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If I have a magnesium atom and a chlorine atom why can't the magnesium atom combine with just one chlorine atom? Wouldn't the magnesium atom become more stable, than it was before combining, as it gets closer to octet after combining with one chlorine atom?

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    $\begingroup$ I don't know how are you going to stop the second chlorine atom from attaching. $\endgroup$ Mar 6, 2018 at 1:20
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    $\begingroup$ There isn't a second chlorine atom as i have mentioned in the question. I just want to know why wouldn't the Mg atom combine with one Cl atom. $\endgroup$
    – Hark
    Mar 6, 2018 at 2:08
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    $\begingroup$ If you had bare Mg and Cl atoms it gas phase then they would bind (if their energy wasn't too high), just like you say. You have a misunderstanding, probably based on ordinary behavior in more common situation. $\endgroup$
    – Mithoron
    Mar 6, 2018 at 15:19
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    $\begingroup$ MgCl molecule certainly exist but in gas phase and ordinarily also high temperature, so it's a bit exotic and reactive. What is true for other elements? Molecules without full octet are found for most of elements, but are often very reactive or very high or low temperature is needed to make/detect them $\endgroup$
    – Mithoron
    Mar 6, 2018 at 15:58
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    $\begingroup$ Very little chemistry is done with single atoms. In almost all practical chemistry you have to worry about what all the other atoms in your system will do. $\endgroup$
    – matt_black
    May 7, 2021 at 11:52

2 Answers 2

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In solid state $\ce{MgCl2}$, can be approximately described assuming fully ionic bond, in which $\ce{Mg}$ atoms lose two electrons to form $\ce{Mg^2+}$, while a $\ce{Cl}$ atoms gain one electron to form $\ce{Cl-}$. To balance the two electrons released by $\ce{Mg}$, you need two chlorine atoms. The electrostatic force of attraction (aka the "ionic bond") binds them, and it has the formula $\ce{MgCl2}$.

Now, you wish to form $\ce{MgCl}$. To maintain charge neutrality, you'll want the $\ce{Mg}$ to lose only one electron. This means the valence shell of $\ce{Mg}$ would still have a single electron left in it. This is a very unstable configuration.

However, it is incorrect to say that $\ce{MgCl}$ cannot form. A research paper1 has explored the very $\ce{MgCl}$ molecule. Note that the conditions were very drastic, $\pu{500^\circ C}$ for pyrolysis and $\pu{2500^\circ C}$ for vaporization respectively. They ensured "the complete formation of $\ce{MgCl}$ molecular species, since $\ce{Cl}$ was the limiting reactant" as you had correctly identified yourself.

Another paper3 describes the formation of the $\ce{MgCl}$ molecule:

Excited $\ce{MgCl}$ molecules were produced by two different techniques: (i) at Orsay, they were created by mixing Mg vapor with a flow of $\ce{He/Cl2}$ and excited in a “heated” Schuller-type discharge tube, and (ii) at Waterloo, they were generated by using a copper hollow cathode lamp loaded with $\ce{MgCl2}$ powder.

Note that these techniques are indeed quite "unnatural", in the sense that you might not find $\ce{MgCl}$ naturally, but you can certainly create it through artificial conditions. In fact, $\ce{MgCl}$ is fairly popular otherwise. Several papers2 have already discussed the emission spectrum of the $\ce{MgCl}$ molecule. Some4 also explore its laser cooling. Hence, by no means it is an unknown compound and in fact, can definitely be formed.


References:

  1. Rina Lourena da S. Medeiros, Sidnei de Oliveira Souza, Rennan Geovanny Oliveira Araujo, Djalma Ribeiro da Silva and Tatiane de A. Maranhão, Chlorine determination via MgCl molecule in environmental samples using high resolution continuum source graphite furnace molecular absorption spectrometry, Talanta, http://dx.doi.org/10.1016/j.talanta.2017.08.026
  2. Darji, A.B., Shah, N.R., Shah, P.M. et al. Pramana - J Phys (1985) 25: 571. https://doi.org/10.1007/BF02847235
  3. Vervloet, M. Fourier Transform Spectroscopy 2001. https://doi.org/10.1006/jmsp.2002.8514
  4. The Journal of Chemical Physics 143, 024302 (2015); https://doi.org/10.1063/1.4926389
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Aside from the reactive gas phase monochloride, magnesium is well known in the +1 oxidation state. Magnesium(I) compounds are generally dimeric, $\ce{Mg2^{2+}}$ (similar to mercury) complexes with complex organic, anionic ligands. The magnesium atoms are covalently bound to each other with one valence, and the other valence is used for ionic bonding to the ligand.

An example is taken from the Wikipedia article linked above*:

enter image description here The structure of Magnesium Complex by Makecat

Calcium has been made in the +1 oxidation state, too.

*This structure shows aromatic rings, which is misleading because of the ionic bonding between magnesium and nitrogen. There are six pi electrons in each ligand, but the ionic bond prevents them from being well-delocalized all the way around the ring.

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    $\begingroup$ This is a nice answer, so I apologize for nitpicking technicalities. May I assume the WP is an abbreviation for Wikipedia? en.wikipedia.org/wiki/WP Apart from that, it is not necessary to point to the source of an image in the public domain, but when you do, and for any other cases where attribution is required, a link under the word "source" is not the proper way to do it. When in doubt, apply scientific quoting and citing rules on Stack Exchange, too. $\endgroup$ Jun 6, 2022 at 16:40
  • $\begingroup$ How should I source the image, then? If you can point me to an example I can aim for it. $\endgroup$ Jun 6, 2022 at 16:43
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    $\begingroup$ Well, the image is in the public domain, so just use it. I think structural diagrams are not copyrightable anyway, I remember Loong writing that somewhere. In other cases, like with a creative commons license, attribution is required and the conditions are explained in their licensing information. Which is also the license our content uses. Something like the following: Title by User at Link (Date or version). $\endgroup$ Jun 6, 2022 at 16:51

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