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18 (eighteen) is the natural number following 17 and preceding 19. It is an even composite number.

Mathematics

18 is a semiperfect number and an abundant number. It is a largely composite number, as it has 6 divisors and no smaller number has more than 6 divisors. There are 18 one-sided pentominoes. In the classification of finite simple groups, there are 18 infinite families of groups.

In science

Chemistry

• 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 (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, 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.

As lucky or unlucky number

• In Chinese tradition, 18 is pronounced 十八; shí bā and is considered a lucky number due to similarity with 實發; shì fā 'definitely get rich', 'to get rich for sure'.
• According to applications of 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 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 websites restrict adult content to visitors who claim to be at least 18 years old.

• In the United States, 18 is the:
  • 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 Adults Only rating.
• In the UK, 18 is the legal age to purchase a BBFC "18" rated film.
• In Japan, 18 is the minimum age at which one can purchase, rent, or buy tickets to R18+ rated movies or buy video games with a Z rating, although 20 is their cultural age of adulthood.

In sports

• In association football (soccer), "the 18" is a slang term for the penalty area.
• In Nippon Professional Baseball (Japanese baseball), No.18 is known as the Ace number.

Notes

References

1. Sloane, N. J. A. (ed.). "Sequence A005835 (Pseudoperfect (or semiperfect) numbers)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
2. Sloane, N. J. A. (ed.). "Sequence A067128 (Ramanujan's largely composite numbers)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
3. Wilson, Robert A. (2009). The finite simple groups. Graduate Texts in Mathematics. Vol. 251. Berlin, New York: Springer-Verlag. p. 3. doi:10.1007/978-1-84800-988-2. ISBN 978-1-84800-987-5. Zbl 1203.20012.
4. Benjaminson, Chani. "What is the reason for the custom to give money gifts in multiples of 18 -- 18, 36, 54, etc.?". Chabad.org. Archived from the original on 2007-12-27. Retrieved January 19, 2022.
5. 言必有「中」：「十八」「實發」 人見人愛. 文匯報. Accessed 2015-01-12. Archived 2015-01-15.
6. "Chai, Its Meaning and Significance | Shiva, Jewish Mourning". www.shiva.com. Retrieved 2020-11-18.
7. "STATUTORY RAPE: A GUIDE TO STATE LAWS AND REPORTING REQUIREMENTS. SEXUAL INTERCOURSE WITH MINORS". Aspe. 23 November 2015.
8. "Minimum Age". Giffords.

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