3.1 The periodic table

• Classification of the first twenty elements of the periodic table
• Ion formation; stability and electron affinity
• Valency and oxidation number
• Names and formulae of common radicals
• Chemical formulae of common compounds
• Writing chemical equations

• examine the electron arrangement of an atom to identify its period and group
• classify elements into groups and periods of the periodic table
• explain the stability and electron affinity of atoms of elements
• describe ion formation for the first 20 elements of the periodic table
• formulate chemical formulae of compounds using valencies of elements and radicals
• appreciate the importance of the structure of the atom in the development of the periodic table

“Why does sodium react violently with water while argon does nothing at all?”

“How can you work out a compound's formula without being told it?”

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1. The periodic table and why atoms form ions
   40 min · 1 practice tasks · quiz
   How electron arrangement places every element, and why some atoms give electrons away while others take them.
2. Valency, formulae and equations
   40 min · 2 practice tasks · quiz
   A method for working out any formula, and how to write a reaction down so it balances.
3. Electron arrangements of the first twenty elements
   40 min · 1 practice tasks · quiz
   Fill shells 2, then 8, then 8; then read period and group from the written arrangement.
4. Metals, non-metals and the step
   40 min · 1 practice tasks · quiz
   How the first twenty split into metals that lose electrons and non-metals that gain them, with hydrogen as the odd one.
5. Groups I and II: metals that lose electrons
   40 min · 1 practice tasks · quiz
   Alkali and alkaline-earth metals of the first twenty, their ions, and Kenyan uses of sodium and calcium compounds.
6. Groups VII and VIII: gainers and the unreactive
   40 min · 1 practice tasks · quiz
   Halogens gain one electron; noble gases already have a full shell — the two right-hand columns of the first twenty.
7. Stability and electron affinity
   40 min · 1 practice tasks · quiz
   A full outer shell is stable; electron affinity is how strongly an atom pulls an extra electron — high on the right, low on the left.
8. Ion formation for the first twenty elements
   40 min · 1 practice tasks · quiz
   A complete working list of the ions you are expected to write, and why carbon and silicon are not on it.
9. Oxidation numbers
   40 min · 1 practice tasks · quiz
   A signed count that often matches valency in size, with rules for elements, compounds and radicals.
10. Radicals and more chemical formulae
    40 min · 1 practice tasks · quiz
    The common radicals by heart, then fluent formulae including ammonium, hydroxides, nitrates, sulphates and carbonates.
11. From word equations to symbol equations
    40 min · 1 practice tasks · quiz
    Correct formulae first, state symbols if asked, then an arrow from reactants to products — before any balancing.
12. Balancing equations and the table as a tool
    40 min · 1 practice tasks · quiz
    Balance by numbers in front only; then use the whole sub-strand to read a fertilizer, a kiln and a salt from the table.