Chemical Periodicity of Other Elements

Full Notes

Predicting Properties Using Periodic Trends

The Periodic Table is structured so that elements with similar properties appear in the same group (vertical columns).

Because of this, we can predict how an element behaves based on its group (e.g. Group 1, Group 7) and its period (horizontal row).

This gives general trends such as:

• Electronegativity (increases across a period, decreases down a group)
• Atomic radius (decreases across a period, increases down a group)
• Reactivity (e.g. Group 1 gets more reactive down the group, Group 7 gets less reactive)

Example:
Potassium (K) is reactive and forms +1 ions, rubidium (Rb) and cesium (Cs), found below it in Group 1, will also form +1 ions and be even more reactive.

Deducing Unknown Elements from Properties

If you're given:

• An element's chemical behaviour (e.g. reacts with water to form a metal hydroxide and hydrogen)
• The type of compound it forms (e.g. NaCl-type salts, oxides that are acidic or basic)
• Its electrical conductivity, melting point, pH of its oxide, etc.

You can often place the element in a:

• Specific group (based on valence electrons and similar chemistry)
• Likely period (based on atomic radius or number of energy levels)
• General block (s, p, d) from its bonding and oxidation state

For Example:

• An element forms an oxide with pH ~13, conducts electricity as a solid, and forms +1 ions
• likely in Group 1, probably s-block (e.g. Na, K).
• An element forms a colourless acidic gas, forms −1 ions, and reacts with metals to form salts
• likely in Group 17 (a halogen).

Summary

• Elements in the same group:
  • Have the same number of outer electrons
  • React in a similar way and form similar compounds
• Use trends like atomic size, ionisation energy, and reactivity to place an unknown element.
• Look for patterns in structure, bonding, and physical properties to work out group or period.