Chemistry for Life

OCR B Chemistry for life

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  • Created by: Joanna
  • Created on: 21-05-11 14:39

Atomic Structure


Relative Charge: +1

Relative Mass: 1


Reltive Charge: 0

Relative Mass: 1


Relative Charge: -1

Relative Mass: 0

  • Rutherford discovered the layout of the atom by firing alpha particles at gold leaf particles.
  • The number of protons is deines the atomic number of an element. It is different for each element.
  • Number of protons + neutrons= mass.
  • Number of protons= number of electrons.
  • Relative atomic mass (Ar)= Average mass of an atom compared to 1/12 the mass of an atom of Carbon.
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Groups one and two- The Alkali Metals

  • Weak bonds between atoms
  • React vigorously with water to form a metal hydroxide and hydrogen
  • he elements towards the bottom of the group are more reactive, because the one outer electron is further away from the positive nucleus, so the force needed to take the electron away is comparatively less than the elements toward the top of the group.
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Mass Spectrometer

The abundance of isotopes are determined using a Mass Spectrometer.


  1. Atoms vaporised, and sent into spectrometer.
  2. Heated tungsten wire produces electrons, which “knock electrons” from the sample, providing ions with a positive charge.
  3. Particles are accelerated using an electric field.
  4. A Magnetic field is placed 90o to the tube, the particles move; they are deflected.
  5. The larger particles are deflected least, and the lighter particles most.
  6. The number of ions hitting the detector is measured, and the magnetic field is changed so that the different isotopes can hit the detector. This gives an abundance of each separate isotope.
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  • When atoms are given energy, their electrons jump up levels. The energy that is needed to cause the electrons to jump energy levels is specific.
  • The atoms are “excited”, meaning they gain energy.
  • After the atom electrons have been promoted, they get demoted again, that is they move back down the energy levels. When being demoted, the atoms emit the specific amounts of energy.
  • This energy is in the form of light. When the light is viewed through a spectroscope, the light emitted is split up into an emission spectrum. The spectrum consists of a series of lines; the colour of these lines is specific to the wavelength.
  • Elements can be identified by their light emissions. An absorption spectrum is when the light absorbed is analysed, and an emission spectrum is when the light produced is analysed.
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Ionic Bonding

  • Most metal atoms have three or fewer outer electrons. A noble gas configuration is reached if these are lost to form positively charged ions (cations). 
  • Most non-metal atoms have more than three outer shell electrons. To become stable, they must become negatively charged ions (anions). 
  • There are limits to how many electrons an atom can pick up. If one electron is gained, the atom becomes an anion with a negative charge. This will repel any more electrons wanting to join the energy level, and so atoms gaining two or three electrons are rare. 
  • It is also hard to remove three or more electrons from an atom, as the ionisation energy increases after each electron is removed. 
  • When metals bond with non-metals, electrons are transferred from the metal atoms to the non-metal atoms. The metal atoms become cations with a positive charge, and the non metals become anions with a negative charge. Opposites attract, and the atoms are held together by an electrostatic attraction. 
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Covalent Bonding

  • Non-metallic elements bond with each other by “sharing” electrons. This is called covalent bonding. Shared electrons count as the outer electron for both elements.
  • Electron pairs which form covalent bonds are called bonding pairs
  • Electrons not involved are called lone pairs
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Shapes of Molecules

  • All electrons have a negative charge; like charges repel, so the electrons are arranged so that they are as far away as possible from each other. This is what determines the shape of molecules.
  • It is not only the shared electrons that affect the shape of the molecule, but the lone pairs also affect the shape, in fact they have a greater effect as their negative charge is stronger than that of the shared electrons
  • The various shapes of molecules are as follows:
    • Triginal Planar
    • Tetrahedral
    • V-Shaped
    • Linear
    • Pyridimal
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Triangular Planar

  • Boron tri-fluoride (BH3) has a dot and cross diagram as below:


  • The bonding electron pairs around the central Boron atom have the same repulsion, and so an equilateral triangle is formed around the central Boron molecule. This means that the angle between the fluorine molecules is 120o.
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  • Methane (CH4) has a dot and cross diagram as below:


  • The four bonding electron pairs around the central carbon atom in this molecule have the same negative repulsion, and so they are formed so that they are as far apart as possible. The angle found between the molecules in 109.5o.
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  • Water (H2O) has a dot and cross diagram as below:


  • In this molecule around the central oxygen atom, the shared pair of electrons repel each other, but the two lone electrons also repel the two shared pairs and so a v-shape is formed, with an angle of 104.4o in between the atoms.
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  • Beryllium chloride (BeCl2) has a dot and cross diagram as below:


  • As there are only two bonding pairs of electrons, they are found directly opposite each other.
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  • Ammonia (NH3) has a dot and cross diagram as below:


  • The lone pair of electrons around the central Nitrogen atom repel the three bonding electron pairs forming a pyramid shape, with an angle of 107o between each atom.
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