Elements, Compounds & Mixtures C2 (Part 1)

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  • Elements, Compounds & Mixtures C2 (Part 1)
    • The Periodic Table
      • Dmitri Mendeleev 1869
        • Mendeleev took the elements known at the time and arranged them into his Table of Elements with various gaps for undiscovered elements
        • Organised the elements in order of atomic mass and kept elements with similar properties in the same vertical groups
      • The modern periodic table shows the elements in order of ascending atomic number and they fit the same patterns that Mendeleev worked out
      • The periodic table is laid out so elements with similar chemical properties form columns called groups
      • The group to which the element belongs corresponds to the number of electrons it has in its outer shell
      • The rows are called periods. The period to which the element belongs corresponds to the number of shells of electrons it has
    • Electron Shells
      • Electrons occupy shells
      • The lowest energy levels (shells) are always filled first
      • Only a certain number of shells are allowed in each shell: 1st shell - 2 electrons. 2nd shell - 8 electrons. 3rd shell - 8 electrons
      • The number of shells which contain electrons is the same as the period of the element
      • The group number tells you how many electrons occupy the outer shell of the element
    • Ionic Bonding
      • Simple Ions form when Atoms Lose or Gain Electrons
      • When atoms lose or gain electrons, they are trying to have a full outer shell or stable electronic structure
      • When metals form ions, they lose electrons to form positive ions
      • When non metals form ions, they gain electrons to form negative ions
        • Group 6 & 7 elements are non - metals
      • The number of electrons lost or gained is the same as the charge on the ion
        • Group 1 & 2 elements are metals
      • The elements in Groups 1, 2, 6 and 7 most readily form ions
        • Group 1 & 2 elements are metals
        • Group 6 & 7 elements are non - metals
        • As you go down each group, you add electron shells, so the outer electrons get further from the nucleus
          • This means that it is harder for the nucleus to attract extra electrons to form ions - so the elements get less reactive as you go down the groups
            • This means that it is easier to remove the outer electrons to form ions - so the elements get more reactive as you go down the groups
          • Ions with opposite charges form Ionic Bonds
            • When a metal and a non - metal react together, the metal can lose electrons to form a positively charged ion and the non - metal can gain electrons to form a negatively charged ion
              • These oppositely charged ions are then strongly attached to one another by electrostatic forces and form an ionic bond
            • To find the formula of an ionic compound, you balance the positive and the negative charges
        • Ionic Compounds
          • Dot and cross diagrams
            • Dot and cross diagrams show the arrangement of electrons in an atom or ion
            • They can also show what happens to the electrons when atoms react with each other
            • Each electron is represented by a dot or a cross
          • Ionic compounds have giant ionic lattice structures
            • The ions form a closely packed regular lattice
              • There are very strong electrostatic forces of attraction between oppositely charged ions in all directions
          • Have similar properties
            • High melting and boiling points due to the strong attraction between the ions. It takes a large amount of energy to overcome this attraction
            • Solid ionic compounds don't conduct electricity because the ions are fixed in place and can't move
              • When an ionic compound melts, the ions are free to move and carry an electric current
            • Dissolve easily in water - the ions separate and are free to move in the solution, so they carry an electric current
        • Simple Molecules
          • Covalent Bonds
            • When non - metal atoms combine together they form covalent bonds by sharing pairs of electrons
            • This way both atoms have a full outer shell
            • Each covalent bond provides one extra shared electron for each atom
            • Covalent bonds are strong because there's a strong electrostatic attraction between the positive nuclei of the atoms and the negative electrons in each shared pair
            • Each atom involved makes enough covalent bonds to fill up its outer shell
            • Dot and cross diagrams can be used to show covalent bonds
          • Simple Molecular Substances
            • Substances formed with covalent bonds usually simple molecular structures
            • The atoms within the molecules are held together by very strong covalent bonds
            • The forces of attraction between these molecules are very weak, these weak intermolecular forces are overcome by melting or boiling
            • The melting and boiling points are very low because the molecules are easily parted from each other
              • The forces of attraction between these molecules are very weak, these weak intermolecular forces are overcome by melting or boiling
            • Most are gases or liquids at room temperature
            • Do not conduct electricity because they don't have free electrons or ions7
        • Giant Covalent Structures
          • Similar to giant ionic lattices except that there are no charged ions
          • Atoms bonded by strong covalent bonds
          • Do not conduct electricity, even when molten EXCEPT GRAPHITE, GRAPHENE AND FULLERENES
          • Graphite
            • Graphite is black and opaque but still shiny
            • Each carbon atom forms three covalent bonds, creating sheets of carbon which are free to slide over each other
