atomic structure and the periodic table

• Atom


  The smallest part of an element that can exist
• Periodic table


  Elements are arranged in order of atomic (proton) number (smaller number) and so that elements with similar properties are in columns, known as groups

  
• Elements in the same periodic group

  Have the same amount of electrons in their outer shell, which gives them similar chemical properties
• John Newlands


  • Ordered his table in order of atomic weight
  • Realised similar properties occurred every eighth element – 'law of octaves' but broke down after calcium
• Dmitri Mendeleev


  • Ordered his table in order of atomic mass, but not always strictly – i.e. in some places he changed the order based on atomic weights
  • Left gaps for elements that he thought had not been discovered yet
• The table is called a periodic table because similar properties occur at regular intervals
• Elements with similar properties are found in the same column (groups)
• Elements with properties predicted by Mendeleev were discovered and filled the gaps
• Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct
• When electrons, protons and neutrons were discovered in the early 20th century, elements were ordered in atomic (proton) number
• When this was done, all elements were placed in appropriate groups
• Metals


  Elements that react to form positive ions
• Chemical symbol


  Represents an atom of an element e.g. Na represents an atom of sodium
• Non-metals


  Elements that do not form positive ions
• Group 1 - Alkali metals


  • They have characteristic properties due to the single electron in their outer shell
  • Metals in group one react vigorously with water to create an alkaline solution and hydrogen
  • They all react with oxygen to create an oxide
  • They all react with chlorine to form a white precipitate
  • The reactivity of the elements increases going down the group
• Group 0 - Noble gases

  • They have 8 electrons in their outer shell (except helium, which has 2). All of them (including helium) have full outer shells
  • They are unreactive and do not easily form molecules, because they have a stable arrangement of electrons (full outer shell)
  • The boiling points of the noble gases increase with increasing relative atomic mass (going down the group)
• Group 7 - The halogens


  • Similar reactions due to their seven electrons in their outer shell
  • Non-metals and exist as molecules made of pairs of atoms (e.g. Cl2)
  • They react with metals to form ionic compounds in which the halide ion carries a -1 charge
  • They react with nonmetals to form covalent compounds, where there is a shared pair of electrons
  • As you go down the group, relative molecular mass, melting point and boiling point all increase
  • Reactivity decreases down the group because halogens react by gaining an electron (to increase their number of outer shell electrons from 7 to 8) and the number of shells of electrons increases down the group, so down the group the element attracts electrons from other atoms less, so can't react as easily
  • A more reactive halogen (one from higher up group 7) can displace a less reactive one in an aqueous solution of its salt
• Compound


  Formed from elements by chemical reactions, contains two or more elements chemically combined in fixed proportions, can be represented by formulae
• Chemical reactions always involve the formation of one or more new substances, and often involve an energy change
• Transition metals compared to group 1

  • Harder and stronger
  • Higher melting points (except for mercury) and higher densities
  • Much less reactive and don't react as vigorously with oxygen or water
• Transition metals


  • Chromium
  • Manganese
  • Iron
  • Cobalt
  • Nickel
  • Copper
• Chromium


  Lustrous, brittle, hard metal
• Manganese


  Hard and very brittle, difficult to fuse, but easy to oxidise
• Iron


  Good conductor, rusts easily in air, strong, ductile
• Cobalt


  Brittle, hard, high melting point
• Nickel


  Hard, malleable, and ductile, fairly good conductor of heat and electricity
• Copper


  Highly ductile and conductive, malleable and soft
• Typical properties of transition metals

  • They have ions with many different charges
  • Form coloured compounds
  • Are useful as catalysts
• Ion charges of transition metals

  • Chromium: +2 +3 +4 +5 +6
  • Manganese: +2 +3 +4 +5 +6 +7
  • Iron: +2 +3 +4 +5 +6
  • Cobalt: +2 +3 +4
  • Nickel: +2 +3 +4
  • Copper: +1 +2 +3
• Colours of compounds with transition metals

  • Chromium: +2 +3 +6
  • Manganese: +2 +4 +6 +7
  • Iron: +2 +3
  • Cobalt: +2 +3
  • Nickel: +2
  • Copper: +2
• Uses of transition metals as catalysts

  • Chromium: Decomposition of hydrogen peroxide (2H2O2 -> 2H2O + O2)
  • Manganese: Haber process (N2 + 3H2 ↔ 2NH3)
  • Nickel: Manufacture of margarine (adding H2 to double bonds)
• Compounds can only be separated into elements by chemical reactions
• Mixture


  Two or more elements or compounds not chemically combined together, chemical properties of each substance are unchanged
• Methods to separate mixtures

  • Filtration
  • Crystallisation
  • Simple distillation
  • Fractional distillation
  • Chromatography
• Separation methods are physical processes, so do not involve chemical reactions and no new substances are made
• Development of the model of the atom

  1. Atoms thought to be tiny spheres that could not be divided

  2. Discovery of electron -> plum pudding model

  3. Alpha particle scattering experiment -> conclusion that mass of atom concentrated at centre (nucleus) and nucleus charged

  4. Bohr: electrons orbit nucleus at specific distances

  5. Later experiments: positive charge of nucleus subdivided into protons

  6. Chadwick's work: provided evidence for existence of neutrons
• Atomic number


  The number of protons in an atom of an element
• All atoms of a particular element have the same number of protons, atoms of different elements have different numbers of protons
• Relative electrical charges of subatomic particles

  • Proton: +1
  • Neutron: 0
  • Electron: -1
• An atom has an overall charge of 0, so number of protons = number of electrons