What is the wavelength of a wave?

Wavelength (λ) – The distance of one complete cycle in the wave. The distance between two consecutive crests and /or troughs. S.I. Unit: metre (m).

In other words:

Wavelength is the distance from crest to crest (or trough to trough).

Atoms of elements that are nonmetals tend to gain electrons and atoms of metallic elements tend to lose electrons. Metals have few electrons in their valence shells. By losing those electrons, these metals achieve noble gas configuration and satisfy the octet rule. Nonmetals that have close to 8 electrons in their valence shells readily accept electrons to achieve noble gas configuration. An example is the reaction between calcium and oxygen. Calcium is a metal and has 2 valence electrons. Oxygen is a nonmetal and has 6 valence electrons. Calcium gives up its two valence electrons and oxygen accepts them and an ionic bond is established resulting in the formation of anew compound namely calcium oxide.