Atoms, Elements, Compound

• All substances are made up of..

  • atoms
  • smallest part of an element to exist
• Elements are found in

  • periodic table
  • around 100 elements
• Elements

  • pure substance
  • consists of one type of atom
• Compounds

  • Formed by chemical reactions
  • Two or more elements chemically combined in fixed proportions
• Chemical reaction v Physical reaction

  • Physical changes are reversible
  • Chemical changes involve formation of new substance
• Separation of compounds

  • By chemical reactions
  • More difficult to separate
• Elements are represented by

  • Chemical symbol
• Compounds are represented by

  • Chemical formulae
• Mixture

  • Consists of two or more elements that are not chemically combined in fixed proportions
  • Chemical properties of the mixture remain the same
• Simple distillation

  1. The mixture is heated to make the liquid evaporate.
  2. The vapour is then cooled to make it condense

  
• Simple distillation : Temperatures

  • Liquids that have boiling points significantly apart
• Describe how the process of distillation produces pure water from salt solution.

  1. Solution is heated
  2. Water evaporates
  3. Vapour cools in the condenser
  4. The vapour condenses
• Fractional distillation

  • Occurs between liquids with boiling points in close proximity to each other.
  • Liquid with lowest boiling point evaporates and condenses first.
• Filtration equipment

  • Filter funnel
  • Filter paper
  • Beaker
  • Mixture
• First thought of atoms by John Dalton 


  • Tiny solid spheres that could not be divided
• What changed the model of the atom?

  • Discovery of electrons
• Plum Pudding model

  • Ball of positive charge
  • Electrons embedded in it
• Alpha particle scattering experiment by Rutherford

  Most alpha particles passed straight through the gold foil

  • atom is mostly empty space
  • mass of an atom is concentrated at the center

  Some alpha particles were deflected

  • Atom had a positively charged nucleus / center
• Niel Bohr

  • suggested that electrons orbit the nucleus at specific distances
  • in energy shells
• Describe the change that Bohr made to the nuclear model. (2)

  • Electrons orbit the nucleus
  • At specific distances from the nucleus
• What did the Alpha Particle Scattering Experiment lead to?

  • The Nuclear Model

  
• 3 statements about the Nuclear Model

  1. Mostly empty space
  2. Mass concentrated in the nucleus
  3. Positive charge is all in the nucleus
• Plum Pudding Model v Nuclear Model

  Plum Pudding

  • ball of positive charge spread throughout
  • no empty space
  • mass spread throughout

  Nuclear Model

  • positive charge concentrated at the centre
  • mostly empty space
  • mass concentrated in the centre
• Chadwick’s experimental work on the atom led to a better understanding of isotopes. Explain how his work led to this understanding

  1. James Chadwick provided evidence for the existence of neutrons
  2. This was necessary because isotopes have the same number of protons, but different number of neutrons.
• Proton discovery

  • Positive charge of nucleus could be subdivided into a whole number of smaller particles
  • With same positive charge
• Proton

  • positive charge
  • relative size : 1
• Neutron

  • no charge
  • relative size : 1
• Electron

  • negative charge -1
  • relative size : very small
• Why do atoms have no overall electrical charge?

  • the number of electrons is equal to the number of protons in the nucleus.
• All atoms of a particular element

  • Have the same number of protons
• Atomic Number 


  • Proton number
  • The number of protons of an atom an element as
• Atomic Weight

  • Relative atomic mass
• Atoms of different elements

  • different numbers of protons.
• Isotopes

  • Atoms with the same number of protons
  • But a different number of neutrons
  • Therefore, have different mass number
• Mass number

  • total number of neutrons and protons in an atom
• Location of nucleus

  • In the centre of the atom
  • Neutrons + Protons
• Charge of nucleus

  • Positive
• Radius of an atom

  • 0.1 nm (1 x 10^-10 m)
• Radius of a nucleus

  • less than 1/10 000 of that of the atom (approx. 1 x 10-14 m).