Atomic structure and the periodic table

• What are atoms?


  smallest part of an element
• What are compounds?


  Substances containing two or more different elements that are chemically bonded together
• What does CO2 mean

  CO2 - for every caron (C) atom there are 2 Oxygen (O) atoms
• What are mixtures?


  Two or more elements or compounds not chemically combined together

  -chemical properties of each substance in the mixture are unchanged

  Examples: salt solution, air , sandy water
• Methods for separating mixtures


  filtration, crystallisation, simple distillation, fractional distillation and chromatography
• Filtration

  -separates an insoluble solid from a liquid, for example sand & water

  -use a filter funnel and filter paper (inside a conical flask)

  -the water is the filtrate which is separated out from this mixture

  -pour the mixture into the filter paper, water molecules and other small particles pass through tiny pores in the filter paper

  -solid material and large particles like the sandy water cannot pass through the filter paper so it is trapped
• Crystallisation method:

  -separates a soluble solid from a liquid

  -pour some salt solution in an evaporating dish

  -use a water bath to heat the salt solution and the water evaporates out from the solution, leaving behind crystals of the salt solution
• Fractional distillation
• Plum Pudding Model


  J.J Thomsons model of an atom

  -The discovery of the electron led to the plum pudding model of the atom.

  -An atom is ball of positive charge with negative elctrons embedded in it
• Alpha sattering experiment


  -very thin layer of gold foil

  -beam of alpha particles were fired at the foil (positive charge)

  -Most alpha particles passed straight through, some were deflected (changed direction) and a very few were reflected back

  -these results led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model replaced the plum pudding model.
• Alpha scattering experiment results

  Most of the αlpha-particles passed straight through the gold foil - so atoms are mainly empty space

  Some particles were deflected from their path - positive charged centre, ( two positive charges so repell)

  Some bounced straight back - centre of atom (nucleus) must contain a amount of mass and charge

  
  

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• Nuclear Model of the Atom

  -Most of the atom is empty space but there is a tiny Positvley charged nucleus in the centre and electrons orbit that nucleus
• Compare nucleur and plum puddig model
• Niels Bohr


  adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances (in shells)

  -Positivley charged particles in the nucleus called protons

  -James Chadwick discoverd that the nucleus also contains neutral particles called neutrons
• Atoms charge

  0 - neutral because the number of electrons is always equall to the number of protons
• Relative electrical charges for sub-atomic particles

  Proton +1

  Neutron 0

  Electron -1
• Atomic number


  The number of protons
• All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.
• Radius of an atom

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

  10,000 x smaller (1 x 10^-14 m)
• Relative masses of protons neutrons and electrons

  Proton 1

  Neutron 1

  Electron very small
• Mass number:


  The sum of the protons and neutrons in an atom
• How to calculate the numbers of protons, neutrons and electrons in an atom

  Protons = atomic number

  Neutrons = atomic number

  Electrons = mass number - atomic number
• Isotopes


  -same number of protons but different number of neutrons

  -same chemical properties but different physical properties
• The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
• Calculating relative atomic mass with given percentage abundance of isotopes

  (% abundance x RAM)+(% abundance x RAM) / 100
• How were elements originally arranged

  In order of atomic weights
• What was wrong with that arrangement and how was it fixed?

  -some elements were placed in inappropriate groups

  -Mendeleev followed atomic weights but left gaps for elements he thought had not been discovered which were eventually discovered and filled the gaps
• Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.
• -Elements are arranged in order of atomic (protons) number -Elements with similar properties are in groups-Elements in the same group have the same number of electrons in their outermost shell which gives them similar chemical properties
• Periodic table

  -Elements are arranged in order of atomic (protons) number

  -Elements with similar properties are in groups

  -Elements in the same group have the same number of electrons in their outermost shell which gives them similar chemical properties
• What are metals?


  Elements that react to form positive ions by loosing an electron
• What are non-metals


  Elements that do not form positive ions
• Metalsreactivity


  -Group 1 & 2 elements are highly reactive

  -transition metals are less reactive than groups 1 & 2

  -group 0 elements are unreactive
• Differences between metals and non-metals

  Metals- hard & shiny, high density, Good conductors of electricity and heat, malleable, high melting & boiling points

  Non-metals - Low density, poor conductors, low melting & boiling points soft, brittle and dull
• Group 0 - the noble gases

  -unreactive and do not easily form molecules because they already have a stable arrangements of electrons (full outer shell)

  -8 electrons in their outer shell except helium which has 2
• Why do the boiling points of noble gases increase as you go down the group?

  -because the atoms get larger

  -so the forces of attraction between atoms become

  -more energy is needed to overcome those forces and separate atoms
• Group 1 - Alkali metals 


  -all have 1 electron on their outer shell giving them their properties
• Reactions of lithium sodium and potassium with oxygen


  -react to make metal oxides (white solids)

  Lithium + oxygen -> Lithium oxide

  Sodium + oxygen -> sodium oxide

  Potassium + oxygen -> potassium oxide
• Reaction with chlorine


  -react vigorously when heated to form metal chlorides (white salts)