gcse aqa chemistry higher

• Word equations and balanced chemical equations represent the changes that happen in chemical reactions
• Atom


  The smallest part of an element that can exist
• Element


  A substance made of one type of atom only
• Chemical symbol


  • Consists of one or two letters
  • Always starts with a capital letter, with any other letter in lower case
• In the periodic table, metals are on the left of the stepped line, and non-metals are on the right
• A solution with a pH of 7 has an equal concentration of hydrogen ions (H+) and hydroxide ions (OH-).
• A solution that has a pH less than 7 is acidic, while one greater than 7 is alkaline (basic).
• Chemists use symbols and formulae to represent elements and compounds
• Word equations and balanced chemical equations represent the changes that happen in chemical reactions
• Chemical formula


  A combination of symbols that indicates the chemical composition of a substance
• Element


  A substance made of one type of atom only
• Compound


  A substance formed by the chemical union of two or more elements
• Balanced chemical equation


  A chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow
• The formula for most elements is just its chemical symbol
• Examples of chemical symbols for elements


  • helium, He
  • lithium, Li
  • beryllium, Be
  • boron, B
  • carbon, C
  • neon, Ne
  • sodium, Na
  • magnesium, Mg
• It's important to use the names and chemical symbols for the first 20 elements in the periodic table, as well as the elements in groups 1 and 7
• Molecule


  A collection of two or more atoms held together by chemical bonds
• Some non-metal elements exist as molecules that are made up of two atoms joined together
• Examples of elements that exist as molecules


  • iodine, I2
  • bromine, Br2
  • chlorine, Cl2
  • fluorine, F2
  • oxygen, O2
  • nitrogen, N2
  • hydrogen, H2
• It's important to remember which elements exist as molecules, and which do not. Take care to use the correct form in chemical equations involving these elements
• Chemical formulae of elements


  • neon, Ne
  • nitrogen, N2
  • sodium, Na
• Compound


  A substance formed by the chemical union of two or more elements
• Element


  A substance made of one type of atom only
• Chemical formula


  A combination of symbols that indicates the chemical composition of a substance
• The elements in a compound are present in fixed proportions
• Most chemical reactions involve energy changes
• It is not easy to split up a compound into its elements - the only way to do this is in chemical reactions
• Subscript


  A letter or number written below the line
• In compounds made up of non-metal elements only, the second word of the compound's name starts with mon-, di-, or tri-, eg carbon dioxide. This shows the number of atoms of this element for every one atom of the first element in the name.
• Ion


  Electrically charged particle, formed when an atom or molecule gains or loses electrons
• Ions


  • The number and sign of its electrical charges are shown in superscript text
• Names and formulae of some common ions


  • Sodium (Na+)
  • Magnesium (Mg2+)
  • Chloride (Cl-)
  • Oxide (O2-)
• Ionic compound


  An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron)
• Simple formulae of ionic compounds


  • Sodium chloride (NaCl)
  • Sodium oxide (Na2O)
  • Magnesium oxide (MgO)
  • Magnesium chloride (MgCl2)
• Chemists use symbols and formulae to represent elements and compounds
• Word equation


  An equation in which only the names of the reactants and products are used to model a reaction
• Reactants


  Substances that react together in a chemical reaction
• Atom


  The smallest part of an element that can exist
• Ion


  Electrically charged particle, formed when an atom or molecule gains or loses electrons
• Products


  Substances formed in a chemical reaction