Zepto-

Zepto (unit symbol z) is a unit prefix in the metric system denoting a factor of 10−21 or 0.000000000000000000001.

Adopted in 1991, it comes from the Latin septem, meaning "seven", since it is equal to 1000−7.

Examples of its use:

  • The electric charge on one electron: 160.217657 zeptocoulombs (zC).
  • One zeptomole (zmol) of substance contains 602 particles.
  • The human insulin molecule has a mass of roughly 10 zeptograms.[1]
  • Travel time of photon across molecular hydrogen: 247 zeptoseconds (for the average bond length of molecular hydrogen).[2][3]
SI prefixes
Prefix Base 10 Decimal English word Adoption[nb 1]
Name Symbol Short scale Long scale
yotta- Y-  1024 1000000000000000000000000  septillion  quadrillion 1991
zetta- Z-  1021 1000000000000000000000  sextillion  trilliard 1991
exa- E-  1018 1000000000000000000  quintillion  trillion 1975
peta- P-  1015 1000000000000000  quadrillion  billiard 1975
tera- T-  1012 1000000000000  trillion  billion 1960
giga- G-  109 1000000000  billion  milliard 1960
mega- M-  106 1000000  million 1873
kilo- k-  103 1000  thousand 1795
hecto- h-  102 100  hundred 1795
deca- da-  101 10  ten 1795
 100 1  one –
deci- d-  10−1 0.1  tenth 1795
centi- c-  10−2 0.01  hundredth 1795
milli- m-  10−3 0.001  thousandth 1795
micro- μ-  10−6 0.000001  millionth 1873
nano- n-  10−9 0.000000001  billionth  milliardth 1960
pico- p-  10−12 0.000000000001  trillionth  billionth 1960
femto- f-  10−15 0.000000000000001  quadrillionth  billiardth 1964
atto- a-  10−18 0.000000000000000001  quintillionth  trillionth 1964
zepto- z-  10−21 0.000000000000000000001  sextillionth  trilliardth 1991
yocto- y-  10−24  0.000000000000000000000001  septillionth  quadrillionth 1991
  1. Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.

References
  1. John Dell BE PhD (2009-03-25). "Mechanical engineering gone small - how to measure Zeptograms and Yoctograms using MEMS technology" (PDF). University of Western Australia. Engineers Australia. Retrieved 2020-05-14.
  2. Zeptosecond birth time delay in molecular photoionization
  3. [https://www.sciencenewsforstudents.org/article/photon-journey-molecule-shortest-event-zeptosecond-physics Physicists have clocked the shortest time span ever. It's how long it took a chunk of light to cross a hydrogen molecule]
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