[chemistryrules (OP)]

Why is MgCl2 formed, as opposed to MgCl?

[Bill.D.Katt.]

I believe it is because of the position of Mg on the periodic table. It has to do with the electron orbitals, Mg is an alkali earth metal and therefore only has two more electrons than the previous noble gas (very stable electron config). Since it only has two more electrons it can lose both of them very easily. There is of course an equilibrium that forces some amount of MgCl to form, but it is very unfavored.

[CliffordK]

Yes.

Mg (Magnesium) tends to form Mg⁺⁺ ions with two extra unshared electrons.
Cl (Chlorine) tends to form Cl^- ions, capturing one electron.

So, it takes two chlorine ions to capture the two extra electrons from the magnesium ion.

Note, these are ionic bonds, not covalent bonds, so the atoms are found more in 2:1 proportions without a distinct molecule, rather than in a molecular form like water or carbon dioxide.

[Bored chemist]

I believe that Mg is in the second column of the periodic table because it forms compounds like MgCl2 rather than the other way round.

The reason for this is that the first and second ionisation potentials for Mg are relatively low and that energy can be recovered by the lattice energy of MgCl2 and the electron affinity of Cl.
The 3rd ionisation potential is too high for this. So it stops reacting at the second chlorine.

[lightarrow]

Why is MgCl2 formed, as opposed to MgCl?

Why NaCl is formed instead of NaCl₅?  []
What kind of question is your? I can't find the meaning. Why shoud Mg form MgCl? What do you think people study chemistry for?

[CliffordK]

Why is MgCl2 formed, as opposed to MgCl?

Why NaCl is formed instead of NaCl₅?  []
What kind of question is your? I can't find the meaning. Why shoud Mg form MgCl? What do you think people study chemistry for?

Don't people study Chemistry to take a break between Basket Weaving classes?

Since it is ionic...  one could just as easily say:
MgCl₂, Mg₂Cl₄, Mg₃Cl₆, Mg₄Cl₈, etc.

One just reduces it to MgCl₂ for convention.