Chemistry EL

• Atomic structure model


  Small, dense central nucleus surrounded by orbiting electrons in electron shells
• Atomic nucleus


  • Contains protons and neutrons
  • Has overall positive charge
  • Contains almost the entire mass of the atom
• Neutral atom


  Number of electrons is equal to the number of protons
• Electron shells


  Maximum number of electrons that can be held by a single shell depends on the number of the shell, calculated as 2n^2 where n is the shell number
• Electron shells must fill before the next one can hold any electrons
• Mass number (A)


  Sum of protons and neutrons in an atom
• Atomic number (Z)


  Number of protons in an atom, also known as proton number
• Fundamental particles


  • Proton
  • Neutron
  • Electron
• Relative charge


  • Proton: +1
  • Neutron: 0
  • Electron: -1
• Relative mass


  • Proton: 1
  • Neutron: 1
  • Electron: 1/1840
• Ion


  Atom that has lost or gained electrons, resulting in an overall charge
• Number of electrons in an ion

  Equal to the number of protons minus the ionic charge
• Isotopes


  Atoms of the same element with the same atomic number but different number of neutrons, resulting in different mass numbers
• Neutral atoms of isotopes react chemically in the same way as their proton number and electron configuration is the same</b>
• Different mass numbers of isotopes mean they have different physical properties
• Relative atomic mass (Ar)


  Weighted mean mass of an atom of an element, relative to one twelfth of the mean mass of an atom of the carbon-12 isotope
• Relative isotopic mass


  Isotopic mass of an isotope relative to one twelfth of the mean mass of an atom of the carbon-12 isotope
• Relative molecular mass (Mr)


  Weighted mean mass of a molecule of a compound, relative to one twelfth of the mean mass of an atom of the carbon-12 isotope
• Relative formula mass is similar to Mr but is used for compounds with giant structures
• Amount of substance


  Measured in moles, the number of particles in a substance
• Mole


  Unit of measurement for substances, always contains the same number of particles (Avogadro's constant)
• Molar mass


  Mass per mole, with units g mol^-1
• Molar gas volume


  Gas volume per mole, with units dm^3 mol^-1
• Empirical formula


  Simplest whole number ratio of atoms of each element present in a compound
• Molecular formula


  True number and type of atoms of each element in a molecule
• Water of crystallisation


  Water that is part of the crystalline structure, stoichiometrically chemically bonded into the crystal structure
• Anhydrous substance


  Contains no water of crystallisation
• Hydrated substance


  Contains water of crystallisation
• Percentage composition


  Percentage of a compound made up by a single element
• Balanced chemical equation


  Must have the same number and type of each atom present on both sides of the equation
• State symbols


  • (s) - solid
  • (l) - liquid
  • (g) - gas
  • (aq) - aqueous (dissolved in water)
• Ionic equation


  Shows just the reacting particles that undergo a change during the reaction, with the spectator species not shown
• Titration


  Practical method where a standard solution of known concentration is reacted with a solution of unknown concentration to determine a property of the unknown solution
• Volumetric solution - simple method

  1. Weigh sample bottle

  2. Transfer solid to beaker and reweigh

  3. Record mass difference

  4. Dissolve solid in distilled water

  5. Transfer to volumetric flask and make up to mark
• Titration - simple method

  1. Fill burette with standard solution

  2. Use pipette to transfer 25cm^3 of unknown solution into conical flask
• Volumetric Solution - simple method

  1. Weigh the sample bottle containing the solid on a (2 d.p.) balance

  2. Transfer solid to beaker and reweigh sample bottle

  3. Record the difference in mass

  4. Add distilled water and stir with a glass rod until all the solid has dissolved

  5. Transfer to a volumetric flask with washings

  6. Make up to the 250cm3 mark with distilled water

  7. Shake flask
• Titration - simple method

  1. Fill the burette with the standard solution of known concentration, ensuring the jet space in the burette is filled and doesn't contain air bubbles

  2. Use a pipette filler and pipette to transfer 25cm3 of the solution with unknown concentration into a conical flask

  3. Add two to three drops of indicator

  4. Record the initial burette reading

  5. Titrate the contents of the conical flask by adding solution to it from the burette until the indicator undergoes a definite, permanent colour change

  6. Record the final burette reading and calculate the titre volume

  7. Repeat until at least two concordant results are obtained (within 0.1cm3 of each other)
• Balance


  • Use a balance with a suitable resolution for your experiment
• Measuring cylinder


  • Use a measuring cylinder to measure the volume of a solution
  • Use a volumetric flask of the required volume when making up a standard solution
• Gas syringe


  • Use a gas syringe to measure the volume of gas produced in an experiment
  • Alternatively, measure the mass lost on a balance and calculate the moles of gas produced from this