Writing Chemical Formulae

Writing Chemical Formulae

1. Elements and Symbols

   • Element is a substance that consists of atoms with the same number of protons in their nucleus
   • When you write the name of an element you do not use a capital letter unless you are starting a sentence
     • Example: hydrogen oxygen iron gold copper
   • Symbol always starts with a capital letter
   • Monatomic Elements
     • Group 0 elements are a family of elements called the noble gases. These elements are very unreactive and do not join to other atoms or even themselves
     • They are called monatomic because they exist as single atoms
     • The formulae of the monatomic elements are just their symbol
     • Examples
       • He = helium
       • Ne = neon
       • Ar = argon
       • Kr = krypton
       • Xe = xenon
   • Diatomic elements
     • There are a small number of elements, which exist as molecules made from two atoms. These elements are called the diatomic elements
     • The diatomic elements are
       • iodine = I2
       • bromine = Br2
       • chlorine = Cl2
       • flourine = F
       • oxygen = O2
       • nitrogen = N2
       • hydrogen = H2

2. Molecules

   • Element molecule - all the atoms in the molecule are the same type
   • Compound molecule - different atoms are joined together
   • Molecule = two or more atoms held together by covalent bonds

3. Bonds

   • The atomic forces, which hold the atoms together in a molecule, are called molecular bonds. In drawings of molecules bonds are often drawn as lines between the atoms

4. -IDE Compounds

   • When 2 elements join together the chemical compound will end in ide
   • The elements present in potassium oxide
     • potassium
     • oxygen

5. -ATE & -ITE compounds

   • Compounds, which end their name in -ate or -ite are made from 3 elements. One of the elements is always oxygen.
   • sodium sulfate
     • Na2SO4
       • oxygen
       • sulfur
       • sodium
   • potassium nitrite
     • KNO2
       • potassium
       • nitrogen
       • oxygen

6. Formulae & Names

   • Sometimes you will find prefixes like 'mono', 'di', 'tri', or 'tetra' in the name of the compound. These prefixes provide information about the formula of the compound
     • you do not use valencies to work out the formulae of these compounds
   • Mono = 1
   • di = 2
   • tri = 3
   • tetra = 4
   • penta = 5
   • hexa = 6

7. Valency

   • The valency is the number of bonds an atom can make
   • Valency of most atoms can be worked out fro their position on the periodic table
   • group 1 = valency 1
   • group 2 = valency 2
   • group 3 = valency 3
   • group 4 = valency 4
   • group 5 = valency 3
   • group 6 = valency 2
   • group 7 = valency 1
   • symbols Li O
     • Valency 1 2
       • Swap 2 1
         • Divide 2 1
           • Formula Li2O
   • The crossover method works when the valency of both atoms are different
     • Be careful when a valency 2 and 4 meet. This has to be simplifies to 1 and 2
   • formulae with groups 1
     • Ammonium compounds and compounds ending with hydroxide, -ate or -ite contain groups of atoms. These groups of atoms each have a valency.
     • ammonium group
       • NH4
       • Valency = 1
     • hydroxide group
       • OH
       • valency = 1
     • nitrate
       • NO3
       • valency = 1
     • carbonate group
       • CO3
       • Valency = 2
     • sulfate group
       • SO4
       • valency = 2
     • phosphate group
       • PO4
       • Valency = 3
     • symbols k OH
       • valency 1 1
         • swap 1 1
           • divide 1 1
             • formula = KOH

8. roman numerals

   • Transition elements ( or metals )
     • Many familiar elements like iron, copper and gold belong to the transition metals
   • most transition metals have more than one valency. To inform you what the valency of the transition metals is, a Roman numeral is inserted into the name of the compound immediately after the name of the transition metals
   • for example
     • iron(II) oxide
       • Fe2O2
         • FeO
           • Valencies are the same, one of each other
   • roman numerals you need to know are
     • I = one
       • II = two
         • III = three
           • IV = four
             • V = five
               • VI = six
   • Zinc and silver are exceptions as they only have one valency
     • zinc = valency is always 2
       • Zn2
     • silver - valency is always 1
       • Ag1