            • The layers are held together weakly so they are slippery and can be rubbed off onto paper to eave a back mark - that's how a pencil works
              • Also makes graphite a good lubricating material
            • High melting point - the covalent bonds need a lot of energy to break
            • Since only three out of  each carbon's four outer electrons are used in bonds, there are lots of delocalised free) electrons that can move making graphite an electrical conductor
            • Graphene
              • Graphene is a single sheet of graphite
              • Its covalent bonds make it extremely strong
              • A sheet of graphene is so thin that it is transparent and incredibly light
              • Its delocalised electrons are completely free to move about which makes it even better at conducting electricity than graphite
          • Diamond
            • Pure diamonds are lustrous (sparkly) and colourless - ideal for jewellery
            • Each carbon atom forms four covalent bonds in a very rigid giant covalent structure, which makes diamond really hard. This makes diamonds ideal as cutting tools
            • All those covalent bonds take a lot of energy to break and give diamond a very high melting point, which is another reason diamond is a good cutting tool
            • It doesn't conduct electricity because it has no free electrons or ions
          • Fullerenes
            • Another form of carbon
            • Not giant covalent structures, they're large molecules shaped like hollow balls or tubes
            • Different fullerenes contain different numbers of carbon atoms
            • The carbon atoms in fullerenes are arranged in rings, similar to those in graphite. They also have delocalised electrons so they can conduct electricity
            • Their melting and boiling points aren't as high as diamond and graphite but still very high for molecular substances because they're big molecules and bigger molecules have more intermolecular forces
          • Carbon is an example of a Giant Covalent Structure
            • Carbon can form lots of different types of molecule, because carbon atoms can form up to four covalent bonds and bond easily to other carbon atoms
        • Polymers
          • Formed when lots of small molecules called monoers joined together
            • This reaction is called polymerisation - usually needs high pressure and a catalyst
          • Plastics are polymers. They're usually carbon based and their monomers are often alkenes (a type of hydrocarbon containing a carbon - carbon double bond)
          • Strong covalent bonds hold the atoms together in polymer chains
          • The forces between the different chains that determine the properties of the plastic
          • Weak Forces
            • Polymer chains will be free to slide over each other
            • The plastic can be stretched easily
            • Low melting point
          • Strong Forces
            • High melting points
            • Rigid, cannot be stretched
        • Properties of Materials
          • All different types of materials have their own special properties
          • Their properties are down to the structures and bonding in the material
          • The individual atoms in the material do not have these properties themselves
            • It is the type and strength of the bonds in a material that determines its properties
        • Metals
          • Metals have a Crystal Structure
            • All metals have the same basic properties, due to the special type of bonding that exists in metals
            • In metals, the outer electrons of each atom can move freely - the atoms become positive ions in a 'sea' of delocalised (free) electrons
              • Metallic bonding is the electrostatic attraction between these ions and electrons - the ions are surrounded by the electrons, so the attraction acts in all directions
            • Metallic bonding is the electrostatic attraction between these ions and electrons - the ions are surrounded by the electrons, so the attraction acts in all directions
            • Metallic Bonding is what gives rise to many of the properties of metals
          • Metals are generally found on the left side of the Periodic Table
          • Very hard, dense and lustrous (shiny)
          • Strong attraction between the delocalised electrons and the closely packed positive ions - causing very strong metallic bonding
          • High melting and boiling points because of the strong metallic bonds
          • Have high tensile strength - they're strong and hard to break
          • Malleable - can be hammered into different shapes
          • Good conductors of heat and electricity due to the sea of delocalised electrons which are free to move
          • React with oxygen to form metal oxides
          • Pure metals can be mixed with other elements to make alloys. Alloys have different properties from the main metal they contain
        • States, Structure and Bonding
          • Affect Melting and Boiling Points
            • The type of bonding in a substance affects its melting point and boiling point. It's all to do with how much energy is needed to get the atoms, ions or molecules apart
            • The stronger the bonds that keep the particles together in a solid or liquid, the more heat energy is required to overcome these bonds and separate the particles
            • Simple Covalent Structures have strong bonds within each molecule but only weak intermolecular forces between the molecules so the melting and boiling points are low because it does not take much energy to overcome these bonds
            • Metals have high melting and boiling points because the metal ions are strongly attracted to the delocalised electron 'sea'
            • The strong electrostatic attraction in ionic lattices means ionic substances have high melting and boiling points
            • Giant covalent lattices have strong covalent bonds and high melting and boiling points

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