Complete the statement.
Krypton is ().

 - an elementary substance
 - a compound
You can tell whether krypton is an elementary substance or a compound by counting the number of symbols in its chemical formula. A symbol contains either one capital letter or a capital letter followed by one or two lowercase letters.
The chemical formula for krypton is Kr. This formula contains one symbol: Kr. So, the formula tells you that krypton is made of one chemical element.
Substances made of only one chemical element are elementary substances. So, krypton is an elementary substance.