Atomic Structure

• Atom


  Mainly made up of protons and neutrons, contained in the middle, with electrons orbiting in shells
• Atom


  • Protons have a positive charge
  • Neutrons have a zero charge
  • Electrons have a negative charge
• Relative mass


  Protons and neutrons have a relative mass of 1, electrons have a relative mass of 1/2000
• Element


  Top number is the mass number (protons + neutrons), bottom number is the atomic number (protons)
• Atoms are neutral because the number of protons equals the number of electrons
• Ion


  Has a different number of electrons and protons, forming a stable charge
• Negative ion


  • O2- has gained 2 electrons for a -2 charge
• Positive ion


  • Na+ has lost 1 electron for a +1 charge
• Isotopes


  Elements with the same number of protons but different numbers of neutrons
• Isotopes react chemically the same but have slightly different masses
• John Dalton proposed atoms were solid spheres


  1803
• JJ Thompson proposed the plum pudding model with electrons in a positive pudding


  Later
• Rutherford discovered the small, positive nucleus and surrounding electron cloud


  Later
• Niels Bohr proposed fixed energy levels for electrons

  1913
• Time-of-flight mass spectrometry


  1. Vaporize sample

  2. Ionize sample

  3. Accelerate ions

  4. Ions drift at constant speed

  5. Detect ions
• Relative atomic mass


  Average mass of an atom of an element, relative to carbon-12
• Relative molecular mass


  Average mass of a molecule, relative to carbon-12
• Relative isotopic mass


  Mass of an isotope, relative to carbon-12
• Mass spectra show isotopes of an element, with the mass/charge ratio on the x-axis and the abundance on the y-axis
• Isotopic mass


  The mass of an isotope which makes it relatively straightforward if it had two electrons knocked off which would be quite rare
• If an isotope has two electrons knocked off

  The mass to charge ratio will be half as much
• Abundance


  Always shown on the left, can be written as a percentage or a nominal value
• If the abundance is a percentage, the total abundance of all isotopes must be 100%
• Mass spectrometer spectrum


  • Shows two isotopes of one element
  • One isotope has a mass of 35
  • One isotope has a mass of 37
• Most abundant isotope


  The most common isotope
• Calculating relative atomic mass

  1. Abundance of isotope A * Mass to charge ratio of A

  2. + Abundance of isotope B * Mass to charge ratio of B

  3. Divide by total abundance
• The calculated relative atomic mass can be used to identify the element
• Molecular fragments


  When molecules are sent through a mass spectrometer, they break into smaller bits called fragments
• Molecular ion peak


  The last significant peak on a mass spectrum, representing the unfragmented molecular ion
• Electron subshells


  • S
  • P
  • D
  • F
• Electron subshells


  • S has 1 orbital, can hold 2 electrons
  • P has 3 orbitals, can hold 6 electrons
  • D has 5 orbitals, can hold 10 electrons
  • F has 7 orbitals, can hold 14 electrons
• Electron configuration


  Represents the arrangement of electrons in an atom's subshells, written as 1s2 2s2 2p6 etc.
• Filling electron configurations


  1. Fill from lowest energy level upwards

  2. Fill orbitals singly first before pairing up
• Transition metal electron configurations

  Electrons can move from higher energy 4s orbitals to partially fill 3d orbitals to create more stable configurations
• Determining ion electron configurations

  1. Remove electrons from highest energy levels first

  2. For transition metals, remove 4s electrons before 3d
• Ionization energy


  The minimum energy required to remove one mole of electrons from one mole of atoms in the gaseous state
• Removing electrons from an ion

  1. Remove electrons from 4s first

  2. Then remove from 3D

  3. Check the numbers to ensure total matches
• Ionization energy


  Minimum amount of energy required to remove one mole of electrons from one mole of atoms in the gaseous state
• Ionization energy is always endothermic and has a positive value
• Shielding


  • The more shells or electron shells between the positive nucleus and the outer electron, the less energy is required and the weaker the attraction