3.1.1 Atomic Structure

• The mass number is the number of protons and neutrons in a nucleus of an atom and is given the symbol A
• An isotope is atoms of the same element with the same number of protons and same electronic configuration but different number of neutrons.
• The mass spectrometer is a machine that gives you information about which isotopes are present in a sample, the Ar or Mr of the sample.
• An element's relative atomic mass is the average mass of an atom of the element compared to one-twelfth of the mass of an carbon 12 atom
• Electron Spray Ionisation:

  the sample molecule are ionised by gaining a proton(H+). A fine mist leaves the needle and the solvent evaporates leaving only ions. Electro spray does not lead to fragmentation.
• Ion Drift:

  The positive ions then travel along a tube, called the flight tube to the detector (field free drift region). The ToF of each ion through this flight tube depends on its velocity, which in turn depends on its mass.
• Detection:

  When ions arrive at the detector, the lighter ions are first as they traveled faster. The positive ion hits the electrically charged plate and pick up an electron which causes an electric current to flow. The size of the current tells us the number of ions hitting the plate.
• Mass Spectrum:

  A mass spectrum shows the mass/charge (m/z) ratio of each ion on the x axis.
• Mass Spectrum:

  If the charge on the ion is 1+ then the m/z ratio is the same as the mass number of the ion.
• An element's relative molecular mass is the average mass of a molecule compared to one-twelfth of the mass of one carbon 12 atom
• The atomic radius decreases across period 3 (from Na->Cl)because the number of protons and neutrons increase but the shielding stays the same.
• The first ionisation energy of krypton is greater than bromine because Krypton has more protons than bromine but similar shielding.
• One reason that it is necessary to ionise isotopes of atoms before they are analysed in a TOF mass spectrometer is so that the isotope of the element can be detected. Another reason is so that ions are accelerated to the negative plate and the ion create a current
• There are two methods of ionisation:

  Electron Impact

  Electro spray
• Period 3:

  Sodium has the highest second ionisation energy
• Electron Impact Ionisation:

  The sample must be gaseous and a high energy electron from an electron gun knocks one electron of the molecule to produce a positive ion. The positive ions are accelerated by an electric field.

  e-M(g)-->2e-M+(g)
• First deviation: Period 3

  There is a deviation in the first ionisation energies between Aluminium and Magnesium in Period 3. This is because Aluminium’s outer electron is in the 3p orbital and Magnesium’s outer electron is in the 3s orbital. The 3p orbital has a higher energy level and therefore less energy is required to remove the electron. Therefore Aluminium has a lower first ionisation energy than Magnesium.
• Electrospray ionisation:

  The sample is dissolved in a volatile solvet e.g ethanol and is injected through a fine hypodermic needle at a high voltage supply. Each molecule gains a h+ or proton.
• Detection:

  When the ion hits the detector the ion gains an electron. The relative abundance depends on the size of the current. As current is proportional to abundance