Atomic structure

• Compounds are formed from elements by chemical reactions
• Chemical reactions always involve the formation of one or more new substances, and often involve an energy change
• Compounds can only be separated into elements by chemical reactions
• A mixture consists of two or more elements or compounds not chemically combined together
• Mixtures can be separated by:

  • filtration
  • crystallisation
  • simple distillation
  • fractional distillation
  • chromatography
• Discovery of electron-> plum pudding model (atom is a ball of positive charge with negative electrons embedded in it)
• Alpha particle scattering experiment-> conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged
• Neil Bohr: suggested electrons orbit the nucleus at specific distances
• James Chadwick’s work provided the evidence to show the existence of neutrons within the nucleus
• atomic number: the number of protons in an atom of an element.
• All atoms of a particular element have the same number of protons
• An atom has an overall charge of 0, so number of protons = number of electrons
• 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
• elements are arranged in order of atomic (proton) number
• elements in the the same group have the same number of electrons in their outer shell, which gives them similar chemical properties
• John Newlands ordered his periodic table in terms of atomic weight
• Dmitri Mendeleev ordered his table in atomic mass but not strictly
• Mendeleev left gaps in his table to allow for undiscovered elements
• Knowledge of isotopes made it possible to explain why ordering elements in order of atomic weight was not always correct
• Metals are elements that react to form positive ions
• non metals are elements that do not form positive ions