131
Iodine-131 is the isotope released in reactor accidents.
- Odd
- Prime
Where 131 fits
Why 131 is special
It concentrates in the thyroid and has an eight-day half-life, which is why potassium iodide is distributed quickly or not at all.
131 is part of
Factors & digit properties
- Divisors (2)
- 1, 131
- Prime factorisation
- 131
- Digit sum
- 5
- Digital root
- 5
Representations
- Decimal
- 131
- Scientific notation
- 1.310 × 10^2
Roman numerals
C X X X I
Binary
1 0 0 0 0 0 1 1
1 × 128 + 0 × 64 + 0 × 32 + 0 × 16 + 0 × 8 + 0 × 4 + 1 × 2 + 1 × 1 = 131
Octal
2 0 3
2 × 64 + 0 × 8 + 3 × 1 = 131
Hexadecimal
8 3
8 × 16 + 3 × 1 = 131
Read as powers of two: 128 + 2 + 1 = 131.
In historical numeral systems
Egyptian
1 coil of rope, 3 hobbles, 1 stroke
1 × 100 + 3 × 10 + 1 × 1 = 131. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
2 vertical wedges (sixties place), 1 corner wedge + 1 vertical wedge (units place)
2 × 60 + 11 = 131. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
rho, then lambda, then alpha, marked as a numeral
rho-lambda-alpha + keraia
rho + lambda + alpha written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
qof, then lamed, then alef
qof-lamed-alef
qof (100) + lamed (30) + alef (1) = 131, written in descending order.
Maya
1 bar + 1 dot (upper, twenties place) over 2 bars + 1 dot (lower, units place)
6 × 20 + 11 = 131. Places stack vertically with the smallest at the bottom, so the upper group is worth twenty times the lower.
Systems that simply cannot reach this value are left out. Where a script needs a font most readers do not have, the reading is given instead of the glyphs — see number systems for each system in full.
Other mathematical patterns
- Palindromic number
- Deficient number
- Sophie Germain prime
Related numbers & connections
Both are released in reactor accidents, one determining the immediate response and one the length of the exclusion.
View 137 →Numbers Around Us
- appears alongside137
why?
Strength 0.50, confidence "high".
Science
Iodine-131 has a half-life of about eight days and concentrates in the thyroid gland. Stable iodine prophylaxis saturates the gland and is effective only if given close to exposure.
DocumentedScienceGeography: GlobalPeriod: 1938–
Common questions
Is 131 a prime number?
Yes, 131 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 131?
The factors of 131 are 1, 131.
What is 131 in Roman numerals?
131 in Roman numerals is CXXXI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 1 reviewer-confirmed record, sourced and reviewed before publication.
Last reviewed: 15 July 2026