18 (eighteen) is the
natural number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country").
Numbers used for counting are called ''cardinal ...
following
17 and preceding
19.
In mathematics
* Eighteen is a
composite number
A composite number is a positive integer that can be formed by multiplying two smaller positive integers. Equivalently, it is a positive integer that has at least one divisor other than 1 and itself. Every positive integer is composite, prime, ...
, its
divisor
In mathematics, a divisor of an integer n, also called a factor of n, is an integer m that may be multiplied by some integer to produce n. In this case, one also says that n is a multiple of m. An integer n is divisible or evenly divisible by ...
s being
1,
2,
3,
6 and
9. Three of these divisors (3, 6 and 9) add up to 18, hence 18 is a
semiperfect number
In number theory, a semiperfect number or pseudoperfect number is a natural number ''n'' that is equal to the sum of all or some of its proper divisors. A semiperfect number that is equal to the sum of all its proper divisors is a perfect number.
...
. Eighteen is the first inverted square-prime of the form ''p''·''q''
2.
* In
base ten
The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numeral ...
, it is a
Harshad number.
* It is an
abundant number
In number theory, an abundant number or excessive number is a number for which the sum of its proper divisors is greater than the number. The integer 12 is the first abundant number. Its proper divisors are 1, 2, 3, 4 and 6 for a total of 16. Th ...
, as the sum of its proper divisors is greater than itself (1+2+3+6+9 = 21). It is known to be a
solitary number, despite not being
coprime
In mathematics, two integers and are coprime, relatively prime or mutually prime if the only positive integer that is a divisor of both of them is 1. Consequently, any prime number that divides does not divide , and vice versa. This is equivale ...
to this sum.
* It is the number of one-sided
pentomino
Derived from the Greek word for ' 5', and " domino", a pentomino (or 5-omino) is a polyomino of order 5, that is, a polygon in the plane made of 5 equal-sized squares connected edge-to-edge. When rotations and reflections are not considered ...
es.
* It is the only number where the sum of its written digits in
base 10
The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numer ...
(1+8 = 9) is equal to half of itself (18/2 = 9).
* It is a Fine number.
In science
Chemistry
* Eighteen is the
atomic number
The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
of
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...
.
*
Group
A group is a number of persons or things that are located, gathered, or classed together.
Groups of people
* Cultural group, a group whose members share the same cultural identity
* Ethnic group, a group whose members share the same ethnic ide ...
18 of the
periodic table is called the
noble gases
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...
.
* The
18-electron rule is a
rule of thumb in
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...
chemistry for characterising and predicting the stability of Metal complex#Metal complexes, metal complexes.
In religion and literature
* The Hebrew language, Hebrew word for "life" is (''Chai (symbol), chai''), which has a gematria, numerical value of 18. Consequently, the custom has arisen in Jewish circles to give donations and monetary gifts in multiples of 18 as an expression of blessing for long life.
* In Judaism, in the Talmud; Pirkei Avot (5:25), Rabbi Yehudah ben Teime gives the age of 18 as the appropriate age to get married (''"Ben shmonah esra lechupah"'', at eighteen years old to the Chupah (marriage canopy)). (See Coming of age, Age of majority).
* Shemoneh Esrei (sh'MOH-nuh ES-ray) is a prayer that is the center of any Jewish religious service. Its name means "eighteen". The prayer is also known as the Amidah.
* In Ancient Roman custom the number 18 can symbolise a blood relative.
* Joseph Heller's novel ''Catch-22'' was originally named ''Catch-18'' because of the Hebrew meaning of the number, but was amended to the published title to avoid confusion with another war novel, ''Mila 18''.
[N James. ''The Early Composition History of Catch-22''. In Biographies of Books: The Compositional Histories of Notable American Writings, J Barbour, T Quirk (edi.) pp. 262-90. Columbia: University of Missouri Press, 1996]
* There are 18 chapters in the Bhagavad Gita, which is contained in the Mahabharata, which has 18 books. The Kurukshetra War which the Indian epic poetry, epic depicts, is between 18 armies (11 on the Kuru (kingdom), Kuru side, 7 on the Pandava). The war itself lasts for 18 days. In the other Hindu epic, the Ramayana, the war between Rama and the demons also lasted 18 days.
* In Babism the first 18 disciples of the Báb were known as the Letters of the Living.
As lucky or unlucky number
* In Numbers in Chinese culture, Chinese tradition, 18 is pronounced and is considered a lucky number due to similarity with 'definitely get rich', 'to get rich for sure'.
*According to applications of Gematria, numerology in Judaism, the letters of the word ''Chai (symbol), chai'' ("living") add up to 18. Thus, 18 is considered a lucky number and many gifts for Bar and bat mitzvah, B'nai Mitzvot and weddings are in $18 increments.
Age 18
In most countries, 18 is the age of majority, in which a minor becomes a legal adult. It is also the voting age, marriageable age, drinking age and smoking age in most countries, though sometimes these ages are different than the age of majority. Many Website, websites restrict adult content to visitors Minor (law), who claim to be aged over 18.
*In the United States, 18 is the:
**age of consent, Age for sexual consent in eleven states and under federal law.
**Minimum age to purchase firearms in thirty-eight states with the exception of handguns (21 under federal law).
**Marriageable age without parental consent except for Nebraska (19), Mississippi and Puerto Rico (21).
**The minimum age at which one can purchase, rent, or buy tickets to NC-17-rated films or buy video games with an Entertainment Software Rating Board, Adults Only rating.
*In the UK, it is the legal age to purchase a British Board of Film Classification, BBFC "18" rated film.
In sports
* In association football (soccer), "the 18" is a slang term for the penalty area.
* In Australian rules football (except in AFL Women's), each team has 18 players on the field during play.
* There are 18 holes on a regulation golf course.
* In Nippon Professional Baseball (Japanese baseball), No.18 is known as Ace (baseball), Ace number.
See also
* Numeral system
Notes
{{Integers, zero
Integers