## Sunday, July 21, 2013

### Science Clock Series: Part V

Today's number comes from chemistry, and is given by:

$\approx\text{sp. gr. Fe}^{2+}\,\text{Fe}_2^{3+}\,\text{O}_4$ The letters "sp. gr." stand for the term "specific gravity." Specific gravity  is the ratio of the density of a substance to the density of another substance, usually a reference substance of some kind.  The most common reference substance is liquid water which, as you may remember from the first post in this series, has a density of one gram per cubic centimeter.

The chemical formula $$\text{Fe}^{2+}\,\text{Fe}_2^{3+}\,\text{O}_4$$ stands for the chemical compound iron(II,III) oxide with the chemical name ferrous-ferric oxide, found in nature as the mineral magnetite. (“Fe” and “O” being the chemical symbols for iron and oxygen, respectively.) The Roman numerals II and III refer to the oxidation state of the iron atoms in the compound, which are represented in the formula by the superscript +2 and +3 respectively. The oxidation state is basically how many electrons an atoms gains or loses while in a compound. Positive numbers indicate that an atoms has lost electrons (which have a negative charge), and negative means an atom gains electrons.

The subscript 2 and 4 refer to the number of atoms of that kind, so there are two $$\text{Fe}^{3+}$$ atoms and four oxygen atoms. In a stable compound the oxidation numbers should come out to zero (no net electrical charge). Since oxygen atoms almost always have a $$-2$$ oxidation state, they add up to give $$-8$$ to the oxidation state of the compound. There is one iron atom giving +2, and two iron atoms giving +3, for a total of $$2+(2\times3)=+8$$ to the oxidation state, which nicely balances the oxygens and helps ensure the compound is balanced and stable.

Ferrous-ferric oxide as it appears in nature in the form of magnetite has a blackish-brown color with a metallic sheen and has a density of approximately 5.17 grams per cubic centimeter, which gives it a specific gravity of 5.17 (relative to water). Magnetite is the most magnetic naturally-occurring material, and is also where the name magnetism comes from.

Check back next time for another number from chemistry! Click here to jump directly to it.