139

139 is a natural number that lies between 138 and 140. It is an odd integer used in counting, ordering, and various numerical contexts.

In mathematics, 139 is prime. It is the 34th prime and forms a twin prime pair with 137, since the two primes differ by two. As a prime congruent to 3 modulo 4, 139 cannot be expressed as a sum of two squares. Its numeral representations include decimal 139; binary 10001011; octal 213; and hexadecimal 8B. In Roman numerals, it is written as CXXXIX.

In science, 139 is notable as the mass number of the stable isotope 139La, the most abundant and essentially only stable isotope of the element lanthanum. This gives natural lanthanum its characteristic isotopic composition, with 139La comprising the majority of the element on Earth.

Other numeric properties include its digit pattern arising from the digits 1, 3, and 9, which sometimes appear in educational discussions about base-10 representations and simple prime examples. As a prime number with a modest size, 139 appears in recreational mathematics and number theory discussions about twin primes and prime gaps.

Overall, 139 is recognized primarily for its status as a prime number (and its twin-prime relation to 137), its various base representations, and its role as the mass number of the stable lanthanum isotope 139La.

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