Chemical Symbols & Formulae (OCR Gateway GCSE Chemistry: Combined Science)

• An element is a substance made of atoms that all contain the same number of protons and cannot be split into anything simpler
• So, if you had 500g of pure carbon and divided it into 500 x 1g piles, each pile would contain the same substance and would not differ from any other pile
• There are 118 elements on the Periodic Table e.g., hydrogen, carbon, nitrogen
• Each element is represented by its own unique symbol as seen on the Periodic Table e.g., N stands for nitrogen and for nitrogen only
• The symbols of elements can have one, two or three letters
• Where a symbol contains two or more letters, the first one is always written in uppercase letters and the others in lowercase e.g., sodium is Na, not NA

The Formulae of Metallic Elements

• The formula of a metallic element is always an empirical formula and cannot be made any simpler

The Formulae of Non-metallic Elements

• Non-metallic elements in Group 0 exist as individual atoms, so their formula is the same as their symbol
• Many other non-metallic elements exist as molecules and giant structures, so their formulae are not always empirical
• For example sulfur is S₈ and phosphorus is P₄
• The following elements must be written as molecules as they exist in nature as two atoms joined together: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂
• The atomic number and mass number are also shown on the periodic table

The symbol key for Carbon as represented in the periodic table - C is the symbol for Carbon and 12 is the mass number and 6 is the atomic number

• Elements take part in chemical reactions in which new substances are made
• In these reactions atoms combine together in fixed ratios that will give them full outer shells of electrons, often producing compounds
• A compound is a pure substance made up of two or more elements chemically combined and which cannot be separated by physical means
• The properties of compounds are usually quite different from the elements that form them
• There is an unlimited number of compounds, and the chemical formula is what tells you the ratio of atoms in a compound
• For example, H₂O is a compound containing 2 hydrogen atoms and 1 oxygen atom
• The chemical formula can be deduced from the relative number of atoms present
  
• For example, a molecule containing 3 atoms of hydrogen and 1 atom of nitrogen is NH₃
  
• Diagrams or models can be used to represent the chemical formula

The ammonia molecule consists of a central nitrogen atom bonded to 3 hydrogen atoms

• Ionic compounds contain metal and non-metal elements joined together as particles called ions
• In the chemical formula of an ionic compound the symbol of the metal element is always written first
• For example when sodium and chlorine react together they form common salt or sodium chloride, NaCl

The properties of sodium chloride are quite different from sodium and chlorine

• The formula of an ionic compound is always an empirical formula
• This is because ionic compounds are giant structures containing millions of ions so the formula used is the simplest possible ratio of the ions in the structure
• Occasionally ionic compounds don't contain any metals at all
  • For example ammonium chloride, NH₄Cl, contains positive ammonium ions and negative chloride ions
  • In these cases we write the positive ion first