Atomic structure and the periodic table

• Atoms


  The smallest part of an element that can exist
• Chemical symbols

  Represent an atom of an element e.g. Na represents an atom of sodium
• Compounds

  Formed from elements by chemical reactions, contain two or more elements chemically combined in fixed proportions, can be represented by formulae
• Mixtures

  Consist of two or more elements or compounds not chemically combined together, can be separated by physical processes
• Development of the model of the atom

  1. First thought to be tiny spheres that could not be divided

  2. Discovery of electron led to plum pudding model

  3. Alpha particle scattering experiment led to conclusion mass concentrated in nucleus

  4. Bohr suggested electrons orbit nucleus at specific distances

  5. Later experiments showed positive charge of nucleus could be subdivided into protons

  6. Chadwick's work provided evidence for existence of neutrons
• Atomic number

  The number of protons in an atom of an element, all atoms of a particular element have the same number of protons
• Relative electrical charges of subatomic particles

  • Proton: +1
  • Neutron: 0
  • Electron: -1

  
• An atom has an overall charge of 0, so number of protons = number of electrons
• Size and mass of atoms

  • Atoms are very small (radius about 0.1 nm), nucleus is less than 1/10,000 of atom size but holds almost all the mass
  • Relative mass: Proton 1, Neutron 1, Electron very small
• Mass number

  The sum of the protons and neutrons in an atom
• Isotopes

  Atoms of the same element with different numbers of neutrons
• Relative atomic mass

  An average value that takes account of the abundance of the isotopes of the element
• Calculating relative atomic mass

  • Carbon has 2 isotopes: carbon-14 with abundance 20% and carbon-12 with abundance 80%. Relative atomic mass = ((14 x 20) + (12 x 80)) / 100 = 12.4
• Electronic structure

  Electrons occupy the lowest available energy levels (shells closest to nucleus), electronic structure tells you how many electrons are in each shell

  
• Example electronic structure for sodium: 2,8,1