atomic structure and the periodic table

• elements which come in pairs

  • iodine, I2
  • bromine, Br2
  • chlorine, Cl2
  • fluorine, F2
  • oxygen, O2
  • nitrogen, N2
  • hydrogen, H2

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• The formula of an ionic compound can be predicted using the formulae of its ions. The numbers of ions in a formula must give an equal number of positive and negative charges.
• use the valency to find the charge

  
• • a pure substance consists only of one element or one compound
  • a mixture consists of two or more different substances, not chemically joined together
• Mixtures can be separated by physical processes. These processes do not involve chemical reactions, and no new substances are made.
• Filtration is used to separate an insoluble solid from a liquid

  Filtration works because the filter paper has tiny holes or pores in it. These are large enough to let small molecules and dissolved ions through, but not the much larger particles of undissolved solid.
• filtration:

  1. Set up the equipment: Place a filter funnel into the neck of a conical flask.
  2. Prepare filter paper: Fold the filter paper into a cone and place it inside the funnel.
  3. Pour the mixture: Pour the mixture (insoluble solid and liquid) into the funnel - pour it carefully to avoid splashing.
  4. Filtration process: The liquid (filtrate) will pass through the filter paper into the flask, while the solid (residue) will stay on the filter paper.
• 1. Heat solution: gently heat solution in an evaporating dish to remove some solvent (e.g., water). This makes the solution more concentrated.
  2. saturation test: Dip a glass rod into solution, if crystals start forming on it, the solution is saturated.
  3. Cool solution: Allow the hot, saturated solution to cool slowly. As it cools, crystals will start forming
  4. Collect the crystals: Once crystals have formed, filter the mixture to separate the crystals from the remaining solution.
  5. Dry the crystals: Place the crystals in a warm, dry place or blot them with filter paper to remove any remaining liquid.
• distillation

  • Heat the mixture: The solution is heated in a round-bottomed flask. The component with the lower boiling point evaporates first.
  • Condense the vapor: The vapor passes into a condenser, where it cools and condenses back into a liquid.
  • Collect the distillate: The condensed liquid (now called the distillate) is collected in a separate container, leaving the dissolved solids or other liquid behind.
• distillation process:

  
• Simple distillation (complex is fractional distillation) works because the dissolved solute (solid) has a much higher boiling point than the solvent. When the solution is heated, solvent vapour leaves the solution. It moves away and is cooled and condensed. The remaining solution becomes more concentrated as the amount of solvent in it decreases.
• Paper chromatography is used to separate mixtures of soluble substances. These are often coloured substances such as food colourings, inks, dyes or plant pigments.
• Phases

  Chromatography relies on two different ‘phases’:

  • the stationary phase, which in paper chromatography is very uniform, absorbent paper
  • the mobile phase is the solvent that moves through the paper, carrying different substances with it

  The different dissolved substances in a mixture are attracted to the two phases in different proportions. This causes them to move at different rates through the paper.
• Separation by chromatography produces a chromatogram. A paper chromatogram can be used to distinguish between pure and impure substances:

  • a pure substance produces one spot on the chromatogram
  • an impure substance, or mixture, produces two or more spots
• A paper chromatogram can also be used to identify substances by comparing them with known substances. Two substances are likely to be the same if:

  • they produce the same number of spots, and these match in colour
  • the spots travel the same distance up the paper
• how chromotography works

  It works based on the different solubilities of substances in a solvent and their attraction to the chromatography paper.
• Measure the distance each component traveled from the pencil line to determine its Rf value (Retention factor)

  rf = distance moved by substance / distance moved by solvent front

  Solvent front: The furthest point reached by the solvent.
• chromotography

  1. draw line: draw a pencil line across the chromatography paper, 1 - 2 cm from the bottom
  2. place ink: use a pipette or capillary tube to add small spots of each ink to the line on the paper
  3. place in solvent: place the paper into a container with a suitable solvent in the bottom
  4. rest: allow the solvent to move through the paper, but remove the chromatogram before it reaches the top
  5. drying: allow the chromatogram to dry, then measure the distance travelled by each spot and by the solvent
• test for oxygen

