2.1.8 Chemical Periodicity of Other Elements

• If the chemical and physical properties of an element are known, the position of that element in the Periodic Table can be predicted
• Similarly, predictions can be made about the physical and chemical properties of elements if the position of the element in the Periodic Table is known

The diagram shows a flowchart of how to use the position and/or properties of an element to make predictions about its behaviour

Worked example: Deducing the position of an element in the Periodic Table

Answer

Break the question down and systematically approach the question

• Step 1: ‘Element X forms a chloride, which reacts with water to form a solution of pH 1’

  The low pH of the solution formed suggests that the chloride is a non-metallic chloride (group 13 to 17)

• Step 2: ‘The oxide does not dissolve in or react with aqueous sodium hydroxide’
  Since aluminium oxide does reaction with sodium hydroxide, element G cannot be Group 13

• Step 3: It forms an oxide which has a melting point of 1610 °C’

  This suggests a giant molecular (covalent) structure which corresponds to Group 14

• Step 4: Element X cannot be carbon (which is in Group 14) as carbon dioxide is a gas whereas the element X oxide is a solid (with a melting point of 1610 °C)

• Step 5: Element X is therefore a Group 14 element in Period 3 or lower

• Note that this is an example of predicting the position of an element based on its physical and chemical properties

Worked example: Predicting physical and chemical properties of selenium

Answer

• Step 1: Selenium is a nonmetal in Group 16 and if compared to sulfur, its structure is predicted to be a simple molecular structure

• Step 2: Physical properties of simple molecular structures are low melting points, do not conduct electricity and are insoluble in water

• Step 3: Chemical properties are:
  • Reaction with water: simple molecules do not react with water
  • Reaction with oxygen: reacts with oxygen to form an oxide of SeO₂ (if compared with sulfur)
  • Reaction with chlorine: reacts with chlorine to form simple molecule of SeCl₂ which can react with the water vapour in air to form hydrogen chloride (if compared with sulfur)

• Note that this is an example of predicting the chemical and physical properties of an element based on its position in the Periodic Table

Worked example: Determining an unknown chloride compound

Answer

Answer 1:

Group 15 chlorides on the other hand react with water to form strong acidic solutions

Element X therefore belongs to Group 15 of the Periodic Table

Answer 2:

Answer 3: