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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. $\endgroup$ – Avnish Kabaj Mar 6 '18 at 1:20
  • $\begingroup$ 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 '18 at 2:08
  • $\begingroup$ 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 '18 at 15:19
  • $\begingroup$ So we can have MgCl atom, right? Is it true for all other elements? $\endgroup$ – Hark Mar 6 '18 at 15:48
  • $\begingroup$ 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 '18 at 15:58
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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. The $\ce{Mg^+}$ ions, like similar ions of mercury, form diatomic pairs $\ce{Mg2^{2+}}$ in which each magnesium atom is covalently bound to the other with one valence, and the other valence is used for ionic bonding to the anionic ligand.

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

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