The Chemistry of Iron

Welcome back to metallurgy Monday! Today we’re going to explore metallic bonds and why iron is super special.

If you can remember high school chemistry, you probably remember what a covalent bond is - like water a.k.a H2O - where there is one oxygen atom with two hydrogen atoms bonded to it, making a polar covalent bond. The oxygen atom has two free spaces and the hydrogen atoms bond to those spaces. So, oxygen shares one of its outer electrons with each of the two hydrogen atoms, and each of the two hydrogen atoms shares it's one and only electron with the oxygen, creating a covalent bond. Each hydrogen atom thinks it has two electrons, and the oxygen atom thinks that it has 8 outer electrons, completing the outer shell. 

Metals are TOTALLY different. 

Metals form "metallic bonds", and these take the form of a lattice where free electrons are traded from atom to atom instead of orbiting their specific atoms like in a covalent bond. This "sea of electrons" allows for things like electrical conductivity and even the ability to weld two pieces of metal together.

Iron is special because, at its lowest energy state, the atoms arrange into a cube; at room temperature we call it Body Centered Cubic (BCC). When stacked into a lattice the cube has one atom in the center and atoms sharing space on each corner, about ⅛ of an atom per corner.

On its own, pure iron is pretty standard, as far as metals go. It is ductile, meaning it can be drawn out and stretched very thin into shapes like wire, and it’s malleable, meaning it can be hammered or otherwise deformed without cracking or breaking. What makes iron special is its ability to bond with a non-metal: Carbon. Carbon is what makes up most of the universe. It’s the 4th most abundant element by mass. It’s what makes organic systems work by giving us a crazily diverse amount of organic compounds, and the only thing you need more of in your body to be able to live is oxygen. The special connection between iron and carbon wasn’t formally defined until after Antoine Lavoisier published his experiments surrounding carbon in the mid-1770s. Even then, it took almost 80 years to really start to unlock the secrets and begin producing reliable steel. If we go back to how hydrogen and oxygen make water, we know that hydrogen and oxygen alone have no properties that even remotely resemble water. Together they make molecules and the emergent property of those molecules result in what we call water. Iron and carbon have a similar relationship. Iron is a metal with certain properties, carbon is a non-metal with certain properties, together they form an alloy with properties different than the two parts have separately. Carbon is easy to alloy with iron because the carbon atom is JUST the right size to fit in the gaps, or INTERSTICES between the iron atoms. There is a lower and upper limit to how much carbon is beneficial vs how much is detrimental, but for now we can see how adding carbon will strengthen or stiffen the structure by filling in the empty spaces, right?

The ability of iron and carbon to bond as an alloy is one of the leading factors that changed the world and gave us everything we have today.

Next Monday we’ll look at how carbon ratios affect steel and how that affects our every day lives!