Chemistry

• Atom

  Mainly made up of protons and neutrons in the nucleus, with electrons orbiting in shells
• Protons


  • Positive charge
• Neutrons


  • Zero charge
• Electrons


  • Negative charge, very small relative mass
• Mass number


  Number of protons and neutrons in the nucleus
• Atomic number


  Number of protons in the nucleus
• 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
• Ions


  • Negative ion O2-
  • Positive ion Na+
• Isotopes


  Elements with the same number of protons but different numbers of neutrons
• Isotopes


  • Carbon-12
  • Carbon-13
  • Carbon-14
• History of atomic models

  1. Dalton's sphere model

  2. Thomson's plum pudding model

  3. Rutherford's nucleus model

  4. Bohr's shell model
• Time-of-flight mass spectrometer


  Vaporizes sample, ionizes it, accelerates ions, ions drift at constant speed, detected based on mass-to-charge ratio
• 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 spectrum


  Shows abundance of isotopes based on mass-to-charge ratio
• Isotope


  Atoms of the same element with the same number of protons but different numbers of neutrons
• Mass spectrometer analysis of isotopes

  • Provides information about the mass-to-charge ratio of the isotopes
  • Provides information about the abundance of the isotopes
• Calculating relative atomic mass from isotope data

  1. Multiply abundance of isotope A by mass-to-charge ratio of A

  2. Multiply abundance of isotope B by mass-to-charge ratio of B

  3. Add the results

  4. Divide by total abundance
• Relative atomic mass calculated from isotope data can be used to identify the element
• Molecular ion peak


  The peak on a mass spectrum that corresponds to the unfragmented molecular ion
• Electron configuration


  • Electrons are split into 4 subshells: s, p, d, f
  • Each subshell can hold a maximum number of electrons
• Writing electron configurations


  1. Start with the 1s orbital

  2. Fill orbitals in order of increasing energy

  3. Fill orbitals singly first before pairing up electrons
• Ion


  An atom that has lost or gained one or more electrons
• Determining electron configuration of an ion

  1. Remove electrons from the highest energy level first

  2. For transition metals, an electron may move from the 4s to the 3d orbital to create a more stable configuration
• 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 it adds up correctly
• Ionization energy


  The 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 is
• Atomic size


  • The bigger the atom, the further away the electrons are from the nucleus, the weaker the attractive force, and the less energy is required to remove the outer electron
• Nuclear charge


  • The more protons in the nucleus, the bigger the attraction between the nucleus and the outer electron, and the more energy is required to remove the electron
• Successive ionization energy


  The removal of more than one electron from the same atom
• Successive ionization energies


  Generally increase as more electrons are removed, due to the increasing positive charge of the ion
• Ionization energy trends down a group

  Atomic radius increases, shielding increases, so the attractive force between the nucleus and outer electron decreases, requiring less energy to remove the electron
• Ionization energy trends across a period

  Generally increases as nuclear charge increases, though there are some exceptions due to electronic configuration and electron repulsion
• Exceptions to the general trend of increasing ionization energy across a period include the decrease at aluminium, due to the outer electron being in a higher energy subshell, and the decrease at sulfur, due to electron repulsion in the 3p orbital
• First ionisation energy


  The energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
• Ionisation energies are measured for gaseous atoms