18

18 (eighteen) is the natural number following 17 and preceding 19. In mathematics Eighteen is the tenth 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·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 (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 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.