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OCR Gateway GCSE Chemistry

Revision Notes

Home / GCSE / Chemistry / OCR Gateway / Revision Notes / 3. Chemical Reactions / 3.1 Introducing Chemical Reactions / 3.1.1 Chemical Symbols & Formulae


3.1.1 Chemical Symbols & Formulae


Chemical Formulae of Elements

  • 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

Carbon atom structure, downloadable IGCSE & GCSE Chemistry revision notes

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

Chemical Formulae of Simple Covalent Compounds

  • 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


Ammonia-Displayed-Formula, downloadable IGCSE & GCSE Chemistry revision notes

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

Chemical Formulae of Ionic Compounds

  • 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


  • 1. Particles
    • 1.1 The Particle Model
      • 1.1.1 The Particle Model
        • 1.1.2 Limitations of the Particle Model
        • 1.2 Atomic Structure
          • 1.2.1 Atomic Models Through Time
            • 1.2.2 Atomic Structure
              • 1.2.3 Calculating the Particles
            • 2. Elements, compounds & mixtures
              • 2.1 Purity & Separating Mixtures
                • 2.1.1 Pure & Impure
                  • 2.1.2 Relative Masses
                    • 2.1.3 Empirical Formula
                      • 2.1.4 Alloys
                        • 2.1.5 Separation Techniques
                          • 2.1.6 Chromatography
                          • 2.2 Bonding
                            • 2.2.1 Metals & Non-Metals
                              • 2.2.2 Electronic Structure
                                • 2.2.3 Ionic Bonding
                                  • 2.2.4 Covalent Bonding
                                    • 2.2.5 Giant Covalent Structures
                                      • 2.2.6 Polymers
                                        • 2.2.7 Metals
                                          • 2.2.8 Atomic Structure & the Periodic Table
                                          • 2.3 Properties of materials
                                            • 2.3.1 Carbon
                                              • 2.3.2 Changes of State
                                                • 2.3.3 Bulk Properties
                                                  • 2.3.4 Nanoparticles
                                                • 3. Chemical Reactions
                                                  • 3.1 Introducing Chemical Reactions
                                                    • 3.1.1 Chemical Symbols & Formulae
                                                      • 3.1.2 Deducing Formulae Using Common Ions
                                                        • 3.1.3 Balancing Chemical Equations
                                                          • 3.1.4 Writing Balanced Ionic Equations
                                                            • 3.1.6 Conservation of Mass
                                                              • 3.1.7 Using Moles to Balance Equations
                                                                • 3.1.8 Calculating Masses from Balanced Equations
                                                                • 3.2 Energetics
                                                                  • 3.2.1 Exothermic & Endothermic Reactions
                                                                    • 3.2.2 Calculating Energy Changes
                                                                    • 3.3 Types of Chemical Reactions
                                                                      • 3.3.1 Oxidation & Reduction
                                                                        • 3.3.2 OxidatIon & Reduction in Terms of Electrons
                                                                          • 3.3.3 Acids & Alkalis
                                                                            • 3.3.4 Neutralisation
                                                                              • 3.3.4 Strong & Weak Acids
                                                                                • 3.3.5 Effect of Hydrogen Ion Concentration on pH
                                                                                • 3.4 Electrolysis
                                                                                  • 3.4.1 Electrolysis
                                                                                    • 3.4.2 Electrolysis of Aqueous Solutions
                                                                                      • 3.4.3 Reactions at Electrodes
                                                                                        • 3.4.4 Half Equations
                                                                                      • 4. Predicting & Identifying Reactions & Products
                                                                                        • 4.1 Predicting Chemical Reactions
                                                                                          • 4.1.1 Group 1
                                                                                            • 4.1.2 Group 7
                                                                                              • 4.1.3 Group 0
                                                                                                • 4.1.4 Transition Metals
                                                                                                  • 4.1.5 The Reactivity Series
                                                                                                  • 4.2 Identifying the Products of Chemical Reactions
                                                                                                    • 4.2.1 Testing for Common Gases
                                                                                                      • 4.2.2 Testing for Aqueous Cations
                                                                                                        • 4.2.3 Testing for Aqueous Anions
                                                                                                          • 4.2.4 Flame Tests
                                                                                                            • 4.2.5 Instrumental Analysis
                                                                                                          • 5. Monitoring & Controlling Chemical Reactions
                                                                                                            • 5.1 Monitoring Chemical Reactions
                                                                                                              • 5.1.1 Concentration in g/dm3
                                                                                                                • 5.1.2 Concentration in mol/dm3
                                                                                                                  • 5.1.3 Titrations
                                                                                                                    • 5.1.5 Gas Volume Calculations
                                                                                                                      • 5.1.6 Yields
                                                                                                                        • 5.1.7 Atom Economy
                                                                                                                          • 5.1.8 Choosing a Reaction Pathway
                                                                                                                          • 5.2 Controlling Reactions
                                                                                                                            • 5.2.1 Measuring Rates
                                                                                                                              • 5.2.2 Rate Graphs
                                                                                                                                • 5.2.3 Change Rates
                                                                                                                                  • 5.2.4 Catalysts & Enzymes
                                                                                                                                  • 5.3 Equilibria
