Elements of Life

Elements of Life

1. MODEL OF THE ATOM

   • Mass number == number of protons + number of neutrons
   • atomic number == number of protons/electrons
   • ISOTOPES
     • Elements with the same atomic number but different mass number.
     • There are more or less neutrons
     • Some are radioactive.
       • This is due to an unstable nuclei
       • All types of emission are dangerous and can cause cancer
       • HALF LIFE == the time taken for half the radioactive nuclei to decay.
         • Can be used to determine the age of archaeological artefacts
       • TRACERS == Radioactive isotopes whose decay is monitored; used in medicine to aid diagnosis.
         • Path is followed by a Geiger counter
         • The half life of it cant be too long or too short.
     • There is one isotope which is more abundant than the rest but you can find the average isotopic mass.
   • Nuclear fusion is the joining together of 2 or more nuclei to form a heavier nucleus
     • Need high temperatures and pressures to overcome the repulsion between positive nuclei.

2. LIGHT AND ELECTRONS

   • 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.
   • ABSORPTION SPECTRA
     • Electrons absorb a photon.
       • Excited electrons move up to a higher energy level ( producing lines on an absorption spectra.
     • The electromagnetic radiation absorbed by each of the hydrogen atoms has a definite frequency, which gives the specific colour.
     • Coloured background with black lines
   • EMISSION SPECTRA
     • black background with coloured lines.
     • Electrons first absorb a photon
       • Excited electrons move up to a higher energy level
         • Electrons then drop back to lower energy levels emitting a photon with a certain frequency.
     • The colour of the lines depends on the frequency of the photon emitted.
   • ELECTRON SHELLS
     • They fill up lowest first.
     • period 3 can have more than 8 electrons

3. SHAPES OF MOLECULES

   • 2 ELECTRON PAIRS
     • Linear
       • 180 degress
   • 3 ELECTRON PAIRS
     • Trigonal planar
       • 120 degrees
       • no lone pairs
     • bent
       • 120 degrees
       • 1 lone pair
   • 4 ELECTRON PAIRS
     • Tetrahedral
       • 109.5 degrees
       • no lone pairs
     • trigonal pyramid
       • 107 degrees
       • 1 lone pair
     • bent
       • 2 lone pairs
       • 104.5 degrees
   • 5 ELECTRON PAIRS
     • trigonal bypyramid
       • 120 degrees
       • 109 degrees
       • no lone pairs
   • 6 ELECTRON PAIRS
     • Octahedral
       • no lone pairs
       • all bond angles are 90 degrees
   • double bonds repel further than single bonds
   • lone pairs repel more than bonds
   • All around a central atom

4. CHEMICAL BONDING AND PROPERTIES

   • Metal and Metal == Metallic
     • Metal ions are arranged regularly in a lattice
     • Outer shell of electrons are all shared; they are delocalised.
     • Electrons free to move about therefore can carry a current.
   • Metal and Non Metal == Ionic
     • The Transfer of electrons
       • Usually from the metal to the non metal.
     • Form 2 charged ions in the process
       • Cations == positive
       • Anions == negative
       • Form a giant ionic lattice, held together by the electrostatic attraction between anions and cations.
   • Non Metal and Non Metal == Covalent
     • Sharing of electrons between two atoms
     • Double bonds are where two parsr of electrons are shared
     • Want to gain a full outer shell.
     • Dative Covalent Bonding
       • A dative covalent bond is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom

5. GROUP 2 REACTIONS

   • React with Water
     • metal + water --> metal hydroxide + hydrogen
     • more reactive as you go down the group
   • Oxides
     • metal oxide + water --> metal hyrdroxide
       • Alkaline solution is formed
     • metal oxide + acid --> salt + water
   • Hydroxides
     • metal hydroxide + acid --> salt + water
       • solutions produced are alkaline
     • More soluble as you go down the group
   • Carbonates
     • Less soluble as you go down the group
     • metal carbonate --> metal oxide + carbon dioxide
       • thermal decomposition occurs
         • Thermal stability increases down the group

6. MASS SPECTROMETER

   • 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
   • Sample inlet ~ gases or liquids are simply injected but solids are heated to vaporise them.
     • 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.
       • Acceleration area ~ an electric field is used to accelerate any ions so that they all have the same kinetic energy.
         • 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
   • The mass spectrum for a compound can be much more complex than the mass spectrum on an element.
     • 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.
       • The BASE PEAK is the most intense peak; there are many peaks because fragments are formed in the ionisation chamber.
       • 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.
   • % relative isotopic abundance = (relative abundance/total relative abundance)*100%

7. Number of moles = number of particles you have/number of particles in a mole

   • Avogadro's constant == 6.02 x10^23

8. THE PERIODIC TABLE

   • Periodic Trends
     • 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.
     • 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 .
     • 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.
     • 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.
     • The noble gases have the lowest melting/boiling points because they exist as individual atoms resulting in very weak intermolecular forces.
   • The modern periodic table is arranged according to atomic number(proton number)
   • In the 1800s, they ordered the periodic table by relative atomic mass.
     • John Newlands discovered the law of octaves as there were similar elements appearing at regular intervals.
       • However the pattern broke down in the third row
     • Mendeleev produced a better table, ordered by by atomic mass but he left gaps so the elements with similar chemical properties were together.
       • He made predictions about the undiscovered elements, which later on turned out to be correct

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