Elements of Life

MODEL OF THE ATOM
1. Mass number == number of protons + number of neutrons
2. atomic number == number of protons/electrons
3. ISOTOPES
  1. Elements with the same atomic number but different mass number.
  2. There are more or less neutrons
  3. Some are radioactive.
    1. This is due to an unstable nuclei
    2. All types of emission are dangerous and can cause cancer
    3. HALF LIFE == the time taken for half the radioactive nuclei to decay.
      1. Can be used to determine the age of archaeological artefacts
    4. TRACERS == Radioactive isotopes whose decay is monitored; used in medicine to aid diagnosis.
      1. Path is followed by a Geiger counter
      2. The half life of it cant be too long or too short.
  4. There is one isotope which is more abundant than the rest but you can find the average isotopic mass.
4. Nuclear fusion is the joining together of 2 or more nuclei to form a heavier nucleus
  1. Need high temperatures and pressures to overcome the repulsion between positive nuclei.
LIGHT AND ELECTRONS
1. ENERGY LEVELS == Electrons in a hydrogen atom must be in one of the allowed energy levels. If an electron is in the first energy level, it must have exactly -13.6 eV of energy. If it is in the second energy level, it must have -3.4 eV of energy.
2. ABSORPTION SPECTRA
  1. Electrons absorb a photon.
    1. Excited electrons move up to a higher energy level ( producing lines on an absorption spectra.
  2. The electromagnetic radiation absorbed by each of the hydrogen atoms has a definite frequency, which gives the specific colour.
  3. Coloured background with black lines
3. EMISSION SPECTRA
  1. black background with coloured lines.
  2. Electrons first absorb a photon
    1. Excited electrons move up to a higher energy level
      1. Electrons then drop back to lower energy levels emitting a photon with a certain frequency.
  3. The colour of the lines depends on the frequency of the photon emitted.
4. ELECTRON SHELLS
  1. They fill up lowest first.
  2. period 3 can have more than 8 electrons
SHAPES OF MOLECULES
1. 2 ELECTRON PAIRS
  1. Linear
    1. 180 degress
2. 3 ELECTRON PAIRS
  1. Trigonal planar
    1. 120 degrees
    2. no lone pairs
  2. bent
    1. 120 degrees
    2. 1 lone pair
3. 4 ELECTRON PAIRS
  1. Tetrahedral
    1. 109.5 degrees
    2. no lone pairs
  2. trigonal pyramid
    1. 107 degrees
    2. 1 lone pair
  3. bent
    1. 2 lone pairs
    2. 104.5 degrees
4. 5 ELECTRON PAIRS
  1. trigonal bypyramid
    1. 120 degrees
    2. 109 degrees
    3. no lone pairs
5. 6 ELECTRON PAIRS
  1. Octahedral
    1. no lone pairs
    2. all bond angles are 90 degrees
6. double bonds repel further than single bonds
7. lone pairs repel more than bonds
8. All around a central atom
CHEMICAL BONDING AND PROPERTIES
1. Metal and Metal == Metallic
  1. Metal ions are arranged regularly in a lattice
  2. Outer shell of electrons are all shared; they are delocalised.
  3. Electrons free to move about therefore can carry a current.
2. Metal and Non Metal == Ionic
  1. The Transfer of electrons
    1. Usually from the metal to the non metal.
  2. Form 2 charged ions in the process
    1. Cations == positive
    2. Anions == negative
    3. Form a giant ionic lattice, held together by the electrostatic attraction between anions and cations.
3. Non Metal and Non Metal == Covalent
  1. Sharing of electrons between two atoms
  2. Double bonds are where two parsr of electrons are shared
  3. Want to gain a full outer shell.
  4. Dative Covalent Bonding
    1. A dative covalent bond is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom
GROUP 2 REACTIONS
1. React with Water
  1. metal + water --> metal hydroxide + hydrogen
  2. more reactive as you go down the group
2. Oxides
  1. metal oxide + water --> metal hyrdroxide
    1. Alkaline solution is formed
  2. metal oxide + acid --> salt + water
3. Hydroxides
  1. metal hydroxide + acid --> salt + water
    1. solutions produced are alkaline
  2. More soluble as you go down the group
4. Carbonates
  1. Less soluble as you go down the group
  2. metal carbonate --> metal oxide + carbon dioxide
    1. thermal decomposition occurs
      1. Thermal stability increases down the group
MASS SPECTROMETER
1. Mass spectrometry is an analytical chemistry technique that helps identify the amount and type of chemicals present in a sample by measuring the mass-to-charge ratio and abundance of gas-phase ions
2. Sample inlet ~ gases or liquids are simply injected but solids are heated to vaporise them.
  1. Ionisation area ~ a heated filament produces high-energy electrons. These electrons bombard any atoms or molecules in the sample and knock electrons out. Cations are formed.
    1. Acceleration area ~ an electric field is used to accelerate any ions so that they all have the same kinetic energy.
      1. Drift region ~ there is a vacuum here so that ions do not collide with air molecules which would change the direction of their flight path. Since kinetic energy = mass*velocity^2 and all ions have the same kinetic energy , heavier ions more through this region more slowly.
        Ion detector ~ light ions reach the detector before the heavier ones. A computer system converts the informations into a mass spectrum. Only positive ions are detected after fragmentation
3. The mass spectrum for a compound can be much more complex than the mass spectrum on an element.
  1. The ion with the greatest mass corresponds to the molecular mass; this is called the MOLECULAR ION and corresponds to the parent molecule minus an electron.
    1. The BASE PEAK is the most intense peak; there are many peaks because fragments are formed in the ionisation chamber.
    2. The MOLECULAR ION breaks down into fragments; if the fragment has a positive charge then it will be accelerated by the electrical field and detected later.
4. % relative isotopic abundance = (relative abundance/total relative abundance)*100%
Number of moles = number of particles you have/number of particles in a mole
1. Avogadro's constant == 6.02 x10^23
THE PERIODIC TABLE
1. Periodic Trends
  1. For the metals; melting and boiling points increase across the period because the metal-metal bonds get stronger as there are more delocalised electrons and decreasing radius; higher charge density.
  2. The elements with giant covalent structures have strong covalent bonds linking all their atoms together. A lot of energy is needed to break these bonds .
  3. Simple molecular substances; their melting and boiling points depend on the strength of the intermolecular forces. The stronger the intermolecular forces, the higher the melting and boiling point.
  4. More atoms in a molecule mean stronger intermolecular forces. e.g. in period 3 sulfur is the biggest molecule so its got higher melting/boiling points than chlorine.
  5. The noble gases have the lowest melting/boiling points because they exist as individual atoms resulting in very weak intermolecular forces.
2. The modern periodic table is arranged according to atomic number(proton number)
3. In the 1800s, they ordered the periodic table by relative atomic mass.
  1. John Newlands discovered the law of octaves as there were similar elements appearing at regular intervals.
    1. However the pattern broke down in the third row
  2. Mendeleev produced a better table, ordered by by atomic mass but he left gaps so the elements with similar chemical properties were together.
    1. He made predictions about the undiscovered elements, which later on turned out to be correct

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