chemistry - atomic structure / periodic table

• Atom


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


  Represents an atom of an element, e.g. O for oxygen, Na for sodium
• There are about 100 different elements
• Elements


  Shown in the periodic table
• Compounds


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


  Always involve the formation of one or more new substances, often involve a detectable energy change
• Representing chemical reactions


  1. Word equations

  2. Equations using symbols and formulae
• Students will be supplied with a periodic table for the exam and should be able to: use the names and symbols of the first 20 elements in the periodic table, the elements in Groups 1 and 7, and other elements in this specification; name compounds of these elements from given formulae or symbol equations; write word equations for the reactions in this specification; write formulae and balanced chemical equations for the reactions in this specification
• Mixture


  Two or more elements or compounds not chemically combined together, chemical properties of each substance unchanged
• Separating mixtures


  • By physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography
  • Physical processes do not involve chemical reactions and no new substances are made
• New experimental evidence may lead to a scientific model being changed or replaced
• Plum pudding model of the atom

  Atom is a ball of positive charge with negative electrons embedded in it
• Nuclear model of the atom

  Mass of an atom concentrated at the centre (nucleus) and the nucleus is charged
• Bohr model of the atom

  Electrons orbit the nucleus at specific distances
• Proton


  Particle with the same amount of positive charge, making up the nucleus
• Neutron


  Particle within the nucleus
• Atomic number


  Number of protons in an atom of an element
• All atoms of a particular element have the same number of protons
• Atoms of different elements have different numbers of protons
• Relative charge


  Proton = +1, Neutron = 0, Electron = -1
• Atom size


  Radius of about 0.1 nm, nucleus radius less than 1/10,000 of atom
• Almost all of the mass of an atom is in the nucleus
• Mass number


  Sum of the protons and neutrons in an atom
• Isotopes


  Atoms of the same element with different numbers of neutrons
• Representing atoms


  1. Atomic number

  2. Mass number
• Calculating numbers of protons, neutrons and electrons


  Given atomic number and mass number
• Relative atomic mass


  Average value taking account of the abundance of the isotopes
• Calculating relative atomic mass

  Given percentage abundance of isotopes
• Electronic structure


  Electrons occupy the lowest available energy levels (innermost available shells)
• Representing electronic structure


  Numbers or diagrams, e.g. 2,8,1 for sodium
• Periodic table


  Elements arranged in order of atomic (proton) number, with similar properties in columns (groups)
• Elements in the same group have the same number of electrons in their outer shell (outer electrons), giving them similar chemical properties
• Early periodic tables


  Incomplete, some elements placed in inappropriate groups if strict order of atomic weights was followed
• Mendeleev's periodic table


  Left gaps for undiscovered elements, changed order based on atomic weights
• Knowledge of isotopes explained why order based on atomic weights was not always correct
• Metals


  Elements that react to form positive ions
• Non-metals


  Elements that do not form positive ions
• Majority of elements are metals, found to the left and towards the bottom of the periodic table; non-metals are found towards the right and top of the periodic table
• Noble gases


  Group 0 elements, unreactive, have stable arrangements of 8 electrons in outer shell (except helium with 2)
• Going down Group 0

  Boiling points of noble gases increase with increasing relative atomic mass