18 (eighteen) is the
natural number following
17 and preceding
19.
In mathematics
* Eighteen is a
composite number, its
divisors 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. Eighteen is the first inverted square-prime of the form ''p''·''q''
2.
* In
base ten, it is a
Harshad number
In mathematics, a harshad number (or Niven number) in a given number base is an integer that is divisible by the sum of its digits when written in that base.
Harshad numbers in base are also known as -harshad (or -Niven) numbers.
Harshad numbers ...
.
* It is an
abundant number, 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 to this sum.
* It is the number of one-sided
pentominoes.
* It is the only number where the sum of its written digits in
base 10 (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 of
argon.
*
Group 18 of the
periodic table
The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...
is called the
noble gases.
* The
18-electron rule is a
rule of thumb
In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...
in
transition metal chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
for characterising and predicting the stability of
metal complexes
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
.
In religion and literature
* The
Hebrew word for "life" is (''
chai''), which has a
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
Coming of age is a young person's transition from being a child to being an adult. The specific age at which this transition takes place varies between societies, as does the nature of the change. It can be a simple legal convention or can b ...
,
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
The ''Amidah Amuhduh'' ( he, תפילת העמידה, ''Tefilat HaAmidah'', 'The Standing Prayer'), also called the ''Shemoneh Esreh'' ( 'eighteen'), is the central prayer of the Jewish liturgy. Observant Jews recite the ''Amidah'' at each o ...
.
* In
Ancient Roman
In modern historiography, ancient Rome refers to Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–509 BC ...
custom the number 18 can symbolise a
blood relative
Consanguinity ("blood relation", from Latin '' consanguinitas'') is the characteristic of having a kinship with another person (being descended from a common ancestor).
Many jurisdictions have laws prohibiting people who are related by blood fr ...
.
*
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
''Mila 18'' is a novel by Leon Uris set in German-occupied Warsaw, Poland, before and during World War II. ''Mila 18'' debuted at #7 on ''The New York Times'' Best Seller List (the second-highest debut of any Uris novel ever, bested only by th ...
''.
[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
The Bhagavad Gita (; sa, श्रीमद्भगवद्गीता, lit=The Song by God, translit=śrīmadbhagavadgītā;), often referred to as the Gita (), is a 700- verse Hindu scripture that is part of the epic ''Mahabharata'' (c ...
, which is contained in the
Mahabharata, which has 18 books. The
Kurukshetra War
The Kurukshetra War ( sa, कुरुक्षेत्र युद्ध ), also called the Mahabharata War, is a war described in the ''Mahabharata ( sa, महाभारत )''. The conflict arose from a dynastic succession struggle be ...
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'' ("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