Atomic structure and isotopes

• Sub-atomic (fundamental) Particles


  • Proton
  • Neutron
  • Electron
• An atom of Lithium (Li) can be represented as: 7 3 Li, Atomic Number, Atomic Symbol, Mass Number
• Atomic Number (Z)


  The number of protons in the nucleus
• Mass Number (A)


  The total number of protons and neutrons in the atom
• Number of neutrons


  A - Z
• Isotopes


  Atoms of the same element with the same number of protons, but different numbers of neutrons
• Isotopes have similar chemical properties because they have the same electronic structure. They may have slightly varying physical properties because they have different masses.
• There are various models for atomic structure
• Relative atomic mass


  The weighted mean mass of one atom compared to one twelfth of the mass of one atom of carbon-12
• Relative Isotopic mass


  The mass of one isotope compared to one twelfth of the mass of one atom of carbon-12
• Calculating the Relative Atomic Mass of an Element


  1. R.A.M = (isotopic mass x % abundance) / 100

  2. R.A.M = (isotopic mass x relative abundance) / total relative abundance
• Relative molecular mass


  The average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12
• Isotope abundances


  • Cl35 (75%) and Cl37 (25%)
  • Br79 (50%) and Br81 (50%)
• The 160 peak has double the abundance of the other two peaks because there is double the probability of 160 Br79-Br81 + as can be Br79-Br81 and Br81-79
• Summary of most important ions to know


  • Group 1: Hydrogen, Silver, Gold, Ammonium (NH4+)
  • Group 2: Zinc, Copper (II), Iron (II), Tin, Lead
  • Group 3: Iron (III)
  • Group 5: Phosphate (PO4 3-)
  • Group 6: Carbonate (CO3 2-), Sulfate (SO4 2-)
  • Group 7: Nitrate (NO3-), Hydroxide (OH-)
• How to work out the formula from the ionic charge


  1. Identify the ionic charges of the two ions

  2. Combine the ions together to get a neutral compound. i.e. combine so that the total +ve charge cancels out the total –ve charge
• Formulas


  • Lithium sulfate: Li2SO4
  • Calcium phosphate: Ca3(PO4)2
• Writing ionic equations


  1. Take full equation

  2. Separate (aq) solutions into ions

  3. Cancel out spectator ions leaving ionic equation
• Atom


  Made of protons and neutrons in the nucleus, with negative electrons orbiting in shells
• Proton


  • Relative charge of +1, relative mass of 1
• Neutron


  • Relative charge of 0, relative mass of 1
• Electron


  • Relative charge of -1, relative mass of 1/2000
• The number of protons equals the number of electrons in atoms
• To find the number of neutrons, subtract the atomic number from the mass number
• Ion


  Atom that has lost or gained electrons, so has a different number of electrons and protons
• Ions


  • Oxygen ion with 8 protons, 8 neutrons, and 10 electrons (charge of -2)
  • Sodium ion with 11 protons, 12 neutrons, and 10 electrons (charge of +1)
• Isotopes


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


  • Carbon-12 (6 protons, 6 neutrons)
  • Carbon-13 (6 protons, 7 neutrons)
  • Carbon-14 (6 protons, 8 neutrons)
• Relative atomic mass


  Weighted mean mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom
• Relative isotopic mass


  Mass of an atom of an isotope compared to 1/12 the mass of a carbon-12 atom
• Relative molecular mass


  Mean mass of a molecule compared to 1/12 the mass of a carbon-12 atom
• Mass spectrometry


  Technique to measure the mass-to-charge ratio and relative abundance of isotopes in an element
• Calculating relative atomic mass from mass spectrometry data


  Multiply abundance of each isotope by its mass-to-charge ratio, then divide by total abundance
• The relative atomic mass calculated from mass spectrometry data can be used to identify the element on the periodic table