Atomic Structure & The Periodic Table

• Compounds can only be separated into elements through chemical reactions
• John Dalton 19th Century - atoms are solids spheres that make up different elements

  JJ Thompson 1897 - atoms aren't solid spheres. measurements of charge and mass shows they must contain smaller negatively charged particles. (electrons) this new theory was the plum pudding model 1904 - atoms are balls of positive charge with electrons swimming
• Ernest Rutherford and Marsden 1909 - alpha particle scattering experiment. fired positively charged alpha particles at a thin sheet of gold. results explained the nuclear model of the atom - tiny positively charged nucleus + cloud of electrons (lots of empty space)
• Bohr suggested electrons obit the nucleus in fixed shells. Supported by many experiments.
• Experiments by Rutherford + others showed the nucleus can be divided into smaller particles (protons). James Chadwick provided evidence for neutral particles called neutrons - resulted in models close to the modern day ones.
• Mendeleev 1869 - put elements in order of atomic weight but switch it if the properties meant it should be changed. gaps were left to put elements with similar properties in the same group. this meant elements that hadn't been discovered yet could be put in the right place
• An atom is the smallest part of an element that can exist.

  An element is a substance with only one type of atom in it.

  A compound is two or more elements chemically bonded.

  A mixture is is two or more elements not chemically bonded.
• Mixtures can be separated using physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography.

  These physical proccesses don't include chemical reactions and no new substance is made.
• Pure substances consist of one type of element or compound. A mixture is not a pure substance.
• Filtration is used to separate an insoluble solid from a liquid.

  An example is separating sand from water.
• Filtration Method:

  1. Fold up filter paper and place it inside a filter funnel.
  2. Place the filter funnel in a conical flask.
  3. Pour mixture into the filter funnel.
  4. The liquid will pass through the tiny pores in the filter paper whilst the solid will be caught.
• Crystallisation or Evaporation can be used to separate a soluble solid from a liquid.
• Cystallisation Method:

  1. Pour the solution into an evaporating dish and gently heat it. The solvent will evaporate, causing the solution to become more concentrated.
  2. When you see crystals starting to form, remove the dish from heat and leave the solution to cool.
  3. Filter the crystals from the solution and leave them in a warm place to dry.
• During the evaporation method, a solution is heated until there is no more liquid.
• Simple Distillation is used to separate a liquid from a solution.

  An example is separating pure water from seawater.
• Simple Distillation Method:

  1. Pour the solution into the flask.
  2. Gently heat the solution. The liquid will start to evaporate, turning into a vapour.
  3. The vapour rises up the tube, travelling into the condenser.
  4. The vapour condenses and turns back into a liquid.
  5. The liquid is collected in the beaker.
• Label the Simple Distillation apparatus

  A) Themometer

  B) Flask

  C) Cooling water in

  D) Cooling water out

  E) Condenser

  F) Beaker
• Fractional Distillation is used to separate liquids with similar boiling points.

  An example is separating ethanol from water.
• Fractional Distillation Method:

  1. Pour the solution into the flask.
  2. Gently heat the solution.
  3. The liquids will start to evaporate and travel towards the fractionating column. However, the liquid with the lower boiling point will evaporate faster.
  4. As the temperature is cooler at the top, the liquids with higher boiling points will condense and drip back down.
  5. When the thermometer matches the liquid with the lowest boiling point, it will reach the top.
  6. The liquid with the lower boiling point will reach the condenser, condense and collect in the beaker.
  7. Raise the temperature and repeat.
• Paper chromotograhy is used to separate mixtures of souble substances.

  An example is separating the inks in pens.
• Paper Chromotography Method:

  1. Draw a pencil line at the bottom of the chromotography paper.
  2. Place dots of the colours on the line.
  3. Place the bottom of the paper into a solvent.
  4. The solvent will make its way up the paper, dissolving the ink in the dots. The ink is carried up the paper.
• Phases in Chromatography:

  • The paper is called the stationary phase as it doesn't move.
  • The solvent is called mobile phase as it moves.
• In chromatography, a pure substance produces a single spot.

  However, the a mixture may separate into multiple spots.
• History of the Atom (Tiny Spheres & Plum Pudding Model)

  • In the early 19th century, it was believed that atoms were tiny spheres that couldn't be divided.
  • In 1897, JJ Thompson discovered that atoms contained electrons. This led to the plum pudding model, which suggested that the atom was a ball of positive charge with negative electrons embedded in it.
• History of the Atom (Alpha Particle Scattering Experiment)

  • In 1909, Ernest Rutherford and Ernest Marsden conducted the alpha particle scattering experiments. They fired alpha particles at a thin sheet of gold. Some particles went straight through the sheet, but others were deflected at different angles. This was because positively charged alpha particles had been repelled and reflected by a small concentration of charge in the nucleus.
• History of the Atom (Nuclear Model)

  • The alpha particle screening experiment led to the development of the nuclear model. This suggested that the atom had a positively charged nucleus at the centre where the mass of the atom was concentrated. This was surrounded by a 'cloud' of negative electrons.
  • Scientists later realised that if electrons were in a 'cloud' around the nucleus, they would be attracted, and the atom would collapse. Niels Bohr adapted the nuclear model, suggesting that electrons orbit the nucleus at specific distances. This was supported by experimental observations.
• History of the Atom (Protons & Neutrons)

  • Later experiments showed that the positive charge of a nucleus could be divided into a number of smaller particles, each with the same amount of positive charge. These particles were named protons.
  • Around 20 years after scientist accepted the idea of the nucleus, James Chadwick carried out an experiment which showed the existence of neutrons in the nucleus.