  Oxygen supports combustion. If oxygen is present in a test tube, a glowing splint relights when it is held inside.
• test for hydrogen

  Hydrogen ignites in air. If hydrogen is present in a test tube, a lighted splint held near its mouth ignites with a squeaky pop.
• test for carbon dioxide

  Carbon dioxide reacts with calcium hydroxide solution to produce a white precipitate of calcium carbonate. Limewater is a solution of calcium hydroxide. If carbon dioxide is bubbled through limewater, the limewater turns milky or cloudy white.
• test for chlorine (gas)

  Chlorine is an acidic gas that also acts as a bleach. Damp litmus paper is bleached white when it is placed in chlorine.

  If damp blue litmus paper is used, the paper turns red then white.
• development of the atom

  "atomus" greek for "indivisible" was first proposed by democritus who believed tha if you continues cutting objects, there would be a limiy yo how far you can go
• 1803: John Dalton published his idea that atoms were tiny indivisible spheres of matter
• development of the atom

  JJ Thompson: carried out experiments and discovered the electron

  he proposed the plum pudding model, where atoms were made of spheres, and in a positively charged substance sat negatively charged electrons
• development of the atom

  1909: Ernst Rutherford's gold foil experiment

  using gold foil: can be flattened to only a few atoms wide.

  positively charged alpha particles were fired at the gold leaf

  most passed straight through, suggesting most of atom is empty space

  some were deflected suggesting that the nucleus was positively charged (because they're both positively charged: same charges repel)

  some bounced straight back suggesting very massive/ concentrated center

  > leading to a small, dense positively charged nucleus (no neutrons yet) and negatively charged electrons orbiting around it
• development of the atom

  Niels Bohr

  he discovered, through his calculations, that electrons orbit the nucelus at different distances (shells) according to their energy levels

  calculations agreed with observations from experiments
• development of the atom

  further experiments led to the discovery of the proton, tiny particles with positive charge
• development of the atom

  1932: James Chadwick discovered the neutron

  we now use this altered model to this day
• size of the atom

  • the radius of an atom is about 0.1 nm (1 × 10^-10 m)
  • the radius of a nucleus (1 × 10^-14 m) is less than 1/10,000 of the radius of an atom
• we can figure out the number of protons, neutrons and electrons in an atom:

  atomic (proton) number is the number of electrons and protons (atoms have no overall charge, so the amount of positive and negative have to equal)

  atomic number and atomic mass number, the difference is the neutrons
• isotopes

  atoms of the same element have to have the same proton and electron number, but can have different numbers of neutrons, this is an isotope

  same atomic number

  different mass number
• isotopes

  the chemical properties of an isotope stays the same, this is because it is the number of electrons which determines chemical properties
• isotopes

  The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes

  It takes account of the abundance of each of the isotopes of the element.
• relative atomic mass:

  the average mass of its atoms as compared to the mass of 1/12 of carbon 12

  the number of times heavier an atom is that one twelfth, 1/12, of a carbon 12 atom
• calculating relative atomic mass

  (isotope mass x % abundance) + (isotope mass x %abundance)/ total number of atoms (100%)
• calssifying elements

  before the modern periodic table, scientists classed elements according to their mass

  also, many elements had not yet been discovered
• Dmitri Mendeleev

  arranged elements in increasing atomic mass and took into accound the properties of the elements

  • he swapped elements so that they were organised in groups with similar chemical properties
  • he left gaps where elements hadn't been discovered yet

  he used his gaps to predict the mass and properties of the undiscovered element, by using the mass and properties of the elements next to it. a lot of his predictions were accurate
• the groups (vertical) show the number of electrons in the outer shell

  the periods (horizontal) show us how many shells the element has

  > use this to figure out, and draw electron configuration
• metal atoms lose electrons to form positive ions

  elements, when reacting, will always try to gain a full outer shell by exchanging electrons. metals lose electrons (remember: the number of electrons in the outer shell determines whether an element will gain or loose electrons)