C1 - Atomic structure

• what is an atom?

  the smallest particle of a chemical element that can exist. it has a tiny nucleus at its centre surrounded by electrons
• what is an element?

  substances made up of/ containing only one type of atom
• what are compounds?

  a pure substance made from more than one type of element chemically bonded
• what is the law of conservation of mass?

  mass is neither created or destroyed in chemical reactions (the total mass of products formed in a reaction is equal to the total mass of the reactants).
• what is a mixture?

  two or more substances (elements or compounds) that are not chemically combined together
• compounds have fixed compositions. what does this mean?

  the ratio of elements present is always the same in any compound
• mixtures dont have fixed compositions. what does this mean?

  the ratio of elements/substances present varies in mixtures
• what type of reaction must be used to separate the elements in a compound

  chemical reaction
• is it possible to separate different elements in a mixture

  yes because they are not chemically bonded
• what is filtration?

  a technique used to separate insoluble substances in a solvent from those that are soluble
• what is crystalisation?

  the process of obtaining a pure salt from an impure solution
• why do we draw the starting lin in chromatography in pencil?

  pen would interfere and dissolve up the paper as it consists of ink mixed with a solvent
• summarise paper chromatography:

  it separates mixtures of substances dissolved in a solvent as they move up a piece of chromatography paper. the different substances are separated because of their solubilities in the solvent
• what happens to compounds in a mixture in paper chromatography?

  they separate into spots depending on the solvent
• what are the two phases in paper chromatography and why?

  1. stationary phase: paper as it does not move
  2. mobile phase: solvent as it moves
• what happens in a pure compound in chromatography?

  a single spot appears for each solvent
• why does paper chromatography work?

  substances have different solubilites so more soluble substances will travel further than those that are less soluble therefore making it work
• what are the relative charges and masses of protons, electrons and neutrons?

  relative charge:

  • protons: +1
  • neutrons: 0
  • electrons: -1

  relative mass:

  • protons: 1
  • neutrons: 1
  • electrons: very small
• what is the nuclear model?

  the modern day nucleus model, started off with a positive nucleus with empty space but now has electron shells
• what was concluded from the alpha particle experiment

  • atoms are mostly empty space
  • centre of the atom must have a positive charge as alpha particles are positive (like charges repel)
  • centre of the atom must contain a great deal of mass
• explain the alpha scattering experiment:

  alpha particles (positive) fired at a thin gold foil (few atoms thick). most passed straight through but some deflected or bounced back
• what is the plum pudding model?

  a ball of positive charge with negative electrons embedded in it
• what is an atomic number?

  the number of protons in an element
• what is the mass number?

  the number of protons and neutrons in an element
• how do we calculate the number of electrons (using mass and atomic number)?

  mass number - atomic number
• what is an ion?

  a charged atom or group of atoms that has become charged due to a loss or gain of an electron
• what is an isotope?

  atoms of the same element with different numbers of neutrons