Cryogenics for English Majors

Something Special about Helium

All in a hot and copper sky,
The bloody sun, at noon,
Right up above the mast did stand,
No bigger than the moon.

Helium is one of the main ingredients in our sun. It’s cooked to perfection inside the core of the sun (as well as other stars) in a recipe called nuclear fusion: Two hydrogen atoms coalescing into a helium atom.

In fact, “helium” comes from the Greek word for sun (helios) and was named in the late 1800s, when it was discovered. The element was first spotted by an astronomer studying a solar eclipse; he noticed a bright yellow emission line that didn’t correspond to any known element.

Turns out the “new” element was as old as the Big Bang. Further study determined that it was quite light; the second lightest element around, after hydrogen. Its atomic number is a measly 2 – meaning it is home to two protons, two neutrons and two electrons. We’re talking extremely low density. As you’ve witnessed in many a birthday balloon, it’s lighter than air. This is one of helium’s nifty features.

Another distinguishing feature of helium is that it belongs to a category of elements from the Periodic Table called inert gasses. These guys are like old bachelors, set in their ways, very much disinclined to hook up with other atoms. Helium is the inertest of the inert. While other elements feature electron distributions that predispose them to binding with other atoms to form molecules, life for the helium atom feels complete with its own two electrons contentedly orbiting. These fuddy-duddies like things the way they are, and they like their space.

Fellow featherweight hydrogen provides an interesting contrast. It feels incomplete with its single electron and seeks companionship either with other hydrogen atoms (H₂) or by forming a trio with another hydrogen atom and an oxygen atom, an arrangement also known as water. Once it finds this bond, hydrogen is loath to let it go. That’s why, compared to other substances, water has such a high boiling point (100 degrees Celsius, or 212 degrees Fahrenheit): The hydrogen molecules in water are quite attached to each other and hold on as long as they can.

Why else is helium special? It’s the only element capable of becoming, at super low temperatures, a superfluid, an amazing state akin to superconductivity in solids. While superconductive solids can carry an electrical current without resistance, a helium superfluid can flow unhampered by any viscosity whatsoever; it even flows up! This property, much studied at the MagLab and elsewhere, will be discussed later.

Light weight, inert, low density: Pretty neat element! But as far as making it a super cryogen, one more property is needed to clinch the deal.

Next Page Cold Enough for Ya?

1 | 2 | 3 | 4 | 5 | 6 | Links | Full Article