atoms, elements and molecules

• Atom


  The smallest particle of an element that can exist on its own and still retain the properties of the element
• Element


  A pure substance that is made up of one type of atom only
• Elements


  • gold
  • copper
  • magnesium
  • lithium
• Molecule


  Formed when two or more atoms are chemically bonded together
• The Concept of Atoms

  • O2 (oxygen) >> O-O or 2 atoms of O
  • N2 (nitrogen) >> N-N or 2 atoms N
• Molecule (compound)


  A molecule made up of atoms of different elements
• If a molecule is made up of two or more different atoms that are chemically bonded together, it is a COMPOUND
• Subatomic particles


  • Protons
  • Neutrons
  • Electrons
• Nucleus


  • Dense central structure with overall positive charge
• Protons


  • Positively charged
• Neutrons


  • No charge
• Electrons


  • Negatively charged
  • Very light mass
• Electron shells/energy levels


  Where electrons are found in an atom
• Electron shells


  • First shell max 2 electrons
  • Second shell max 8 electrons
  • Third shell max 8 electrons
• Atoms are electrically neutral because the number of protons equals the number of electrons
• Atomic number


  The number of protons in an atom
• Periodic table


  • Classifies elements by atomic number and properties
• Groups


  Vertical columns in the periodic table showing number of electrons in the last shell
• Groups


  • Group 1 (alkali metals)
  • Group 2 (alkaline earth metals)
  • Group 7 (halogens)
  • Group 8 (inert/noble gases)
• Horizontal rows in the periodic table showing number of occupied electron shells are PERIODS
• The present day periodic table arranges elements by atomic number
• Dmitri Mendeleev started developing the periodic table


  1869
• Mendeleev arranged elements by atomic mass
• Robert Moseley arranged the elements by atomic number, ensuring no two elements have the same atomic number
• When is a molecule an element?

  If it is made up of two or more atoms that are chemically bonded together