The element with 18 protons and 21 neutrons is **argon**. The atomic number of argon is 18, which means that it has 18 protons in its nucleus. The number of neutrons in an atom can vary, and different isotopes of an element can have different numbers of neutrons. The isotope of argon with 18 protons and 21 neutrons is called **argon-39**.
Argon is a noble gas, which means that it is chemically inert and does not react with other elements. It is the third most abundant gas in the Earth's atmosphere, after nitrogen and oxygen. Argon is used in a variety of applications, including welding, lighting, and electronics.
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