Atomic Structure

Cards (16)

  • 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=Cr =1s22s22p63s23p64s13d5 1s^22s^22p^63s^23p^64s^13d^5
    Cu=Cu =1s22s22p63s23p64s13dX 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