Why are gold leaves used in a gold-leaf electroscope?

Explanation

The leaves of the gold-leaf electroscope should be thin and light. They diverge due to repulsion of similar charges on them. The divergence of the leaves is more if they are light. Large divergence makes observation easier. Now gold is a ductile metal that can be beaten to produce very thin leaves. Moreover, gold does not react with air. For these reasons gold leaves are used in an electroscope.