoms of an element with different numbers of neutrons- remember: all atoms of an element have the same number of protons 
• Ions are atoms which have an overall charge- they have gained or lost electrons; positive ions have lost electrons and negative ions have gained electrons 
• Relative atomic mass is the average of the mass number of the different isotopes- is weighted of the abundance of each isotope (how common each isotope is) Ar

  (% of isotope b x mass of isotope a)+ (% of isotope b x mass of isotope b)/ 100 = Ar

  
• Development of periodic table: Dobereiner’s Triads 

  • Tried to put elements into some sort of order: Noticed that elements with similar chemical properties often occurred in threes- triads  
  • Lithium, Sodium & Potassium- these three metals react rapidly in water
  • Chlorine, bromine & Iodine- reactive non-metals 
• Development of periodic table: Newlands’ Octaves 

  • Arranged the elements in order of increasing atomic weight 
  • Newlands law of octaves: when Elements are arranged in increasing order of Atomic Mass, the properties of every eighth Element starting from any Element are a repetition of the properties of the starting Element
  • However it had some problems: by always sticking to the exact order of atomic weight sometimes elements were grouped together when they had different properties
• Development of periodic table: Dmitri Mendeleev’s

  • arranged all the elements in order of increasing atomic weight 
  • If needed- would switch the order of specific elements so that they fitted the pattern of the other elements in the same group 
  • realised some elements had not been discovered so he left gaps where he thought they were missing; he predicted properties of the undiscovered elements based on the other elements in the group  
• The modern periodic table is ordered by atomic number (protons)- these were not discovered so the problem with Mendeleev's table is that elements can appear in the wrong order due to the presence of isotope, it also has Group 0, the noble gases and these were not discovered 
• • The noble gases (group 0) are very unreactive: this is because all noble gases have a full outer energy level/shell so they are stable- they have no tendency to lose, gain, or share electrons
  • The boiling points increase as the relative atomic mass increases (going down in the group)
• • When metals react, they lose electrons to achieve a full outer energy level
  • This gives them the same electronic structure as a group 0 noble gas
  • Metals always form positive ions 
• Group 1 metals are soft 

  when they react with..

  • Oxygen: react rapidly with and as we move down the group they react more rapidly because all group 1 metals have one electron on the outer energy level- the further away the electron is from the nucleus the easier it is to break the attraction, they all react to make metal oxides
  • Chlorine: react rapidly, the alkali metal loses the electron in its outermost shell and forms a +1 ion. Chlorine then gains that electron
  • Water; rapid gases produce and (go down the group the more rapid it gets)
• Group 1- alkali metals

  • moving down, the outer electron is less attracted to the nucleus and easier to lose.
  • There Is a greater distance between the positive nucleus and negative outer electron
  • The outer electron is shielded from the nucleus by the internal energy levels  

  
• Group 7- Halogens (non-metals)

  • Group 7 elements have 7 electrons in their outer level 
  • They form molecules with two atoms joined by covalent bonds eg: Fluriourine (F), F2, F-F

  the further down the group the higher its relative molecular mass (size of atom), melting point and boiling point
• • Group 7 elements form covalent compounds when they react with other non-metal atoms
  • They form ionic compounds when they react with metal- the group 7 element gains one electron and forms a -1 ion 
• group 7

  Fluorine is the most reactive; they get less reactive as you move down the group 

  • As we descend the group: there are more shells between the nucleus and the outer electron. the force of attraction between the nucleus and outer electron decreases. it becomes harder to gain the outer electron
  •  it's harder to attract an electron so it's less reactive. 
• A more reactive halogen (G7) can displace a less reactive halogen from an aqueous solution of salt 

  Eg: Sodium Bromide  + Fluorine → Sodium Fluoride + Bromine

  • Fluorine can push out/displace bromine
• Fractional Distillation

  • Used to separate mixture of different liquids 
  • These liquids must have different boiling points 

  
• Simple distillation: used to separate a liquid from a solid if we want to keep the liquid

  1. Heat the liquid using a bunsen burner- this will make it  evaporate turning it to a vapour  
  2. The vapour rises up the glass tube, the thermometer shows that the temp is rising 
  3. Vapour passes through the condenser (its kept cold as cold water is circulating around it) this means the vapour now condenses back into a liquid and through into the beaker 
  4. In the end you're left with the crystal in the original beaker and liquid in the other beaker