Magic is neither a property of atoms, nor molecules (and/or polyatomic ions), but an emergent property resulting from specific arrangements of trillions of parts, most notably in higher-order life forms. In certain instances, it behaves similarly to a particle (Magicka may only be expended in multiples of an exceedingly small number), but it can be generally compared to a charge (albeit, on a much larger scale; charge is measured at the ionic level, whereas magic is measured on a material scale). Magical laws approximating the arrangement of energies (the equivalent of oxidation numbers for charge, AP students :P) are yet to be fleshed out, but are theoretically possible to arrive at through in-character development (assuming your nation cares...)
Some of Magic’s ties to life are quite complex. Living things generally produce a constant flow of magical energies, which generally emanate outwards. The amount correlates loosely to activity, both mental and physical. Some life forms, particularly sentient beings, produce abnormally high levels of magical energy, when compared to their calorie burn and mental activity. These creatures are typically of species with magical talents, whether they be of the innate variety, or the more complex, ritualistic schools. Incredibly, among many of these species (nearly exclusively sentient), this magical excess increases (sometimes rather dramatically) over generations of heavy magical use, and may also decrease over generations of little use. Changes in the individual’s supply over a lifetime, though present, are much less marked. It is even noted that languages (spoken or written), tools, and other constructs can benefit from a similar mechanic. If a language is frequently used for magic, it becomes more attuned to the flow of energies. The same may go for tools and constructs. As a result, many ancient languages and artifacts are powerful conduits for the use of magic.
No non-living material produces magic naturally. However, organic materials (such as deadwood, feces, bone, and so on), metals, and gemstones are able to hold a stable amount of magic. The underlying principle of this property is saturation; materials are able to hold an amount of magic within a specific range only. Below this range, they absorb energies from their environment. Above this range, they release energies, gradually or violently. Changes in physical state (gas, solid, so on) may result in anomalies, wherein a material is stable outside its range.