Given a scientific question, generate a correct answer to it.

Input: Consider Input: What is the wavelength of light expressed in?

Output: nanometers (nm)

Input: Consider Input: Exemplified by cesium, which ignites spontaneously in air, pure elements with a high metallic character tend to be very what?

Output: reactive

Input: Consider Input: What does lattice energy of an ionic solid measure the strength of?
Output: bonds