atomic structure & periodic table

• an atom is the smallest part of an element that can exist.
• all substances are made of atoms
• chemical symbols represent an atom of an element. There are around 100 different elements. Elements are shown in the periodic table.
• compounds are formed from elements by chemical reactions.
• chemical reactions always involve the formation of one or more new substances and often involve a detectable energy change.
• compounds contain 2 or more different elements chemically combined together in fixed proportions and can be represented by formulae using the chemical symbols of the atoms from which they were formed.
• compounds can only be separated into elements by chemical reactions.
• mixture separation -> FILTRATION: filtration separates an insoluble solid from a liquid.

  • a filter funnel lined with filter paper should be placed into an empty beaker 
  • the mixture should be poured through the filter funnel into the beaker 
  • the insoluble solid (or residue) can be collected from the filter paper and the liquid (or filtrate) can be collected from the beaker. 
  • filtration only works because the filter paper's holes are large enough to let small molecules are dissolved ions through but are too small for the larger particles of undissolved solids
• Mixture separation -> CRYSTALLISATION : separates a dissolved solid from a solution, when the solid is much more soluble in hot solvent than cold.

  1. solution should be placed in an evaporating dish; gently heated over a bunsen burner
  2. a saturated solution will form as the solvent evaporates (to test for a saturated solution place a glass rod in the basin and check for crystals around the rod
  3. slowly cool the saturated solution
  4. as the solution cools the solid will start to come out of the solution and form crystals.
  5. to obtain crystals filter the solution, wash with cold distilled water & dry
• Mixture separation -> SIMPLE DISTILLATION : separates a solvent from a solution.

  1. solution should be heated in a conical flask, with a bung and connected to a condensor
  2. as the solution is heated, the solvent evaporates and enters the condensor
  3. the condensor has a cold water jacket which covers it, cooling the condensor. This causes the vaporised solvent to condense back into a liquid
  4. This liquid drips into a beaker
  5. Simple distillation only works because the dissolved solute has a much higher boiling point than the solvent and in a mixture substances retain their properties
• Mixture separation -> FRACTIONAL DISTILLATION: separates different liquids of a mixture, where the liquids and miscible

  1. the mixture should be heated in a flask to the temperature of the liquid with the lowest boiling point(bunsen burner or if mixture is flammable with an electric heater)
  2. the flask should be connected to a fractionating column which is full of little glass rods and has a temperature gradient where the top is cooler than the bottom
• FRACTIONAL DISTILLATION:

  3. as the vapours of the lowest boiling point liquid rise, they pass through the fractionating column into the condenser where they cool, condense and drip into a beaker

  4. If any of the other liquids evaporate as well, the fractionating column causes them to condense and drip back into the flask because the glass rods are cooler than their boiling point meaning they lose energy and condense
• mixtures can be separated by physical processes. These physical processes do not involve chemical reactions and no new substances are made.
• atomic number is the number of protons in an atom of an element. All atoms of a particular element have the same atomic number. Number of protons is equal to the number of electrons in an atom and so there is no overall charge
• mass number is the number of protons and the number of neutrons in the nucleus of an atom
• isotopes are atoms of an element that have the same number of protons but a different number of neutrons
• the relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.

  to calculate relative atomic mass:

  (isotope 1 mass X abundance) + (isotope 2 mass X abundance) / 100
• the early periodic table -- Mendeleev:

  • Mendeleev left gaps for elements that had not been discovered yet
  • in some places the order changed based on atomic weight
  • Mendeleev re-ordered some elements according to relative atomic mass (considering isotopes)
  • Mendeleev grouped elements based on chemical and physical properties.
• none of the early period tables accounted for transition metals.
• elements in the periodic table are arranged in order of atomic number and so that elements with similar properties are in columns, known as groups
• the periodic table is called a 'periodic' table because similar properties occur at regular intervals
• elements in the same group in the periodic table have the same number of electrons in their outer shell. This gives them similar chemical properties.
• the modern periodic table:

  • elements with properties predicted by Mendeleev were discovered and the gaps were filled
  • knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct
  • transition metals were added
  • when sub-atomic particles (proton, neutron, electron) were discovered, elements were ordered in atomic number
• metals are elements that react to form positive ions

  non-metals are elements that react to form negative ions
• majority of elements are metals, found to the left and towards the bottom of the periodic table.
• the early periodic table -- John Newlands:

  • ordered his table strictly in order of atomic weight, and didn't leave gaps for undiscovered elements
  • realised similar properties occurred every eighth elements - 'Law of Octaves' - but this broke down after calcium
  • the early periodic table was incomplete and some elements were placed in inappropriate groups because atomic weight was strictly followed
• group 1 (alkali metals) --

  • group 1 metals have one electron in their outer shell
  • which makes them very reactive
  • the alkali metals are good thermal and electrical conductors
  • they are soft and have low densities
  • as you go down the group
  • reactivity increases because:
  • the outer electron gets further away from the nucleus as the nucleus gets larger so the attraction between the nucleus and outer electron gets weaker, and the nucleus is more shielded so the electron is more easily lost
  • the melting point and boiling point decreases
  • the elements have a higher relative atomic mass
• group 1 : reactions with water --

  • when in water, group 1 metals react vigorously to produce hydrogen gas and metal hydroxides (which dissolve in water to produce an alkaline solution)
  • the amount of energy given out by the reaction increases as you go down the group
  • Lithium - fizzes slowly until it disappears
  • Sodium - fizzes rapidly and gets used up quicker (disappears sooner)
  • Potassium - burns violently with a purple flame and gets used up the quickest (disappears very quickly)
• group 1 : reaction with chlorine --

  • alkali metals react with chlorine to form a white precipitate
  • they react vigorously when heated in chlorine gas to form white metal chloride salts
• group 1 : reactions with oxygen --

  • group one metals react with oxygen to produce metal oxides
  • lithium oxide = L20
  • sodium oxide = Na2O
  • sodium peroxide = Na2O2
  • potassium peroxide = K2O2
• group one metals are more reactive than transition metals
• group 1 metals are less dense, strong and hard than transition metals, and have much lower melting and boiling points
• a mixture contains two or more elements or compounds that are not chemically combined together
• in a mixture, the chemical properties of each substance in it remain unchanged
• new experimental evidence may lead to a scientific model being changed or replaced
• development of the model of the atom --

  • before the discovery of electrons, atoms were thought to be tiny spheres which couldn't be divided
  • J.J. Thompson discovered the electron, which led to the discovery of the 'plum pudding model'. It suggested that the atoms was a positively charged sphere with negative electrons scattered and embedded in it
  • Rutherford conducted the alpha scattering practical, after which Rutherford created the nuclear model. The mass was concentrated at the centre in a positively charged nucleus and electrons moved around the nucleus
• development of the model of the atom --

  • Neils Bohr discovered electrons orbit at fixed energy levels and distances
  • James Chadwick discovered the neutron within the nucleus.
• electrons occupy the lowest available energy levels
• the elements in group 7 are known as the halogens and have similar reactions because they all have seven electrons in their outershell
• group 7 trend:

  • as you go down the group, reactivity decreases because halogen gain electrons and as you go down the group, the number of shells increases meaning the attraction between the nucleus and electrons is weaker and there is more shielding
  • as you go down the group, the relative atomic mass increases and the melting and boiling point increases