Atomic Structure and the Periodic Table

• atoms elements and compounds

  • atoms are the smallest part of an element that can exist.
  • chemical symbols represent an atom.
  • compounds are formed through chemical reactions.
  • compounds contain two or more elements.
• mixtures

  • a mixture has two or more elements not chemically combined.
  • chemical properties of the atoms are not changed.
  • can be separated by: filtration, crystallisation, distillation, chromatography.
• plum pudding model


  believe that an atom was a ball of positive charge with negative electrons embedded in it.
• alpha particle scattering experiment:

  concluded that the mass of an atom was at the centre (nucleus) and that the nucleus was charged.

  • a beam of alpha particles was aimed at thin gold foil.
  • some alpha particles emerged from the foil at different angles and some came straight back.
  • positively charged alpha particles were being repelled and deflected by concentration of positive charge in the atom.
• neil bohr

  suggested electrons orbit the nucleus at specific distances
• later experiments

  positive charge of nucleus could be subdivided into other smaller particles with equal positive charge (protons)
• james chadwick

  provided evidence to show the existence of neutrons within the nucleus
• relative electrical charges of subatomic particles

  • atomic number - the number of protons in an atom of an element.
  • same elements have the same number fo protons.
  • different elements have different numbers of protons.
• relative charges of particles

  proton = +1

  neutron = 0

  electron = -1
• size and mass of atoms

  atoms are very small and the radius is 1/10,000 of the atom, although it holds most of the mass.

  • mass number = the sum of the protons and neutrons in an atom
  • isotopes = atoms of the same element with different numbers of neutrons
• relative mass of particles

  proton = 1

  neutron = 1

  electron = very small
• how atoms are shown in the periodic table:

  .

  
• relative atomic mass

  an average value that takes account of the abundance of isotopes of the element.
• carbon has 2 isotopes: carbon-14 with abundance 20% and carbon-12 with abundance 80%. calculate the relative atomic mass of carbon

  ((14 x 20) + (12 x 80)) / 100 = 1240/100 = 12.4
• electronic structure

  • electrons occupy the lowest available energy levels
  • electronic structure of an atom tells you how many electrons are in each shell.
• the periodic table

  • elements are arranged in order of atomic number so that elements with similar properties are in groups.
  • elements in the same group have the same amount of electrons in their outer shell, giving them similar chemical properties.
• john newlands

  ordered his table in order of atomic weight
• dmitri mendeleev

  left gaps for elements that he thought had not been discovered yet
• modern periodic table

  • elements with properties predicted by mendeleev were discovered and filled the gaps
  • knowledge of isotopes made it possible to explain why the order based off atomic weight wasnt always correct.
• metals and non-metals

  metals = elements that react to form positive ions

  • majority of elements are metals
  • found on the left and bottom of the periodic table

  non-metals = elements that do not form positive ions

  • found on the left and top of the periodic table
• group 1 - alkali metals

  • only have one electron in the outer shell
  • react vigorously with water to create alkaline solution and hydrogen.
  • all react with oxygen to make oxides
  • all react with chlorine to make a white precipitate.
  • reactivity of elements increase asd you go down the group
• alkali metal reactions

  
• group 0 - noble gasses

  • have a full outer shell
  • unreactive and do not usually form molecules as they have stable electrons.
  • boiling points increase with increasing relative atomic mass
• group 7 - halogens

  • non metal and exist as molecules made of pairs of atoms
  • react with metals to form ionic compounds
  • react with nonmetals to form covalent compounds
  • reactivity decreases down the group as halogens react by gaining an electron, the number of shells of electrons increases