Atomic structure and bonding

• Chromatography


  To separate out mixtures
• Evaporation


  To separate a soluble salt from a solution; a quick way of separating out the salt
• Atoms are contained in the nucleus are the protons and neutrons. Moving around the nucleus are the electron shells. They are negatively charged.

  
• Overall, atoms have no charge; they have the same number of protons as electrons. An ion is a charged particle - it does not have an equal number of protons to electrons.
• Atomic Number


  The number of protons in the nucleus of an atom
• Mass Number


  The total number of protons and neutrons in the nucleus of an atom
• To calculate the relative atomic mass


  Relative atomic mass (Ar) = sum of (isotope abundance × isotope mass number) / sum of abundances of all isotopes
• Balancing Symbol Equations


  There must be the same number of atoms on both sides of the equation
• Mixture


  In a mixture there are no chemical bonds, so the elements are easy to separate
• Examples of mixtures


  • Air
  • Salt water
• Separating out salt from rock salt


  1. Grind the mixture of rock salt

  2. Add water and stir

  3. Filter the mixture, leaving the sand in the filter paper

  4. Evaporate the water from the salt, leaving the crystals
• Element


  Made of atoms with the same atomic number
• Isotope


  An element with the same number of protons but a different number of neutrons. They have the same atomic number, but different mass number.
• Compound


  When two or more elements are chemically joined
• Examples of compounds


  • Carbon dioxide
  • Magnesium oxide
• Examples of chemical formulas

  • CO2
  • NaCl
  • HCl
  • H2O
  • Na2SO4
• Word equation for a chemical reaction

  Reactants on the left-hand side, products on the right-hand side
• Symbol equation for a chemical reaction


  Reactants on the left-hand side, products on the right-hand side. Numbers are put in front of the compounds to balance the equation.
• Filtration


  To separate solids from liquids
• Crystallisation


  To separate a soluble salt from a solution; a slower method of separating out salt
• Distillation


  To separate out mixtures of liquids

  
• Simple distillation


  Separating a liquid from a solution

  
• Fractional distillation


  Separating out a mixture of liquids. Can be used to separate out crude oil into fractions.
• In the early 1800s, elements were arranged by atomic mass. The periodic table was not complete because some of the elements had not been found. Some elements were put in the wrong group.

  
• Dimitri Mendeleev (1869) left gaps in the periodic table. He put them in order of atomic mass. The gaps show that he believed there was some undiscovered elements. He was right! Once found, they fitted in the pattern.
• The Modern Periodic Table


  Elements are in order of atomic mass/proton number. It shows where the metals and non-metals are. Metals are on the left and non-metals on the right. The columns show the groups. The group number shows the number of electrons in the outer shell. The rows are periods – each period shows another full shell of electrons.

  
• Alkali Metals


  The alkali metals (group 1 elements) are soft, very reactive metals. They all have one electron in their outer shell, making them very reactive. They are low density. As you go down the group, they become more reactive. They get bigger and it is easier to lose an electron that is further away from the nucleus. They form ionic compounds with non-metals. They react with water and produce hydrogen. They react with chlorine and produce salt. They react with oxygen to form metal oxides.
• Halogens

  The halogens are non-metals: fluorine, chlorine, bromine, iodine. As you go down the group they become less reactive. It is harder to gain an extra electron because its outer shell is further away from the nucleus. The melting and boiling points also become higher.
• Noble Gases


  The noble gases (group 0 elements) include: helium, neon and argon. They are un-reactive as they have full outer shells, which makes them very stable. They are all colourless gases at room temperature. The boiling points all increase as they go down the group – they have greater intermolecular forces because of the increase in the number of electrons.
• Electronic Structure


  Electrons are found in shells. A maximum of two in the most inner shell, then eight in the 2nd and 3rd shell. The inner shell is filled first, then the 2nd then the 3rd shell.
• John Dalton described atoms as solid spheres at the start of the 19th century.
• JJ Thomson proposed the plum pudding model - the atom is a ball of charge with electrons scattered.
• Ernest Rutherford's alpha scattering experiment showed that mass is concentrated at the centre, the nucleus is charged, and most of the mass is in the nucleus. Most atoms are empty space.

  
• Niels Bohr proposed that electrons are in shells orbiting the nucleus.

  
• James Chadwick discovered that there are neutrons in the nucleus.
• Metals


  Are strong, malleable, good conductors of electricity and heat. They bond metallically.
• Non-Metals


  Are dull, brittle, and not always solids at room temperature.
• Transition Metals


  A block of elements found between groups 2 and 3 in the middle of the periodic table. They have all the properties you would expect metals to have, such as being strong, shiny and conductors of electricity and heat. Transition metals make very good catalysts; this means they speed up a reaction without being used up themselves. They can form more than one ion, which are often coloured.