Chemistry

• Atoms


  Incredibly small, make up everything in the universe, smallest part of an element that can exist
• Atoms

  • Composed of three sub-atomic particles: protons, neutrons, and electrons
• Nucleus

  Center of an atom, containing protons and neutrons
• Protons

  Positive charge (+1), relative mass of 1
• Neutrons

  No charge (0), relative mass of 1, similar to protons
• Electrons

  Negatively charged (-1), very small, with virtually no mass when compared to protons and neutrons
• The nucleus is tiny compared to the whole atom, about 1/10,000th of the atom's total size, but it holds nearly all the atom's mass
• Radius of an atom

  0.1nm (1 x 10-10 m)
• Radius of a nucleus

  1 x 10-14 m
• Atoms are neutral and have no overall charge because they have the same number of protons and electrons, which cancels out their charges
• Chemical symbol

  Atoms of each element are represented by a chemical symbol which can be found in the periodic table
• Atomic number

  Indicates how many protons are in an atom's nucleus
• Mass number


  Total count of both protons and neutrons
• To find the number of neutrons

  Subtract the atomic number from the mass number
• Element

  A substance made up of one type of atom
• There are over 100 different discovered elements which can all be found in the periodic table
• All the atoms in an element contain the same number of protons in their nucleus
• When atoms have different numbers of protons, they are classified as different elements
• Isotopes

  Different forms of the same element, having the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers)
• A common example is Carbon-12, Carbon-13 and Carbon-14 which all have six protons but differ in the number of neutrons
• Compound

  Substances that form when two or more elements react and their atoms combine in fixed proportions, held together by chemical bonds
• In compounds, only the electrons are involved in bond formation while the nuclei remain unchanged
• The properties of a compound are usually quite different from the properties of the original elements
• Hydrogen and oxygen

  • Hydrogen is a colourless, odourless gas that is highly flammable, while oxygen is a colourless gas that can burn (combustion). When these two elements react, they form water (H₂O) which is a liquid at room temperature and does not burn. The properties of water are entirely different from those of its constituent gases, hydrogen and oxygen.
• Compound formula

  Represents compounds and shows the elements and the number of atoms of each element in a compound
• Compound formulas

  • H₂O (water), CH₄ (methane)
• Relative atomic mass (Ar)

  The average mass of all the isotopes of an element, taking into account their abundance
• Calculating relative atomic mass

  Use the formula: (Ar = (mass of isotope 1 x abundance of isotope 1) + (mass of isotope 2 x abundance of isotope 2) + ...)
• As an example, the relative atomic mass of chlorine is calculated using the two isotopes Chlorine-35 and Chlorine-37, which have abundances of 75% and 25% respectively
• Chemical equation

  Used to show what happens during a chemical reaction, with the left hand side representing the reactants and the right hand side representing the products
• Word equation


  Expresses the reactants and products of a reaction in words
• Symbol equation

  More concise, using chemical symbols and formulas to represent the reactants and products
• Symbol equations must be balanced, meaning they have the same number of each type of atom on both sides of the equation</b>
• Steps to balance equations

  Identify the number of atoms of each element on either side of the equation. 2. Adjust the coefficients to balance that element, which may cause another element to become unbalanced. 3. Repeat the process, adjusting coefficients as necessary until the equation is balanced.
• Mixture

  Combinations of two or more substances that are not chemically combined, where the substances retain their individual properties
• Compound

  Combinations of two or more substances that are chemically combined
• Compounds can only be separated using chemical reactions, while mixtures can be separated using physical methods
• Physical separation techniques

  • Filtration
  • Evaporation
  • Crystallisation
  • Simple distillation
  • Fractional distillation
  • Chromatography
• Filtration

  A method used to separate an insoluble solid from a liquid, by pouring the mixture through a filter paper where the solid is trapped and the liquid passes through
• Evaporation

  A method to recover soluble solids like salt, by dissolving them in a solvent and then gently heating to evaporate the solvent, leaving the dry crystals behind