Chapter 3 - Atoms, Element, Compound

• Pure substance

  Cannot be broken down into 2 or more simpler substances
• Element

  Pure substance that cannot be broken down into 2 or more simpler substances
• There are many elements in the periodic table
• Sugar

  • Can be broken down into carbon, hydrogen, oxygen
• Compound


  Pure substance made of 2 or more elements chemically combined in a fixed ratio
• Compound

  • After combination, properties are changed
  • Made of 2 or more elements in a fixed ratio
• Atom


  Smallest particle of an element
• Molecule


  Group of 2 or more atoms chemically combined
• Ion


  Smallest particle of an element or group of atoms with an electric charge
• Monatomic element


  Exists as individual atoms
• Diatomic molecules


  • Combination of 2 atoms, e.g. Iodine (I2), Chlorine (Cl2), Oxygen (O2)
• Molecules can contain a group of 2 or more atoms that are chemically combined
• Non-metal


  Particles that gain/lose electrons to form ions
• Metal and non-metal


  Particles that form ions
• Atom


  Made up of protons, neutrons and electrons
• Atom


  • Protons and neutrons form the nucleus
  • Electrons move rapidly around the nucleus
• Atomic mass/nucleon number


  Sum of protons and neutrons
• Proton number/atomic number


  Number of protons
• Electron arrangement


  Can affect chemical properties
• Electron shells


  • First shell closest to nucleus, max 2 electrons
  • Second and third shells up to 8 electrons, filled in order
• Electronic configuration


  Represents the electron arrangement in an atom
• Magnesium atom


  • Electronic configuration: 2,8,2
• Isotopes


  Same element, same proton number, different number of neutrons
• Isotopes


  • Same chemical properties, different physical properties
  • If neutrons > protons, tend to be radioactive
• Relative atomic mass


  Average mass of the isotopes
• Chlorine


  • Contains 75% Cl-35 and 25% Cl-37, relative atomic mass is 35.5
• Carbon-12


  Chosen as the standard atom, all other masses compared to 1/2 mass of 1 atom
• Period


  Number of electron shells
• Group


  Same number of valence electrons
• Group 8


  Full valence electron shell
• Valency


  • Number of electrons needed to gain, lose or share to have a stable configuration
  • Group 1: +1
  • Group 2: +2
  • Group 3-7: +/-3,4,5,6,7
  • Group 8: 0