# Why is the ångström not a metric unit? And why is the ångström spelt with the Scandinavian letters “å” and “ö”?

The website here http://unitsofmeasure.org/ucum.html tells us whether every unit is metric or not. Metric units can be multipled by a power of 10 and can be combined with a prefix. 1 ångström is defined as 0.1 nanometers or 10^-10 meters. But the website above says that the ångström in not a metric unit. I cannot understand why the ångström is not a metric unit and why it is spelt with the Scandinavian letters "å" and "ö"?

• It's named after someone whose name is just written like that. I don't understand what you're asking for here. – ACuriousMind Oct 16 '19 at 20:10
• You must include in your question your full appreciation of the size/diameter of a Hydrogen atom. – Cosmas Zachos Oct 16 '19 at 20:44

$$10^{-10}m$$ actually.

According to Wikipedia, it is metric but not SI. SI is a subset of the metric system.

"The ångström is not a part of the SI system of units, but it can be considered part of the metric system."

Like many units, it is named after a person and this person happened to be Swedish so it is not very surprising that his name contains Swedish letters.

• There is that one sentence currently on the Wikipedia page that you linked, but it contradicts the discussion elsewhere on Wikipedia, the NIST page that I linked, the reference by the OP, and common usage. For example: "The metric system is an internationally recognised decimalised system of measurement. It is in widespread use, and where it is adopted, it is the only or most common system of weights and measures (see metrication). It is now known as the International System of Units (SI)." [Emphasis added] en.wikipedia.org/wiki/Metric_system – Brick Oct 16 '19 at 20:42
• @Brick Interesting, so that suggests that the "metric system" is now a synonym of SI. Where does leave more common units such as litre, centimetre, Celsius, ... – badjohn Oct 16 '19 at 21:04
• The same as the minute? (Which is common, has an equivalent in the SI unit seconds, and definitely not metric by any standard.) The cm, though, I'd say is SI formed from the base unit m and the prefix c. I supposed you could reasonably adopt a broader definition of "metric", but, as regards the question that was asked about why angstrom was called non-metric, I think this is why. They must have adopted the narrower definition of being SI. – Brick Oct 16 '19 at 21:16
• The liter (with this spelling) is addressed, for example, here: nvlpubs.nist.gov/nistpubs/Legacy/SP/… – Brick Oct 16 '19 at 21:17
• @Brick Yes, centimetre is arguably in a different class to angstrom in that it is metre with a prefix but angstrom, minute, and litre are further removed from SI preferred units. I remember an old chemistry teacher who insisted that we said decimetre cubed rather than litre. – badjohn Oct 16 '19 at 21:20

The SI system (aka metric system) has, by definition, seven "base units" that can be scaled by a standardized list of prefixes. The base unit for length is the meter. There is no $$10^{-10}$$ prefix, so you cannot express 1 angstrom as 1 of any such combination. You can convert it, of course, to any such combination, for example the 0.1 nm that you listed or $$10^{-10}$$ m.

As for the spelling, I've seen it both ways - in the original and in unmarked letters as I wrote it above. But it's ultimately the name of a specific person who spelled his name a particular way, and that's maintained by some people. Certainly the unit always has the circle above it, but still because of the historical person with the same name.

Reference:

https://www.nist.gov/pml/weights-and-measures/metric-si/si-units

• Note, those seven SI base units are not the official definition anymore as of 2019. See the official BIPM brochure. Now, all SI units (also the units of mass) are defined based on a set of universal defining constants. – Steeven Oct 16 '19 at 22:22
• See Table 1 at your link, @Steeven. It's explicit about the same seven units being the "base units". You're confounding what units are base units, as defined, and how those units are standardized, I think. The discussion before the table does address how "essential" those are, or are not, with the new definitions though, so your point is noted. – Brick Oct 17 '19 at 12:36
• I don't believe I'm confounding anything. Those seven base units are still kept in the SI system, sure, due to the historical and well-established use of them, as is described in the announcement. I am merely noting that they are not defining the SI unit system anymore, since another definition has taken over. It is just a note. – Steeven Oct 17 '19 at 12:42