Atomic Structure and 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
• Element


  Shown in the periodic table
• Compound


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


  Involves the formation of one or more new substances, often involves a detectable energy change
• Mixture


  Two or more elements or compounds not chemically combined, chemical properties of each substance unchanged, can be separated by physical processes
• Physical process


  Filtration, crystallisation, simple distillation, fractional distillation, chromatography, does not involve chemical reactions and no new substances are made
• 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 atom concentrated at the centre (nucleus) and nucleus is charged
• Bohr model of the atom

  • Electrons orbit the nucleus at specific distances

  
• Proton

  Particle with +1 electrical charge, found in the nucleus
• Neutron

  Particle with 0 electrical charge, found in the nucleus
• Electron


  Particle with -1 electrical charge, orbits the nucleus
• Atomic number

  Number of protons in an atom of an element
• Mass number


  Sum of the protons and neutrons in an atom
• Isotopes


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


  Average value that takes account of the abundance of the isotopes of an element
• Electronic structure


  Representation of the arrangement of electrons in an atom
• Periodic table


  Elements arranged in order of atomic number, with similar properties in columns (groups)
• Early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed
• Metals


  Elements that react to form positive ions
• Non-metals


  Elements that do not form positive ions
• Noble gases


  Group 0 elements, unreactive and do not easily form molecules
• Alkali metals


  Group 1 elements, have characteristic properties due to single electron in outer shell
• Reaction of alkali metals with oxygen

  1. Lithium forms lithium oxide (Li₂O)

  2. Sodium forms sodium peroxide (Na₂O₂)

  3. Potassium forms superoxides (KO₂)
• Reaction of alkali metals with chlorine


  Form metal chlorides (MCl), highly exothermic with bright, vigorous flames
• Reaction of alkali metals with water

  Produce hydrogen gas and the corresponding metal hydroxide
• In Group 1


  Reactivity of the elements increases going down the group
• Halogens


  Group 7 elements, have similar reactions due to seven electrons in outer shell
• In Group 7


  Further down the group, the higher the relative molecular mass, melting point and boiling point, but the lower the reactivity
• Reaction of chlorine with metals

  Forms ionic compounds called metal chlorides, highly exothermic
• Reaction of bromine with metals

  Forms metal bromides, less reactive than chlorine
• Reaction of iodine with metals


  Forms metal iodides, least reactive among halogens
• Reaction of chlorine with non-metals

  Forms covalent compounds, typically exist as diatomic molecules
• Reaction of bromine with non-metals

  Forms covalent compounds, e.g. hydrogen bromide (HBr), bromine dioxide (BrO₂)
• Reaction of iodine with non-metals

  Forms covalent compounds, e.g. hydrogen iodide (HI), iodine pentoxide (I₂O₅)
• Transition metals


  • Have higher melting points, greater densities, strength and hardness, less reactive with oxygen, water and halogens than group one metals, many have ions with different charges, form coloured compounds, useful as catalysts