On 'heating' in the 'earth atmosphere' elemental magnesium in what form?
This is, in my assessment, actually quite a good chemistry essay type question, because required clarification on the chemistry is likely needed. This arises from noted ambiguities, as for example, with respect to the intensity of the heat applied, the form of Mg present (as a powder or sheet metal), and even the composition of the experiment's atmosphere.
In the case of minor heating in the presence of an 'earth atmosphere' per Wikipedia, to quote:
Magnesium occurs naturally only in combination with other elements, where it invariably has a +2 oxidation state. The free element (metal) can be produced artificially, and is highly reactive (though in the atmosphere, it is soon coated in a thin layer of oxide that partly inhibits reactivity – see passivation).
So one quick answer with mild conditions with bulk magnesium is simply a protective coating of MgO.
However, as we are dealing with an 'earth atmosphere', which can contain water vapor, and per the same source:
Magnesium reacts with water at room temperature, though it reacts much more slowly than calcium, a similar group 2 metal. When submerged in water, hydrogen bubbles form slowly on the surface of the metal – though, if powdered, it reacts much more rapidly.
So, especially powdered Magnesium in the presence of water vapor at room temperature could create some Magnesium hydroxide and hydrogen gas also per the reaction:
$\ce{Mg (s) + 2 H2O (l) -> Mg(OH)2 (s) + H2 (g)}$
Further, assuming sufficient heat and amenability of the form of Mg to ignition, as per the same source, to quote:
Magnesium is highly flammable, especially when powdered or shaved into thin strips, though it is difficult to ignite in mass or bulk. Flame temperatures of magnesium and magnesium alloys can reach 3,100 °C (5,610 °F),[16] although flame height above the burning metal is usually less than 300 mm (12 in).[17] Once ignited, such fires are difficult to extinguish, because combustion continues in nitrogen (forming magnesium nitride), carbon dioxide (forming magnesium oxide and carbon), and water (forming magnesium oxide and hydrogen, which also combusts due to heat in the presence of additional oxygen). This property was used in incendiary weapons...
which apparently adds to the list carbon and magnesium nitride (Mg3N2).
However, again in the presence of water vapor, the nitride is not stable possibly reacting as follows:
$\ce{Mg3N2(s) + 6 H2O(l) -> 3 Mg(OH)2(aq) + 2 NH3(g)}$
So, lastly add ammonia to the product list (for a grade of 'A').