                                                                                                                                    • 5.3.1 Equilibrium
                                                                                                                                      • 5.3.2 Le Chatelier's Principle
                                                                                                                                    • 6.1 Global Challenges
                                                                                                                                      • 6.1 Improving Processes & Products
                                                                                                                                        • 6.1.1 Extracting Metals
                                                                                                                                          • 6.1.2 Biological Extraction Methods
                                                                                                                                            • 6.1.3 The Haber Process
                                                                                                                                              • 6.1.4 Making Fertilisers
                                                                                                                                                • 6.1.5 Explaining the Haber Process
                                                                                                                                                  • 6.1.6 The Contact Process
                                                                                                                                                    • 6.1.7 Life Cycle Assessment
                                                                                                                                                      • 6.1.8 Recycling Materials
                                                                                                                                                        • 6.1.9 Properties & Uses of Alloys
                                                                                                                                                          • 6.1.10 Corrosion of Iron
                                                                                                                                                          • 6.2 Organic chemistry
                                                                                                                                                            • 6.2.1 Alkanes
                                                                                                                                                              • 6.2.2 Alkenes
                                                                                                                                                                • 6.2.3 Addition Polymers
                                                                                                                                                                  • 6.2.4 Condensation Polymers
                                                                                                                                                                    • 6.2.5 DNA
                                                                                                                                                                      • 6.2.6 Alcohols
                                                                                                                                                                        • 6.2.7 Carboxylic Acids
                                                                                                                                                                          • 6.2.8 Crude Oil & Fractional Distillation
                                                                                                                                                                            • 6.2.9 Cracking
                                                                                                                                                                              • 6.2.10 Cells
                                                                                                                                                                              • 6.3 Interpreting & Interacting with Earth Systems
                                                                                                                                                                                • 6.3.1 Origins of the Atmosphere
                                                                                                                                                                                  • 6.3.2 The Greenhouse Effect
                                                                                                                                                                                    • 6.3.3 Climate Change
                                                                                                                                                                                      • 6.3.4 Atmospheric Pollutants
                                                                                                                                                                                        • 6.3.5 Potable Water
                                                                                                                                                                                      • 7. Practical Skills
                                                                                                                                                                                        • 7.1 Practical Skills
                                                                                                                                                                                          • 7.1.1 Reactivity Trends
                                                                                                                                                                                            • 7.1.2 Electrolysis
                                                                                                                                                                                              • 7.1.3 Separation Techniques
                                                                                                                                                                                                • 7.1.4 Distillation
                                                                                                                                                                                                  • 7.1.5 Identification of Species
                                                                                                                                                                                                    • 7.1.6 Titration
                                                                                                                                                                                                      • 7.1.7 Production of Salts
                                                                                                                                                                                                        • 7.1.8 Measuring Rates of Reaction


                                                                                                                                                                                                        DOWNLOAD PDF

                                                                                                                                                                                                      Author: Stewart

                                                                                                                                                                                                      Stewart has been an enthusiastic GCSE, A Level and IB Chemistry teacher for more than 30 years in the UK as well as overseas, and has been an examiner for IB and A Level, also. As a long-standing Head of Department, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams.


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