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1.1Chemical Substances, Reactions and Essential Resources

The Nature of Substances and Chemical Reactions — WJEC GCSE Chemistry Revision Notes & Topical Questions

20 min read 38 parts Lesson 1 of 6

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Complete PDF with figures and original page layoutPage 1 WJEC GCSE Chemistry Your notes 1.1 The Nature of Substances & Chemical Reactions Contents Elements & Compounds Chemical Symbols & Formulae Relative Atomic Mass & Relative Molecular Mass Perc…

Part 1 of 9Elements

  • Element = a substance made of only one type of atom
  • Listing
    • all elements are listed in the Periodic Table
  • Forms
    • elements exist as single atoms or as molecules of joined atoms
  • Building blocks
    • elements cannot be broken down into simpler substances by chemical means, but they react to make more complex substances

Part 2 of 9Compounds

  • Compound = a substance made of two or more different types of atom chemically joined
  • Formation
    • in chemical reactions the atoms of elements rearrange to form compounds
  • Properties
    • compounds can differ greatly from their elements, as sodium chloride differs from sodium and chlorine
  • Molecules
    • a compound of non-metal atoms is a molecule containing different types of atom

Part 3 of 9Chemical symbols and diatomic elements

  • Symbols
    • each element has a symbol from the Periodic Table, such as Li, W, Al, Ca, Au and Si
  • Seven diatomic elements
    • they exist as molecules of two atoms with formula X2
    • Hydrogen H2, nitrogen N2, oxygen O2.
    • Fluorine F2, chlorine Cl2, bromine Br2, iodine I2.

Part 4 of 9Simple molecules and their formulae

  • Simple molecules
    • they are made of two or more non-metal atoms chemically joined, and can be elements or compounds
  • Covalent
    • simple molecules are covalent substances and contain no metal atoms
  • Formula
    • the symbols show which atoms are present and the subscript number shows how many of each
SubstanceFormulaTypeAtoms present
HydrogenH2Element2 H
ChlorineCl2Element2 Cl
WaterH2OCompound2 H, 1 O
MethaneCH4Compound1 C, 4 H
AmmoniaNH3Compound1 N, 3 H
Sulfuric acidH2SO4Compound2 H, 1 S, 4 O

Part 5 of 9Representing molecules in diagrams

  • Information shown
    • the number of atoms and how they are arranged
  • Style 1
    • use chemical symbols for the atoms, and no key is needed
  • Style 2
    • use coloured circles for atoms, and a key shows the colour for each element
  • Patterns
    • different patterns can replace colours when none are available
  • Arrangement
    • In water, the two hydrogen atoms are joined to either side of the oxygen atom, not in a row.
    • In carbon dioxide, an oxygen atom is on either side of the central carbon atom.
    • In methane, four hydrogen atoms surround the central carbon atom.

Part 6 of 9Ions used in formulae

  • Ionic compounds
    • a metal and a non-metal react, with the metal losing electrons and the non-metal gaining electrons to form ions, so they are not simple molecules
IonFormula
HydrogenH+
Group 1Li+, Na+, K+
Group 7F-, Cl-, Br-
Copper(II)Cu2+
Iron(II)Fe2+
Iron(III)Fe3+
CarbonateCO32-
SulfateSO42-
HydroxideOH-
AmmoniumNH4+
  • Data
    • a table of common ions is given in the exam, but the knowledge must be applied to any ion given

Part 7 of 9Formulae of ionic compounds

  • Overall charge
    • an ionic compound has no overall charge, so the positive and negative charges cancel
  • Direct comparison
    • balance the charges by choosing numbers of each ion
    • Iron(II) sulfate has Fe2+ and SO42-, so one of each is needed, giving FeSO4.
    • Iron(III) chloride has Fe3+ and Cl-, so three chlorides are needed, giving FeCl3.
  • Swap-and-drop
    • swap the charge numbers and drop them as subscripts
    • Remove the signs and omit the number 1.
    • Copper(II) chloride, Cu2+ and Cl-, gives CuCl2.
    • If the charges are equal, reduce to the simplest whole-number ratio.
  • Polyatomic ions
    • put brackets around the ion when more than one is needed, so ammonium sulfate is (NH4)2SO4
  • Naming
    • the metal keeps its name and the non-metal ending changes, so iron and chlorine give iron chloride

Part 8 of 9Relative atomic mass, A<sub>r</sub>

  • Symbol and location
    • Ar is the larger number above the symbol in the Periodic Table, and it is always larger than the atomic number except for hydrogen
  • Standard
    • carbon-12 is the standard atom with a mass of exactly 12 units
  • Comparison
    • Mg with Ar 24 is twice as heavy as carbon, and H with Ar 1 is one twelfth of a carbon-12 atom
  • Definition
    • Ar is the average mass of an element’s isotopes compared with one twelfth of the mass of a carbon-12 atom

Part 9 of 9Relative formula mass, M<sub>r</sub>

  • Definition
    • Mr is the sum of the relative atomic masses of all atoms in the formula
  • Term
    • relative formula mass is used for ionic compounds
SubstanceCalculationMr
H22 × 12
H2O(2 × 1) + 1618
K2CO3(2 × 39) + 12 + (3 × 16)138
Ca(OH)240 + (2 × 16) + (2 × 1)74
(NH4)2SO4(2 × 14) + (8 × 1) + 32 + (4 × 16)132
NaCl23 + 35.558.5
Mg(NO3)224 + (2 × 14) + (6 × 16)148

Key terms

TermMeaning
ElementA substance made of only one type of atom.
CompoundA substance made of two or more different atoms chemically joined.
Diatomic moleculeA molecule made of two atoms, such as H2 or Cl2.
Simple moleculeA small covalent molecule of non-metal atoms.
Relative atomic massThe average mass of an element’s atoms compared with one twelfth of carbon-12.
Relative formula massThe sum of the relative atomic masses of the atoms in a formula.

Avoid these mistakes

  1. Writing the formula of an ionic compound with unbalanced charges — the total positive and negative charge must cancel.
  2. Forgetting brackets for repeated polyatomic ions — write (NH4)2SO4, not NH42SO4.
  3. Treating ionic compounds as simple molecules — simple molecules form only between non-metals.
  4. Missing atoms inside brackets in Mr — multiply every atom in the bracket by the subscript.

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