eighteen

Eighteen is the integer that follows seventeen and precedes nineteen. In base-10 notation it is written as 18. In other common numeral systems, it appears as 10010 in binary, 22 in octal, and 12 in hexadecimal. In Roman numerals, eighteen is XVIII.

In mathematics, eighteen factors as 2 × 3², and its positive divisors are 1, 2, 3, 6, 9, and 18. It is an even composite number and is classified as an abundant number since the sum of its proper divisors (1 + 2 + 3 + 6 + 9 = 21) exceeds 18. It is also a Harshad number, being divisible by the sum of its digits (18 ÷ 9 = 2).

In science, the element with atomic number 18 is Argon, a noble gas. Argon is colorless, odorless, and chemically inert under standard conditions. It makes up about 0.93% of Earth's atmosphere and is widely used in lighting, welding, and various industrial processes due to its inertness.

In culture and law, eighteen commonly marks the age of adulthood in many jurisdictions, conferring legal rights such as voting or contract capacity. In the United States, the 18th Amendment, ratified in 1919, established Prohibition concerning the manufacture and sale of alcoholic beverages; it was repealed by the 21st Amendment in 1933. Eighteen is also the ordinal form “eighteenth” and is encountered in calendars, sports, and various numerical systems.