# 18 (number)

18 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 add up to 18, hence 18 is a semiperfect number. Eighteen is the first inverted square-prime of the form p·q2.
• In base ten, it is a Harshad number.
• It is an abundant number, as the sum of its proper divisors is greater than itself. 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 is equal to half of itself.

## In science

### Chemistry

• Eighteen is the atomic number of argon.
• Group 18 of the periodic table is called the noble gases.
• The 18-electron rule is a rule of thumb in transition metal chemistry for characterising and predicting the stability of metal complexes.

## In religion and literature

• The Hebrew word for "life" is חי, 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, Rabbi Yehudah ben Teime gives the age of 18 as the appropriate age to get married..
• Shemoneh Esrei 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.
• There are 18 chapters in the Bhagavad Gita, which is contained in the Mahabharata, which has 18 books. The Kurukshetra War which the epic depicts, is between 18 armies. 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 Chinese tradition, 18 pronounced 十八 and is considered a lucky number due to similarity with 實發 'definitely get rich', 'to get rich for sure'.

## Age 18

In most countries, 18 is the age of majority and also the voting age.
In eleven states of the United States, 18 is the age of sexual consent.
In most US states and pursuant to federal law, 18 is the minimum age to purchase firearms
18 is also the minimum age to get married without parental consent.
It is also the age of sexual consent under U.S. federal law.
In addition, Pennsylvania has a corruption of minors law that makes the de facto age of consent there 18. This is also the case in some jurisdictions outside the United States, including Canada.

## In sports

Eighteen is also: