C2 : ELEMENTS, COMPOUNDS AND MIXTURES (GCSE CHEMISTRY 9-1 SPECIFICATION)

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The Periodic Table

  • Mendeleev took the 50 known elements at the time and arranged them into a Table of Elements.
  • He arraned the elements in order of atomic mass and put elements with similar properties in groups.
  • He left gaps for elements yet to be discovered which helped to predict their properties.
  • Today the periodic table is arranged in order of ascending atomic number.
  • Elements with similar properties form groups and each group corresponds the amount of electrons in the element's outer shell.
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Ionic Bonding

  • Ions are charged particles that form when atoms lose / gain electrons in order to achieve a full outer shell of electrons.
  • Metals lose electrons to form positive ions and non-metals gain electrons to form negative ions.
  • Ionic bonding occurs between a metal and a non-metal. These oppositely charged ions are strongly attracted to each other by electrostatic forces and form an ionic bond.
  • You can show this bonding with dot and cross diagrams.
  • Ionic compounds have giant ionic lattice structures which form a closely-packed regular lattice. There are strong electrostatic forces in all directions between the opposite charges.
  • They have high melting and boiling points due to strong attraction between ions. Lots of energy is needed to break this attraction.
  • Solid ionic compounds don't conduct electricity, as they have no free ions, however molten ionic compounds have free ions that can carry an electric current.
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Covalent Bonding and Simple Molecules

  • Covalent bonding occurs between two non-metals by sharing electron pairs in order to achieve a full outer electron shell.
  • Covalent bonds are strong due to the strong electrostatic attraction between positive nuclei and negative electrons.
  • Supstances formed with covalent bonds usually have simple molecular structures.
  • The attraction between these molecules is very weak so the melting and boiling points are very low.
  • They don't conduct electricity because there are no free electrons / ions.
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Giant Covalent Structures

  • In giant covalent structures there are no charged ions and the atoms are bonded together with strong covalent bonds. They therefore have very high melting and boiling points.
  • Carbon can form lots of allotropes as it can form up to four covalent bonds.

Diamond -> Each carbon atom makes four covalent bonds in a rigid structure so it's very hard. Very high melting point as bonds need lots of energy to break. Doesn't conduct electricity (no free elctrons / ions)

Graphite + Graphene -> Each carbon atom only forms three covalent bonds so sheets of carbon are free to slide over each other. High melting point and conducts electricity because there are lots of delocalised electons thatcan move.

Graphene is a single sheet of carbon that is very strong and light. It's electrons are completely free so it's even better at conducting electricity.

Fullerenes -> Large molecules shaped like balls / tubes. Carbon atoms arranged in rings and have delocalised electrons so conduct electricity. High melting point & boiling point as they're large molecules.

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Nanoparticles

  • Nanoparticles are very tiny particles that contain approx. a few hundred atoms.
  • They have a high surface area to volume ratio.

Uses:

  • Catalysts (huge SA:V)
  • Suncreams & deodorants (don't leave white marks)
  • Nanomedicine (absorbed more easily by body)
  • Electrical circuits (conduct electricity)
  • Added to plastic to make them stronger
  • Long-term effects on health are not yet known.
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Polymers and Material Properties

  • Polymers are formed when lots of smaller monomers join together into chains (polymerisation).
  • Plastics are polymers.

Strong covalent bonds hold together the atoms in polymer chains but forces between the chains determine the properties of the plastic.

Weak Forces:

  • Chains can slide over each other
  • Plastic easily stretched
  • Low melting point

Strong Forces:

  • Rigid
  • Not easily stretched
  • Higher melting points
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Metals

  • In metals, the atoms become positive ions in a sea of negative delocalised electrons.
  • Metallic bonding is the electrostatic attraction between ions and electrons, occurs in all directions as ions are surrounded by electrons.

Properties:

  • High melting and boiling points (hard to overcome strong attraction)
  • High density
  • Strong but also malleable and ductile
  • Good thermal and electrical conductors (sea of delocalised electrons)
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Purity

  • A pure substance is made from a single element / compound.
  • Impure substances have a lower melting point and a higher boiling point than the pure substance.
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Chromatography

Chromatography is used to separate and identify substances in a mixture.

  • The mobile phase is where the molecules can move (liquid / gas)
  • The stationary phase is where molecules can't move (solid / viscous liquid)
  • Components separate as the mobile phase moves over stationary phase.
  • Chemicals spend different amounts of time dissolved in the mobile phase and stuck in the stationary phase.
  • Molecles with a higher solubility will spend more time in the mobile phase than the statioary phase so will be carried further up the plate.

Rf = distance travelled by solute ÷ distance travelled by solvent

  • A pure substance won't be separated by chromatography.
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Relative Masses

Relative atomic mass -> The average mass of one atom of the element compared to 1/12 of the mass of one atom of carbon -12. It is the larger number next to an element on the periodic table.

Empirical Formula -> The smallest whole number ratio of atoms in the compound.

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