17

17 is the natural number that follows 16 and precedes 18. It is a prime number, specifically the seventh prime. In number theory, 17 is notable as a Fermat prime (2^(2^n) + 1 with n = 2 yields 17). It can be written as the sum of two squares: 17 = 1^2 + 4^2, and since it is congruent to 1 mod 4, it factors in the Gaussian integers as (4 + i)(4 − i). In common base representations, 17 is 10001 in binary and 11 in hexadecimal.

In science, 17 is the atomic number of chlorine, a halogen element. Chlorine is widely used in disinfection and various chemical processes. Its discovery is attributed to Carl Wilhelm Scheele and, later, to Humphry Davy in the early 19th century.

In culture and history, XVII is the Roman numeral for 17. In Italian superstition, 17 is considered unlucky because the Roman numeral XVII can be rearranged to VIXI, a Latin phrase meaning “I have lived.” The number also appears in governance and geography: the 17th Amendment to the U.S. Constitution, ratified in 1913, established the direct election of U.S. Senators; and the 17th parallel was the provisional military demarcation line dividing North and South Vietnam after the Geneva Accords of 1954.

Other contextual notes include its position as the 17th letter of the Latin alphabet, Q, and its frequent appearance across calendars, sports, and cultural references.