• 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 1 and 2 chlorides react with water to form colourless solutions of pH 6.5-7Group 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:

Group 15 chlorides gets hydrolysed in water, therefore this is a hydrolysis reactionAnswer 3:

The white fumes that are given off during this reaction is hydrogen chloride gas