Atomic structure

• Define isotopes

  Atoms of the same element with the same number of protons but a different amount of neutrons
• Isotopes have similar chemical properties because...

  they have the same electron configuration- chemical properties depend on number of electrons
• Isotopes have varying physical properties because...

  they have different masses
• Mass Spectrometer: Why is it kept in vacuum conditions?

  To prevents the ions produced colliding with molecules of air
• Mass Spectrometer: What happens during ionisation?

  -A vaporised sample is injected at low pressure

  -High energy electrons are fired with an electron gun and bombards with sample, which knocks off an electron, producing positively charged ions.
• Mass Spectrometer: What happens during acceleration?

  Positive ions are attracted to a negatively charged plate, causing them to accelerate. Lighter ions with a higher charge move faster
• Mass Spectrometer: What happens during ion drift?

  The ions pass through a hole in the negatively charged plate, forming a beam, which drift towards the detector
• Mass Spectrometer: What happens during detection?

  The lighter ions which have the fastest velocities arrive at the detector first. The positive ions pick up an electron, causing a current to flow.

  The size of the current gives a measure of the relative abundance
• Mass Spectrometer: What happens during data analysis?

  Signals from the detector are passed to a computer which generates a mass spectrum
• Sub-shell S holds 2 electrons
• Sub-shell P holds 6 electrons
• Sub-shell D holds 10 electrons
• Sub-shell F holds 14 electrons
• Factors affect ionisation energy: Nuclear Charge

  Greater the nuclear charge (the more protons there are) the greater the nuclear attraction

  • more energy needed
• Factors affect ionisation energy: Shielding

  More shielding, less nuclear attraction

  • less energy needed
• Factors affect ionisation energy: The distance of the outer electrons to the nucleus

  Greater nuclear charge, stronger nuclear attraction

  • size decreases

  Greater the shielding, less nuclear attraction

  • size increases
• Why are the successive ionisation energies always higher?

  Because a positive ion is formed from the first ionisation energy, meaning the nuclear attraction increases, making it harder for the second electron to be removed
• Why is He's ionisation energy bigger than H's?

  • Increased nuclear charge (gone from + to +2)
  • Same distance from nucleus to outer electron
  • Same shielding
• Why is there such a large drop of ionisation energy between He and Li?

  • Increased shielding (Li enters 2s orbital, He still on 1s)
  • Outer electrons are further away from the nucleus
  • Less nuclear attraction- electron is easier to remove
• Define relative atomic mass

  the average mass of an atom of an element relative to one twelfth of the mass of an atom of carbon-12
• Explain the trend in first ionisation energy across group 2


  • General decrease in first IE. The outer electron is further away from the nucleus, less nuclear attraction, atomic radius increases.
  • More protons so more nuclear charge, but increased shielding means the electron is easier to remove
• Explain the trend in the first ionisation energy across period 3

  General increase in IE

  • Drop in IE between Mg and Al as Al enter 3p subshell, more sheilding, easier to remove the electron

  Drop in IE between P and S as the subshells in 3p in P all have one electron but in S there is one orbial with an extra electrons meaning there is repulsion so it is easier to remove an electron from there
• Ions are accelerated by an electric field
• Ions are deflected by a magnetic field
• What does the mass spectrum of an element show?

  The element's relative abundance (y axis) and the isotope (m/z ratio on the x axis)
• Define molecular formula

  Number of atoms of each element in a molecule
• Electrospray ionisation

  • Sample is dissolved in a volatile, polar solvent
  • Injected through a fine hypodermic needle giving a fine mist
  • A high voltage is applied to the tip of the needle
  • Each sample molecule, M, gains a proton to form MH+
• Atomic number= proton number = electron number
• Mass spectrometers have been included in planetary space probes so that elements on other planets can be identified. Elements on other planets can have a different composition of isotopes.
• The second ionisation energy of an element is always bigger than the first ionisation energy. When the first electron is removed a positive ion is formed. The ion increases the attraction on the remaining electrons and so the energy required to remove the next electron is larger.
• Define ionisation energy

  The energy required to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous ions