Questions: When elements combine to form compounds: - their properties are average of all elements in the compound. - their properties do not change. - their properties change completely. - their properties are completely random.

Solution Steps

The question asks about the properties of elements when they combine to form compounds. It provides four options regarding how the properties of the resulting compound relate to the properties of the individual elements.

1. Their properties are average of all elements in the compound.
   This option suggests that the properties of the compound are a simple average of the properties of the elements, which is not typically true in chemistry.

2. Their properties do not change.
   This option implies that the properties of the elements remain unchanged when they form a compound, which is incorrect because compounds have distinct properties.

3. Their properties change completely.
   This option indicates that the properties of the compound are entirely different from those of the individual elements, which is generally true. For example, sodium (a reactive metal) and chlorine (a toxic gas) form sodium chloride (table salt), which is neither reactive nor toxic.

4. Their properties are completely random.
   This option suggests that the properties of the compound are random, which is not accurate as the properties are determined by the chemical structure and bonding.

Based on the analysis, the correct answer is that the properties of the compound change completely compared to the properties of the individual elements.

Final Answer