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The website http://ucum.org/ucum.html tells us whether every unit is metric or not. Metric units can be multiplied by a power of 10 and can be combined with a prefix. One ångström is defined as 0.1 nanometers or $10^{-10}$ meters. It is the unit closest to the Bohr radius and the diameter of a hydrogen atom. But the website above says that the ångström is not a metric unit. What are the factors affecting whether units should be classified as a metric or not? And why is the ångström not classified as a metric unit?

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  • $\begingroup$ I first tried asking at physics.stackexchange.com/questions/508572/…. $\endgroup$ Dec 11, 2020 at 23:48
  • $\begingroup$ Re "But the website above says that the ångström in not a metric unit", I cannot find language to that effect on the website. Can you point us? $\endgroup$
    – njuffa
    Dec 12, 2020 at 4:50
  • $\begingroup$ Re "can be combined with a prefix", I note that "milliangstrom" / "milliångström" was in use into the 2000s. $\endgroup$
    – njuffa
    Dec 12, 2020 at 5:05
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    $\begingroup$ @njuffa On the website, in table 21, the column titled "M" contains a "no" ( = not metric) for the Angstrom $\endgroup$
    – jkien
    Dec 13, 2020 at 10:26

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The UCUM website tells us whether every unit is metric or not

No. That website specifies which units are metric according to UCUM. In other contexts the distinction metric-or-not is less clear for some of the units. I suppose many physicsts would consider the Angstrom to be a metric unit, although non-SI.

The Unified Code for Units of Measure (UCUM) is a system of codes for unambiguously representing measurement units. Its primary purpose is machine-to-machine communication. Some of the UCUM base units are different from the SI base units, such as gram instead of kilogram.

Each unit represented in UCUM is identified as either "metric" or "non-metric". Metric units can accept metric prefixes (as in milligram), and non-metric units can not.

In UCUM, nanometer and picometer are metric, whereas Angstrom is not (it is not a base unit with a prefix). So milliAngstrom is not allowed in UCUM.

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