Chemical formulae

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Para educadores: diapositivas de la lección, apuntes de repaso, diagramas listos para usar sobre Chemical formulae (KS3 Science, Chemistry) — úsalos en tu lección, o presenta el tema como una actividad interactiva de clase que tus aprendices juegan como un juego en vivo.

Apuntes de la lección

Element Symbols

  • Each element has a unique symbol, e.g., C for carbon, Cu for copper.
  • The first letter of a symbol is always a capital letter; if a second letter exists, it is lower case.
  • Some symbols come from Latin names, e.g., Cu from×cuprum×.

The Periodic Table key showing where relative atomic mass (Ar) and atomic number (Z) are given for each element, using carbon as the example.

The Periodic Table key showing where relative atomic mass (Ar) and atomic number (Z) are given for each element, using carbon as the example.

Diatomic Molecules

  • Some elements exist as pairs of atoms (diatomic molecules).
  • Example: oxygen gas is written as O₂ – the subscript 2 means two oxygen atoms joined.
  • Other diatomic elements: H₂, N₂, F₂, Cl₂, Br₂, I₂.

Compounds and Formulae

  • A compound is a substance where two or more different elements are chemically joined.
  • Compounds have different properties from the elements they are made from.
  • A chemical formula shows the elements and the number of atoms of each in a compound.
  • Example: H₂O means two hydrogen atoms and one oxygen atom.

Diagrammatic representation of hydrogen, oxygen, carbon dioxide, water and methane, with a key for atom colours (white = hydrogen, red = oxygen, black = carbon).

Diagrammatic representation of hydrogen, oxygen, carbon dioxide, water and methane, with a key for atom colours (white = hydrogen, red = oxygen, black = carbon).

Distinguishing Elements and Compounds

  • Be careful: Co (cobalt, element) vs CO (carbon monoxide, compound).
  • In CO, both letters are capital because it is two elements: C and O.
  • The subscript numbers show the number of atoms: e.g., CO₂ has one C and two O atoms.

Common Compounds Table

  • Al₂O₃ – aluminium oxide: 2 Al, 3 O.
  • CO₂ – carbon dioxide: 1 C, 2 O.
  • HCl – hydrochloric acid: 1 H, 1 Cl.
  • MgO – magnesium oxide: 1 Mg, 1 O.
  • SiO₂ – silicon dioxide: 1 Si, 2 O.
  • NaCl – sodium chloride: 1 Na, 1 Cl.
  • H₂SO₄ – sulfuric acid: 2 H, 1 S, 4 O.
  • H₂O – water: 2 H, 1 O.

Writing Chemical Formulae

  • Use the element symbols from the periodic table.
  • Write the number of atoms as a subscript after the element symbol.
  • If there is only one atom, no subscript is written (e.g., O in H₂O).
  • The order of elements often follows standard conventions (e.g., metal first, then non-metal).

The charges on Cu2+ and Cl- swap and drop to give the formula CuCl2.

The charges on Cu2+ and Cl- swap and drop to give the formula CuCl2.

Examples from Workbook

  • NaOH – sodium hydroxide: 1 Na, 1 O, 1 H.
  • FeS – iron sulfide: 1 Fe, 1 S.
  • NH₃ – ammonia: 1 N, 3 H.
  • CaO – calcium oxide: 1 Ca, 1 O.
  • Li₂O – lithium oxide: 2 Li, 1 O.
  • CaCl₂ – calcium chloride: 1 Ca, 2 Cl.
  • CH₄ – methane: 1 C, 4 H.
  • Na₂CO₃ – sodium carbonate: 2 Na, 1 C, 3 O.
  • K₂SO₄ – potassium sulphate: 2 K, 1 S, 4 O.

Atomic structure of oxygen (O) – 8 protons, 8 neutrons, 2 electrons in inner shell, 6 in outer shell. Oxygen atoms often pair to form O₂.

O — Bohr model (2,6)8p8n

Particle arrangement in solids, liquids, and gases. In compounds, atoms of different elements are chemically joined (e.g., H₂O molecules).

Particle arrangementSolidLiquidGas

Diapositivas

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Preguntas de práctica

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  1. 1.Which of the following is the correct chemical formula for water?

    Easy
    • AH2O
    • BHO2
    • CH2O2
    • DOH2

  2. 2.The symbol 'Co' represents which element?

    Easy
    • ACobalt
    • BCarbon monoxide
    • CCopper
    • DCarbon

  3. 3.How many oxygen atoms are in the formula H2SO4?

    Easy
    • A1
    • B2
    • C4
    • D6

  4. 4.Which of the following compounds has the formula NaCl?

    Easy
    • ASodium hydroxide
    • BSodium chloride
    • CSodium carbonate
    • DSodium oxide

  5. 5.What is the chemical formula for aluminium oxide?

    Medium
    • AAlO
    • BAl2O
    • CAlO3
    • DAl2O3

  6. 6.Which statement is true about compounds?

    Easy
    • ACompounds have the same properties as the elements they are made from.
    • BCompounds have different properties to the elements they are made from.
    • CCompounds are only made from one type of atom.
    • DCompounds cannot be represented by chemical formulae.

  7. 7.What does the subscript 2 in O2 indicate?

    Easy
    • ATwo oxygen molecules
    • BTwo oxygen atoms joined together
    • CTwo different types of oxygen
    • DTwo oxygen electrons

  8. 8.Which of the following is the correct formula for carbon dioxide?

    Easy
    • ACO
    • BCo2
    • CC2O

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