atomic structure

• relative atomic mass

  average mass of an atom relative to 1/12th the mass of an atom of carbon-12
• atomic number

  number of protons in the nucleus
• relative molecular mass

  average mass of a molecule relative to 1/12th the mass of an atom of carbon-12
• mass number

  number of protons and neutrons in nucleus
• isotopes have different number of neutrons but same number of protons
• history of the atom

  Dalton: elements are made of small spheres known as atoms

  Thomson: Plum pudding model, atoms are positive spheres with random negative charges that are electrons

  Rutherford: most of atom's mass is concentrated in positive nucleus

  Bohr: electrons arranged in fixed energy shells

  current model says shells do not have same energy
• Rutherford discovered an atoms mass is concentrated in positive nucleus through gold leaf experiment

  positive alpha particles were fired and most went through suggesting a lot of the atom is empty space, a few deflected as they hit the positive nucleus
• Bohr proved electrons are in fixed energy shells as when EM radiation is absorbed electrons move between shells

  they emit this radiation when moving down to lower energy shells
• we know shells do not have same energy as the discovery of subshells proves difference in ionisation energy
• mass and charge of subatomic particle

  proton has +1 charge and 1 mass

  neutron has 0 charge and 1 mass

  electron has -1 charge and negligible mass
• mass spectrometry can determine relative isotopic mass, relative abundance of isotopes, identify elements and determine relative molecular mass.
• there are 4 steps in mass spectrometry

  1. ionisation (electron impact or electron spray)
  2. acceleration
  3. flight tube
  4. detection
• electron impact ionisation

  vaporised sample is injected at low pressure

  electron gun fires high energy electron at the sample

  knocks out an outer electron to make a positive ion

  Ti (g) --> Ti+ (g) + e-
• electrospray ionisation

  sample dissolved in polar, volatile solvent

  injected through a fine needle to give a fine mist or aerosol

  tip of needle has high voltage and here the sample gains a proton

  M (g) + H+ --> MH+ (g)

  solvent evaporates while MH+ ions move to negative plate
• electron impact is used for elements and substances with low formula mass

  cannot be used for larger organic molecules as they fragment
• electrospray ionisation is used for larger organic molecules as the softer conditions prevent fragmentation
• acceleration is where positive ions are accelerated by an electric field to a constant kinetic energy
• flight tube is where positive ions with smaller m/z have same kinetic energy as those with larger m/z and will move faster

  heavier particles take longer to move through drift area

  ions are distinguished by different flight times
• detection is where ions reach detector and generate a small current which is fed to the computer for analysis

  current is produced by electrons transferring from the detector to the positive ions

  size of current is proportional to abundance of species
• for each isotope the mass spectrometer can measure a m/z ratio and an abundance
• relative atomic mass = sum of (isotopic mass x abundance)/ 100
• electrons have shells

  each shell has sub energy level labelled s,p, d and f

  s holds up to 2 electrons, p holds upto 6 electrons, d holds upto 10 electrons, f holds up to 14 electrons
• orbitals are regions of an atom that can hold upto two electrons with opposite spins
• each orbital has its own approximate 3D shape

  s sublevels are spherical

  p sublevels are shaped like dumbbells
• periodic table is spot into blocks

  whatever letter shell is outermost electron is in shows the block of an element
• first ionisation energy is the enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge

  H (g) --> H+ (g) + e-
• second ionisation energy is the enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions with a double positive charge

  Ti+ (g) --> Ti2+ (g) + e-
• factors that affect ionisation energy

  attraction of nucleus: more protons means greater attraction

  distance of electrons from the nucleus: bigger atoms have weaker attraction between outermost electron and nucleus

  shielding of attraction of nucleus: electron in outer shell is repelled by electron in complete inner shells, weaking nuclear attraction
• second ionisation energies are always larger than the first

  when the first electron is removed a positive ion forms which increases the attraction for remaining electrons

  this means more energy is required to remove the second
• helium has the largest first IE as its first electron is in the first shell closest to the nucleus and has no shielding effects from inner shells

  IE is higher than hydrogen as it has one more proton
• first IE decreases down a group as outer electrons are found in shells further from the nucleus and are more shielded so the attraction of the nucleus becomes smaller
• ionisation energy increases across a period as electrons are being added to the same shell which has the same distance from the nucleus and the same shielding effect

  however number of protons increase making the effective attraction of the nucleus greater
• Na has a lower first ionisation energy than neon because Na has its outer electron in the 3s shell further from the nucleus and is more shielded, this makes its outer electron easier to remove
• there is a small drop in first IE from Mg to Al because Al starts to fill the 3p subshell whereas Mg has its outer electron in the 3s subshell

  electrons in 3p are slightly easier to remove because they are higher in energy and are slight shielded by the 3s electrons
• there is a small drop from p to s because in sulfur there are 4 electrons in the 3p subshell and there is a slight electrostatic repulsion between two negative charged electrons in the orbital which makes the second electron easier to remove