MadSci Network: Chemistry

Re: Equation for burning steel wool?

Date: Thu Mar 9 21:48:27 2000
Posted By: David B. Vance, Staff, Environmental Processes, ARCADIS Geraghty & Miller
Area of science: Chemistry
ID: 951349443.Ch

There are two possible reactions when metallic iron is burned.

The first is:

2Fe0 + O2 = 2Fe0

Which is ferrous (Fe2+) oxide

Another possible reaction is:

4Fe0 + 3O2 = 2Fe2O3

Which is Ferric (Fe3+) Oxide. This is also the mineral Hematite.

Why these reactions occur with iron has to do with the way electrons are arranged around the nucleus of the iron atom. An iron atom has 26 protons (which have a positive charge) in its nucleus and it takes 26 electrons around the iron nucleus to neutralize that charge. These electrons occupy discrete shells around atom. The number of electrons in each shell and the energy level of each shell varies. The name of the electron shells (and energy levels) from the inside out are 1S (with two electrons), 2S (with two electrons), 2P (with 6 electrons), 3S (with 2 electrons), 3P (with 6 electrons), 3D (with 10 electrons), and 4S (with 2 electrons). Which puts us in the range of the iron atom. Iron is called a transition metal, which means that instead of the shells being filled is sequence when the 3D level is reached, the 2 electrons in the 4S shell are filled first. Then the electrons in the 3D shell are filled. Iron has 2 electrons in the 4S shell and 6 in the 3D shell. To add them up from the inside out in the order that they are filled:

1S 2 electrons
2S 2 electrons
2P 6 electrons
3S 2 electrons
3P 6 electrons
4S 2 electrons
3D 6 electrons

For a total of 26.

Read closely as it gets more confusing. Even though the 4S electron shell is filled first as transition metal atoms are formed and then additional electrons go into the 3D shell, when iron loses electrons to become a cation (positively charged, i.e. Fe2+ or Fe3+) the 4S electrons are lost first, then D electrons. Which is the reverse of the electron filling order. It is very easy for the iron atom to lose its two outer most 4S electrons and develop a plus 2 positive charge (the ferrous ion). In the case of iron, the energy of the 6th electron in the 3D shell is almost exactly equal to that of the 4S electrons, so it is also readily lost. After that however, the energy in the remaining 5 electrons in the 3D shell goes up significantly so it is much more difficult to lose them. There are iron ions that have lost 4 electrons, but the number of forms is very limited and they are produced mostly in laboratories under exotic conditions.

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