chemistry paper 1

• Atoms are the basic units of all substances
• Atoms are really tiny, too small to see even with a microscope
• Atoms

  • Contain protons, neutrons and electrons
  • Have a radius of about 0.1 nanometres
• Nucleus

  • The middle part of the atom
  • Contains protons and neutrons
  • Has a radius of around 1 x 10^-14 m
  • Has positive charge due to protons
  • Accounts for almost all the mass of the atom
• Electrons

  • Move around the nucleus in electron shells
  • Are negatively charged
  • Take up a lot of space in the atom
  • The number of electron shells determines the size of the atom
• The number of protons equals the number of electrons in a neutral atom
• Ions have a different number of electrons than protons, giving them an overall charge
• 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
• The number of neutrons can be calculated by subtracting the atomic number from the mass number
• Isotopes

  Atoms of the same element with the same number of protons but different numbers of neutrons
• Relative atomic mass is an average mass taking into account the different masses and abundances of all the isotopes that make up an element
• Atoms of the same element have the same number of protons
• Substances made up of atoms with the same number of protons are called elements
• There are around 100 different elements
• Compounds

  Substances formed from two or more elements, with the atoms of each element distributed throughout the compound and held together by chemical bonds
• Formation of compounds

  1. Atoms combine by giving away, taking or sharing electrons

  2. Atoms of metals lose electrons to form positive ions

  3. Atoms of non-metals gain electrons to form negative ions

  4. Positive and negative ions are strongly attracted to each other (ionic bonding)

  5. Non-metal atoms share electrons (covalent bonding)
• The properties of a compound are usually very different from the properties of the original elements
• Chemical formula

  A way of representing the composition of a compound using the symbols for the elements and numbers to show the relative numbers of atoms
• Examples of chemical formulas

  • CO2 - carbon dioxide
  • H2O - water
  • NaCl - sodium chloride
  • CO - carbon monoxide
  • HCl - hydrochloric acid
• Brackets in a chemical formula indicate that the atoms inside the brackets are grouped together
• molecules

  1. 2 or more atoms held together by chemical bonds with 1 element
• compounds

  1. 2 or more different elements held together by chemical bonds
  2. always found in the same proportion
• mixtures

  1. two or more substances not chemically combined together
• solvent

  a liquid in which substances can dissolve
• There are 100sh elements, which all materials are made of
• Periodic table

  Elements are laid out in order of increasing atomic number
• Periodic table

  • Arranging elements this way means there are specific patterns in the properties of the elements
  • Properties are said to occur periodically, hence the name periodic table
• If it wasn't for the periodic table organising everything, it would be a 'break of a job' to work out the properties
• Periodic table

  A handy tool for working out which elements are metals and which are non-metals
• Properties of metals

  • They are clear
  • They are ductile
  • Most are good conductors of electricity
  • They are malleable
• Properties of non-metals

  • They are poor conductors of electricity
  • They are brittle
• Elements with similar properties form columns called groups in the periodic table
• Group number

  Tells you how many electrons there are in the outer shell of the elements in that group
• The exception to the rule is group 0, where elements like Helium have two electrons in their outer shell
• The way atoms react depends upon the number of electrons in their outer shell
• Elements in the same group are likely to react in a similar way
• In Group 1, the elements react more vigorously as you go down the group


  Reactivity increases
• In Group 7, reactivity decreases as you go down the group


  Reactivity decreases
• Periods


  The rows in the periodic table, where each new period represents another full shell of electrons