Atomic structure

Atomic structure

1. Fundamental particles

   • protons
     • relative mass: 1 relative charge: +1
   • neutrons
     • relative mass: 1 relative charge: 0
     • in nucleus
   • electrons
     • relative mass: 0 relative charge: -1
     • move around nucleus in orbitals
   • development over time
     • Geiger–Marsden/Rutherford
     • Niels Bohr
     • sub-shells

2. Mass number and isotopes

   • mass number (A): total number of protons and neutrons
   • atomic number (Z): number of protons
     • same for same element
   • isotope: same number of protons different number of neutrons
     • same configuration of electrons so same chemical properties
     • different physical properties
     • abundance
   • relative atomic mass (Ar)
   • relative molecular mass (Mr)
   • time of flight (TOF) mass spectrometer
     • 1. vacuum
       • stops collision with air molecules
     • 2. ionisation
       • electrospray
         • sample dissolved in volatile solvent
           • injected through hypodermic needle giving a fine mist
             • tip of needle has high voltage
               • gains a proton (H+) as it leaves the needle
       • electron impact
         • high energy electrons
           • from electron gun
             • fired at sample
               • knocks off one electron
         • fragmentation
       • possibility of 2+ ions
     • 3. acceleration
       • positive ions are strongly attracted to the negative plate
       • all have the same KE
         • KE = 1/2 mv^2
     • 4. ion drift
       • flight tube
       • lighter ions drift faster
         • t = d/v
     • 5. detection
       • lighter ions will arrive first
       • flight times are recorded
       • positive ions gain e- from detector, generates a current
         • current proportional to number of ions arriving
     • mass spectrum
       • plots abundance against m/z ratio
         • assume charge is +1 so m/z = mass/1
         • if charge is +2 m/z = mass/2
           • could cause small peak
       • Σ (% abundance x m/z) / 100 = Ar
         • if relative abundances used (instead of %), divide by sum of relative abundances
         • may be different from periodic table
           • different abundances of isotopes
         • diatomic elements
           • different combinations eg. Cl35 and Cl37
       • molecular sample
         • molecular ion (M+) peak
           • heaviest m/z value
         • organic sample
         • fragmentation

3. Electron configuration

   • electrons have fixed energies
     • move around nucleus in certain regions called shells
       • further away form nucleus, the higher its energy, larger the principle quantum number
       • divided into sub-shells that have slightly different energies
         • have different numbers of orbitals
           • holds 2 electrons
           • electrons fill singly before they start sharing
         • s = 1 orbital
         • p = 3
         • d = 5
         • f = 7
         • 4s sub-shell has lower energy than 3d so 4s fills up first
         • electronic configuration determines chemical properties
           • groups 1 and 2 (s block)
             • easily loses s block electrons
               • form + ions
           • groups 5, 6 and 7 (p block)
             • gain electrons
               • form - ions
           • want noble gas configuration
           • groups 4, 5, 6, 7 (p block)
             • share electrons when forming covalent bonds
           • d block
             • lose s and d electrons
               • form + ions
   • Cr and Cu
     • donate a 4s electron to the 3d sub-shell
       • full/half full d sub-shell is more stable than a partially filled d sub-shell
   • transition metals
     • when becoming ions, lose 4s electrons before 3d electrons
   • ionisation energies
     • first ionisation energy: the energy required to remove one mole of electrons from one mole of gaseous atoms to provide one mole of gaseous +1 ions
       • O(g) > O+(g) + e-
       • endothermic
       • lower ionisation energy = easier to ionise
     • factors affecting
       • nuclear charge
         • more protons means more + nucleus, stronger electrostatic attraction, harder to remove electrons
       • distance from nucleus
         • the closer an electron to the nucleus, the stronger the attraction
       • shielding
         • as shielding increases, the attraction between the outer electron and the nucleus weakens
       • paired or unpaired
         • paired electrons are removed more easily due to repulsion
     • trends
       • 1st IEs decrease down a group
         • group 2
         • shielding increases
         • distance from nucleus increases
       • generally, 1st IEs increase across a period
         • number of protons increases
           • nuclear charge increases
         • period 3
       • drop between groups 2 and 3
         • increased distance between sub-shells
           • evidence for sub-shells
       • drop between groups 5 and 6
         • paired electron in orbital easier to remove due to repulsion
       • generally, 1st IEs stays the same in a transition series
         • electron always removed from s block first
     • successive ionisation energies
       • X^n-1+(g) > X^n+ + e-
       • IE increases each time e- is removed, stronger attraction between e- and more + nucleus
       • big jumps happen when new shell is broken into, electron being removed from shell closer to nucleus
         • used to predict electronic structure

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