Key concepts in chemistry

• Atoms


  • Cannot be created, divided or destroyed
  • Atoms of the same element are exactly the same and atoms of different elements are different
  • Atoms join with other atoms to make new substances
• The discovery of subatomic particles caused the original Dalton model of an atom to change
• How JJ Thomson discovered the electron

  1. Thomson experimented with a cathode ray tube

  2. The beam moved towards the positively charged plate so he knew that the particles must have a negative charge
• JJ Thomson's atomic model

  • Plum pudding model
  • Negatively charged electrons scattered through a positively charged material
• What Ernest Rutherford discovered from his gold foil experiment

  1. He shot a beam of positively charged particles at sheet of gold foil

  2. Most of the particles passed straight through suggesting that atoms were mostly empty space

  3. A few particles were deflected and a few bounced directly back showing that there must be a tiny, dense and positively-charged nucleus
• Rutherford's new model of the atom

  • Mass is concentrated in the central nucleus
  • Mostly empty space
  • Electrons travel in random paths around the nucleus
• Structure of an atom

  • Small central nucleus made up of protons and neutrons
  • Electrons orbit (move around) the nucleus in shells
• Radius of the nucleus

  • 1x10-14 m
  • This is 1/10000 of the atomic radius
• Relative masses of protons, neutrons and electrons

  • Proton: 1
  • Neutron: 1
  • Electron: 1/1836
• Relative charges of protons, neutrons and electrons

  • Proton: +1
  • Neutron: 0
  • Electron: -1
• Why atoms contain equal numbers of protons and electrons

  • Atoms are stable with no overall charge
  • Protons are positively charged and electrons are negatively charged. For the charges to balance, the number of protons and electrons must be equal
• The mass of an atom is concentrated in the nucleus
• Atomic number


  The number of protons
• Mass number


  The number of protons and neutrons found in the nucleus of an atom
• Atoms of the same element have the same number of protons in the nucleus and this is unique to that element
• Isotope


  • Atoms with the same number of protons (so they are the same element) but a different number of neutrons
  • Isotopes of an element have the same atomic number but different mass numbers
• Why relative atomic mass is not always a whole number

  Different isotopes of the same element have different mass numbers. The relative atomic mass is an average of the masses of all these isotopes
• Values required to calculate relative atomic mass of chlorine

  Mass numbers and relative abundances of all the isotopes of chlorine
• How Mendeleev arranged the elements in his periodic table

  1. Elements arranged with increasing atomic masses

  2. Elements with similar properties put into groups (due to periodic trends in chemical properties)

  3. Switched the position of some elements

  4. Gaps left for undiscovered elements
• How Mendeleev was able to predict the properties of new elements


  Mendeleev left gaps in his periodic table. He used the properties of elements next to these gaps to predict the properties of undiscovered elements
• Mendeleev's table lacked some amount of accuracy in the way he'd ordered his elements because isotopes were poorly understood at the time and protons and neutrons had not yet been discovered
• How elements are arranged in the modern periodic table

  In order of increasing atomic number
• Elements in the same group (column)

  Have similar chemical properties
• Why elements in the same column have similar chemical properties

  • Same number of outer shell electrons
  • Number of outer shell electrons determines how an atom reacts
• Elements arranged in the modern periodic table

  In order of increasing atomic number
• Period (row) number


  Tells you the number of electron shells for all elements in that period (e.g. all elements in period 4 have 4 shells of electrons)
• Group (column) number


  Tells you the number of outer electrons for all elements in that group (e.g. all elements in group 2 have 2 electrons in their outer shell)
• Metals are positioned on the left hand side of the periodic table
• What determines whether an element is a metal or non-metal

  Atomic structures of the elements
• Maximum number of electrons allowed in each of the first 3 shells

  • 1st shell: 2
  • 2nd shell: 8
  • 3rd shell: 8
• When atoms are most stable


  When they have full electron shells
• The electron configuration of sodium (Na) with atomic number 11 is 2, 8, 1
• Electron configuration of an element

  Related to its position in the periodic table
• Ionic bond


  A bond between a metal and non-metal involving the transfer of electrons
• Ion


  An atom or group of atoms with a positive or negative charge
• The ion Na+ has 11 protons, 10 electrons, and 12 neutrons
• The ion O2- has 8 protons, 10 electrons, and 8 neutrons
• Type of ions formed by elements in different groups

  • Group 1 and 2 metals form cations (positive)
  • Group 6 and 7 non-metals form anions (negative)
• Ionic compound ending in -ide

  Contains 2 elements
• Ionic compound ending in -ate


  Contains at least three elements, one of which is oxygen