19

The number 19 is a natural number that comes after 18 and before 20. It is a prime number, meaning its only positive divisors are 1 and 19. It is also an odd number.

In mathematics, 19 has several standard representations: decimal 19; binary 10011; octal 23; hexadecimal 13; and the Roman numeral XIX. It is the eighth prime number. The Mersenne prime with exponent 19 is 2^19 − 1 = 524,287.

In science, 19 is the atomic number of potassium, symbol K. Potassium is an essential alkali metal important for nerve function, muscle contraction, and fluid balance in living organisms. It was first isolated as an element by Humphry Davy in 1807.

In calendars and astronomy, the Metonic cycle spans 19 years, a period after which the phases of the Moon repeat on the same days of the year. The 19th day of a month is a common reference point in date notation.

In language, the 19th letter of the Latin alphabet is S. In Roman numerals, 19 is written XIX.