<p>This is the standard astronomer notation for neutral oxygen. Where chemists would write
$\mathrm{O}$ (or $\mathrm{O}^0$),
$\mathrm{O}^{+}$,
$\mathrm{O}^{++}$, and
$\mathrm{O}^{+++}$ (or $\mathrm{O}^{3+}$),
for neutral, once-, twice-, and thrice-ionized oxygen, respectively, astronomers almost exclusively write
$\mathrm{O}\,\mathrm{I}$,
$\mathrm{O}\,\mathrm{II}$,
$\mathrm{O}\,\mathrm{III}$,
$\mathrm{O}\,\mathrm{IV}$, etc.</p>

<p>The same is of course the case for other elements. For instance, you may have an $\mathrm{H}\,\mathrm{II}$ region (i.e. ionized hydrogen) surrounding hot stars, in a larger $\mathrm{H}\,\mathrm{I}$ (i.e. neutral) envelope.</p>

<p>In other words, we start at 1, where chemists start at 0.</p>

CC BY-SA 3.0
2018-03-01T10:23:39.243Z
Follow-on question: why does this oxygen not bond covalently with other oxygen atoms to form O2 in the atmospheres of planets containing O I?
JThistle
CC BY-SA 3.0
2018-03-01T10:40:25.797Z
@JThistle Atmospheres are far from my comfort zone, but in general to form molecules, you must have high densities and low temperatures. The former should be satisfied in a atmosphere, I suppose, but perhaps it's too close to its star so the temperature is too high, or it is exposed to a strong UV field from the star, so $\mathrm{O}_2$ is dissociated?
pela
CC BY-SA 3.0
2018-03-01T11:58:58.750Z
@pela: high densities are the right keyword. If you just mix a bunch of oxygen atoms together and leave them at low densities, they will stay in atomic form instead of forming molecules. Thus, atomic species probe low-density or low-pressure environments in atmospheres.
AtmosphericPrisonEscape
CC BY-SA 3.0
2018-03-01T12:03:57.483Z
@JThistle In general we prefer people to ask *new* questions as proper new full questions, not comments.  You can link to the original question in your new one.
StephenG - Help Ukraine
CC BY-SA 3.0
2018-03-01T10:17:14.563Z
2018-03-04T20:07:05.107Z
2018-03-04T20:07:05.107Z
pela
<p>I've recently been analysing <a href="http://exoplanet.eu/catalog/all_fields/" rel="nofollow noreferrer">the Exoplanet Database</a>, and have been looking at the 'molecules' field (molecules detected on the planet or in its atmosphere). They all make sense apart from "O I". It's not iodine oxide (which would be I<sub>2</sub>O), so I'm at a loss as to what it could be, especially since I'm very new to this field.</p>

<p>Any help is very much appreciated!</p>

CC BY-SA 3.0
2018-03-01T10:17:14.273Z
Hint: Search for "ionization states of *atomic oxygen;*" a single atom molecule.
uhoh
<exoplanet><planetary-atmosphere>
What does "O I" mean in the context of detected molecules?
JThistle
# What does "O I" mean in the context of detected molecules?

(Asked by: username_0 on 2018-03-01)

I've recently been analysing [the Exoplanet Database](http://exoplanet.eu/catalog/all_fields/), and have been looking at the 'molecules' field (molecules detected on the planet or in its atmosphere). They all make sense apart from "O I". It's not iodine oxide (which would be I2O), so I'm at a loss as to what it could be, especially since I'm very new to this field.

Any help is very much appreciated!

**Tags:** exoplanet, planetary-atmosphere
> **Comments:**
> **username_1**: Hint: Search for "ionization states of *atomic oxygen;*" a single atom molecule.  

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## 1 answers

### Answer by username_2 on 2018-03-01. Score: 3 | Accepted answer

This is the standard astronomer notation for neutral oxygen. Where chemists would write
$\mathrm{O}$ (or $\mathrm{O}^0$),
$\mathrm{O}^{+}$,
$\mathrm{O}^{++}$, and
$\mathrm{O}^{+++}$ (or $\mathrm{O}^{3+}$),
for neutral, once-, twice-, and thrice-ionized oxygen, respectively, astronomers almost exclusively write
$\mathrm{O}\,\mathrm{I}$,
$\mathrm{O}\,\mathrm{II}$,
$\mathrm{O}\,\mathrm{III}$,
$\mathrm{O}\,\mathrm{IV}$, etc.

The same is of course the case for other elements. For instance, you may have an $\mathrm{H}\,\mathrm{II}$ region (i.e. ionized hydrogen) surrounding hot stars, in a larger $\mathrm{H}\,\mathrm{I}$ (i.e. neutral) envelope.

In other words, we start at 1, where chemists start at 0.

> **Comments:**
> **username_0**: Follow-on question: why does this oxygen not bond covalently with other oxygen atoms to form O2 in the atmospheres of planets containing O I?  
> **username_2**: @JThistle Atmospheres are far from my comfort zone, but in general to form molecules, you must have high densities and low temperatures. The former should be satisfied in a atmosphere, I suppose, but perhaps it's too close to its star so the temperature is too high, or it is exposed to a strong UV field from the star, so $\mathrm{O}_2$ is dissociated?  
> **username_3**: @pela: high densities are the right keyword. If you just mix a bunch of oxygen atoms together and leave them at low densities, they will stay in atomic form instead of forming molecules. Thus, atomic species probe low-density or low-pressure environments in atmospheres.  
> **username_4**: @JThistle In general we prefer people to ask *new* questions as proper new full questions, not comments.  You can link to the original question in your new one.  

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