Atomic Structure

• History of the atom

  • 1803 'Billard ball' model - atom made of indestructible matter
  • 1897 'Plum Pudding' model - ball of +ve charge with electrons dotted throughout it
  • 1909-1911 - Rutherford 'Nuclear' model - discovery of the nucleus

  - Alpha Scattering Experiment -> alpha particles fired at a thin sheet of gold foil in a vacuum -> some particles deflected at large angles

  - proved there was a dense & strongly charged nucleus

  • 1913 Neils Bohr 'Orbital' model - electrons kept on energy shells
  • 1920s - Modern atomic model - James Chadwick discovered neutrons
• Atomic structure basics

  Mass Number = total no. of protons & neutrons in nucleus of atom

  Atomic number = no. of protons in an atom

  Nucleons = protons, neutrons & anything in the nucleus of an atom

  Ion = charged particle due to the gain/loss of electrons

  Isoelectronic = such all with the same electron configuration as each other

  Isotope = atoms of the same element but with different no. of neutrons and the same no. of protons

  • same no. of electrons in isotopes -> same chemical properties
  • different no. of neutrons -> different physical properties
• Mass Spectrometry

  mass spectrometry = used to determine relative molecular mass/ to identify elements

  • mass spectrometer gives accurate information about relative isotopic mass & relative abundance of isotopes
  • time of flight spectrometer used to find masses
• TOF Stage 1: ionisation


  Method 1: Electron impact ionisation

  1. Sample is vapourised into gas
  2. High speed electrons fired at it
  3. Electron knocked off X(g) -> X+(g) + e-
  4. Positive ions attracted to negative electric plate & accelerate to it

  • Fragmentation may occur
• TOF Stage 1: Ionisation pt 2

  Method 2: Electrospray

  1. Sample dissolved in volatile solvent
  2. Injected through hypodermic needle to give a mist
  3. Tip of needle attached to +ve terminal of high voltage supply
  4. Particles gain a proton from solvent producing XH+ -> X + H+ -> XH+
  5. Solvent evaporates while XH+ is attracted to negative metal plate where they accelerated

  • Fragmentation rarely occurs
• TOF stage 2: Acceleration

  Positive ions are attracted towards a negatively charged plate

  • they accelerate towards it
  • all ions gain the same amount of kinetic energy KE = 1/2(mv^2)
  • lighter ions & more charged ions achieve a higher speed
• TOF stage 3: Drift Stage

  • Ions drift in flight tube where there's no electric field
  • Ions pass through a hole in the negatively charged plate forming a beam & travel through the tube to detector at same speed as in the electric field
• TOF stage 4: detection

  • positive ions gain electrons when they hit the negatively charged electric plate -> creates an electric current
  • size of current gives measure of the no. of ions hitting the plate (ion abundance)
  • time taken to travel tube -> used for mass/charge ratio
• TOF stage 5: results

  for electron impact ionisation

  • penultimate peak gives Mr of compound

  - last peak is Mr+1 as other isotopes may be present

  for electrospray

  • penultimate peak for mass of protonated ion(XH+)

  - must subtract 1 to find Mr

  RAM = ((mass x abundance) + ...) / total abundance
• Energy level basics

  • closest to nucleus -> lowest energy -> fills with electrons first
  • split into 'sub-shells' containing orbitals
  • orbital = 3D area of space that can hold a maximum of 2 electrons
• Energy Levels

  1st energy level = max. 2 electrons

  • contains 1 s orbital

  2nd energy level = max. 8 electrons

  • contains an s orbital
  • contains 3 p orbitals

  3rd energy level = max. 18 electrons

  • contains an s orbital & 3 p orbitals
  • contains 5 d orbitals

  4th energy level = max. 32 electrons

  • contains an s orbital, 3 p orbitals & 5 d orbitals
  • contains 7 f orbitals
• Periodic table

  energy gaps become smaller between higher energy levels

  A) s block

  B) d block

  C) p block

  D) f block

  
• 
  

  Electron configuration

  • electrons have 'spin' property to differentiate electrons on the same orbital (either spin up or spin down)
  • 4s fills before 3d
  • exceptions:

  $Cr =$$1s^22s^22p^63s^23p^64s^13d^5$

  $Cu =$$1s^22s^22p^63s^23p^64s^13d^X$
• Ionisation energy definitions

  first ionisation energy = energy required to form 1 mole of gaseous 1+ ions from 1 mole of gaseous atoms by removing an electron

  second ionisation energy = energy required to form 1 mole of gaseous 2+ ions from 1 mole of gaseous 1+ ions by removing an electron

  as electrons get removed -> energy needed to remove them increases

  • ion gets smaller -> more +ve charge -> less electron shielding -> more electrostatic attraction
• Trends in ionisation energy

  Going down group

  • easier to lose an electron -> ionisation energy decreases
  • reasons:

  - larger atomic radius -> weaker attraction

  - more electron shielding -> weaker attraction

  - more protons -> greater nuclear charge

  Across a period

  • harder to lose an electron -> ionisation (generally) increases
  • reasons:

  - higher nuclear charge

  - smaller atomic radius

  - similar amount of electron shielding
• Exceptions in trends to ionisation energy

  Between Group 2-3

  • electron is in a higher energy orbital -> less energy needed

  Between Group 5-6

  • pair of electrons in p orbital repel each other -> less energy required