3.1.1 atomic structure

• proton

  • relative mass- 1
  • relative charge- +1
  • location- nucleus
  • affected by electric field?- attracted
• neutron

  • relative mass- 1
  • relative charge- +1
  • location- nucleus
  • affected by electric field?- no
• electron

  • relative mass- 1/1840
  • relative charge- -1
  • location- electron shell
  • affected by electric field?— repel
• mass number: the total number of protons and neutrons in an atom of an element
• atomic number: number of protons in an atom's nucleus
• number of neutrons= mass number - atomic number
• positive ions have lost electrons and are called cations
• negative ions have gained electrons and called anions
• isotopes ( in terms of fundamental particles): atom with the same number of protons and different number of neutrons
• isotopes have same chemical properties because they have the same number and arrangement of electrons
• isotopes have different physical properties e.g. rate of diffusion, boiling point (dependent on mass) radioactivity, ability to absorb neutrons (dependent on nucleus)
• relative atomic mass: the average mass of an atom of an element compared to 1/12th the mass of an atom of C12
• relative molecular mass is the average mass of a molecule compared to 1/12th the mass of an atom of C12
• ions and isotope similarities

  • same number of protons
  • same group / period
• ions and isotopes differences

  • ions have a different number of electrons
  • isotopes have different masses
  • isotopes have different number of neutrons
  • isotopes may be radioactive
  • isotopes have same chemical properties, same number and electron configuration
  • isotopes don’t have a charge
  • ions have different electron configurations
• 4 steps of TOF mass spectrometry

  1. ionisation
  2. acceleration
  3. ion drift
  4. detection
• two types of ionisation

  • electron impact ionisation
  • electrospray ionisation
• electron impact ionisation

  • high energy electrons fired at sample and knocks off an electron
  • X (g) ——> X+ (g) + e-
  • ❌ can cause loss of 2 electrons to form 2+ ions, can cause fragmentation of a molecule
  • used to ionise elements and small molecules, no change to Ar/Mr
• electrospray ionisation

  • sample passed through fine needle with solvent containing protons and high voltage applied
  • X + H+ ——> XH+
  • ✅ no fragmentation
  • used to ionise large molecule, Mr increases by +1
• acceleration

  • positive ions are attracted to an electric field and gain energy
  • all 1+ ions gain the same amount of energy
• ion drift

  • positive ions move through vaccum to prevent collisions
  • collisions cause ions to be knocked off course and lose energy
• detection

  • positive ions hit detector and gain electrons
  • X+ (g) + e- ——> X (g)
  • the gaining of an electron produces a current
  • current is proportional to abundance
• the lower the m/z value the higher the velocity and shorter time of flight
• why do the atoms have to be ionised in a mass spec?

  only positive ions can accelerate and be detected
• why are only positive ions detected/ reach the detector?

  only positive ions can regain an electron and be detected
• furthest right peak is called the molecular ion peak in mass spec
• electrospray and electron impact ionisation similarities

  • form positive ions
• electrospray and electron impact ionisation differences

  • electron impact- high energy e- , electron gun, loss of e-
  • electron impact- Mr stays same, may form 2+ ions
  • electron impact- causes fragmentation, elements and small molecules
  • electron impact- X (g) ——> X+ (g) + e-
  • electrospray- gains protons, solvent, fine needle, high voltage
  • electrospray- large molecule, no fragmentation
  • electrospray- Mr= m/z +1
• why might calculated relative atomic mass may be different from on the periodic table?

  not all isotopes have been accounted for

  different abundances of isotopes
• molecules that contain atoms of 35 or 37 Cl or 79 or 81 Br will produce multiple molecular ion peaks, one for each isotope
• orbital: region of an atom that can hold up to 2 electrons with opposite spins
• what are the four different types of orbitals


  s

  p

  d

  f
• what is the max number of electrons allowed in an orbital?

  2
• the 2 electrons in an orbital must spin opposite ways
• why is an element in x block

  because the outermost electrons are in the x orbital
• sub-shell: s

  no. of orbitals: 1

  max no. of electron: 2
• sub-shell: p

  no. of orbitals: 3

  max no. of electrons: 6
• sub-shell: d

  no. of orbitals: 5

  max no. of electrons: 10
• sub-shell: f

  no. of orbitals: 7

  max no. of electrons: 14
• the electron configuration of Cr and Cu only has the 4s orbital half full