The largest portion of the Moon is the mantle. This is the layer between the crust (the part we see) and the inner core. The lunar mantle is believed to consist of olivine, orthopyroxene, and clinopyroxene. The composition of the mantle is similar to that of the Earth, but the Moon may contain a higher percentage of iron.
Scientists have samples of the lunar crust and take measurements of properties of the Moon's surface. The crust consists of 43% oxygen, 20% silicon, 19% magnesium, 10% iron, 3% calcium, 3% aluminum, and trace amounts of other elements including 0.42% chromium, 0.18% titanium, 0.12% manganese, and smaller amounts of uranium, thorium, potassium, hydrogen and other elements. These elements form a concrete-like coating called regolith. Two types of Moon rocks have been collected from the regolith: mafic plutonic and maria basalt. Both are types of igneous rocks, which formed from cooling lava.
Although it is very thin, the Moon does have an atmosphere. The composition is not well known, but it is estimated to consist of helium, neon, hydrogen (H2), argon, neon, methane, ammonia, carbon dioxide, with trace amounts of oxygen, aluminum, silicon, phosphorus, sodium, and magnesium ions. Because conditions contrast sharply between day and night, the composition during the day may be somewhat different from the atmosphere at night. Even though the Moon has an atmosphere, it is too thin to breathe and includes compounds you wouldn't want in your lungs.
Learn MoreNASA's Moon Fact Sheet
What Does the Moon Smell Like?
Chemical Composition of the Earth's Crust
Chemical Composition of Air