What is valency? How does it vary along the period?

What is a valence band?

What is an electric fuse? How does it protects devices and appliances from damage?

Define activation energy. How does it vary with rise in temperature?

What is hearing aid? How does hearing aid work?

What is time period?

What is periderm? How does periderm formation take place in dicot stem?

What is electronegativity of an element? How does electromegativity vary in a period and in a group in the periodic table?