Helium walks into a bar. The bartender tells him “Sorry, we don’t serve noble gases here.” Helium doesn’t react.
Helium (symbol: He) is the second element – both in terms of atomic number and abundance in the universe. It sits at the top right corner of the periodic table.
Helium is the lightest member of the element family known as the noble gases. The noble gases also include neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), and maybe element 118. The noble gases are unique among the elements in that they all have low melting and boiling temperatures (which explains why they are all gases at room temperature). Also, the noble gases are very unreactive; they do not tend to form compounds with other elements.
I could stop right now and you would understand the joke. Helium doesn’t react because it’s a noble gas, and noble gases don’t react. But there’s much more to tell, so please indulge me while I
bore you to tears explain why noble gases are the way they are. This is your last chance to log off before you learn about electron configurations.
Still here? Good.
We’ve already discussed how the electrons in atoms are arranged into concentric shells (or energy levels) around the atomic nucleus. Actually, it’s a little more complicated than that. Each energy level can be further divided into sublevels. The sublevels are given odd names like sharp, principle, diffuse, and fundamental, but we’ll just call them s, p, d, and f.
The first energy level – that is, the one closest to the atomic nucleus – has only one sublevel. It’s an s-type sublevel, so it’s called 1s (read that as “one-S”). All s-type sublevels can hold 2 electrons, so the entire first energy level can hold 2 electrons.
In atomic physics there’s a guideline called the Aufbau principle. Aufbau is a German word meaning construction. It says that the electrons in atoms fall to the lowest-energy sublevel that is available. Once a sublevel is filled with electrons, the next highest sublevel begins to fill, and so on.
If an atom has one or two electrons, the 1s sublevel is perfectly capable of accommodating them. However, if an atom has 3 or more electrons, it must tap into higher-energy sublevels.
The second energy level is made of two sublevels. It also has an s-type sublevel (in fact, all energy levels contain an s-type sublevel) but it also contains a p-type sublevel. These two sublevels are called 2s and 2p (or not 2p? That is the question!)
Like the 1s sublevel, the 2s sublevel also holds 2 electrons. The 2p sublevel can hold 6 electrons. All told, the second energy level can hold up to 8 electrons.
And so it goes. As the electron population grows, so does the number of sublevels and energy levels. If you’re assigning homes to an atom’s electrons, you continue adding energy levels and sublevels until you run out of electrons. The last sublevel may or may not be full, but every sublevel prior to the last one must be full (except for a few special cases that we’re not going to discuss right now).
The sublevels don’t necessarily get filled in the order you might expect. 1s, 2s, 2p, 3s, and 3p fill with electrons in order, but the next sublevel to be filled after 3p is 4s, not 3d. The 3d sublevel fills after 4s, then comes 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, etc, etc. This arrangement may seem bizarre, but it plays a major role in determining if and how an atom will react with other atoms.
See, the most important electrons in any atom are the valence electrons. Valence electrons are the electrons that inhabit the highest energy level – the one farthest from the nucleus. Atoms that have one through seven valence electrons will generally react with other atoms, either by losing, gaining, or sharing electrons. Atoms that have eight valence electrons, however, are special…eight valence electrons is a very stable arrangement. Why? Because when an atom has eight valence electrons, its highest s-type and p-type sublevels are just filled. Let’s take a look at the arrangement of electrons in each of the noble gas atoms:
Helium (2): 1s2
Neon (10): 1s2 2s2 2p6
Argon (18): 1s2 2s2 2p6 3s2 3p6
Krypton (36): 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6
Xenon (54): 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6
Radon (86): 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6
For each element, the number in parentheses tells you the total number of electrons. The raised numbers tell you the number of electrons in each sublevel. I’ve color-coded the sublevels to make it easier to see which electrons are grouped together in an energy level. Take a look at each of the noble gases (except for helium). Notice how they all have a total of 8 electrons in their outermost energy levels, distributed between the outermost s-type and p-type sublevels.
These arrangements are really stable. Because noble gases have full s– and p-type sublevels in their outermost energy levels, they tend not to gain, lose, or share electrons under normal conditions. That’s why we say that noble gases don’t react with other atoms.
Now of course the joke is about helium, and you’ve probably noticed that helium does not have 8 valence electrons; in fact, helium doesn’t have 8 electrons at all. Regardless, helium still has a complete 1s sublevel, which confers its own kind of stability. Chemically, helium reacts (or more appropriately, it doesn’t react) more like the noble gases than like any other family of elements, and so it is considered to be one of them.
Here are two other interesting tidbits about helium which won’t enhance your understanding or appreciation of the joke, but might give you something to think about.
- The name helium comes from the word Helios, who was a Sun god in Greek mythology. The element was so named because it was discovered first in the Sun, via an anomalous spectral signature in sunlight, 25 years before it was isolated on Earth.
- Despite helium’s abundance in the universe, we’re running out of it on Earth. The looming helium shortage has implications far beyond birthday party balloons.