Early wheels were simple wooden disks with a hole for the axle. In the early Roman empire, most horse-carts used a design featuring two chords across the wheel. The spoked wheel was invented much more recently, and allowed the construction of lighter and swifter vehicles.
Chromium(III) sulfide. Chromium(III) Sulfide is the inorganic compound with the formula Cr2S3. It is a brown-black solid. Chromium sulfides are usually nonstoichiometric compounds, with formulas ranging from CrS to Cr0.67S (corresponding to Cr2S3). Preparation.
Phosphate has a charge of 3-, so we will have 3 magnesium cations. We end up with our final formula: Mg3(PO4)2. Ions are not in parentheses unless they are polyatomic, and there is more than one of the polyatomic ions. This is why the NO3 in NaNO3 is not in parentheses, and in parentheses in Mg(NO3)2.