18

18 is the natural number that follows 17 and precedes 19. In base 10 it is written as 18; in Roman numerals, XVIII. It is an even composite number with prime factorization 2 × 3^2.

It has six positive divisors: 1, 2, 3, 6, 9, and 18. As a hexagonal number, 18 equals 2n^2 − n with n = 3. It is also a Harshad (Niven) number, since 18 is divisible by the sum of its digits (1 + 8 = 9; 18 ÷ 9 = 2). In base representations, 18 is 10010 in binary, 22 in octal, and 12 in hexadecimal.

In geometry, an octadecagon is a polygon with 18 sides. In atomic theory, the third electron shell can hold up to 18 electrons, in accordance with the rule that a shell with principal quantum number n can contain 2n^2 electrons (for n = 3, 18).

Culturally, 18 is commonly associated with adulthood and the right to vote in many jurisdictions. In Hebrew gematria, the number 18 represents life, since the word chai (life) sums to 18. The number also appears in various cultural and mathematical contexts as a convenient milestone due to its factor structure and geometric connotations.