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Chemical formulae

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Lesson notes

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×.

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.

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).

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

Slides

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Practice questions

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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?

    Medium
    • A1
    • B2
    • C4
    • D6

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

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

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

    Hard
    • 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?

    Medium
    • 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?

    Medium
    • ACO
    • BCo2
    • CC2O

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