The fifth element, boron, is essential for healthy plants. I once wrote a research proposal focused on that topic. Challenging the notion that boron's role in plants is structural in nature, I proposed instead that boron plays a dynamic role in plant tissue (cellulose) growth instead. The proposal went nowhere, maybe because I was wrong, but also perhaps because I was way out of my element. I'll never know the difference.
Whether we humans require boron is still in question: rats apparently do-though only in miniscule amounts. But boron is absolutely crucial for our silicon-based life support systems, in particular, for our computer chip-driven way of life. Boron's primary use however is boring and mundane: the bulk of it is used in borosilicate (pyrex) glass, commonly used in chemistry labs and in cookware; pretty much the rest of it is used in laundry detergents.
Now the word boron ends in "on" just like carbon, but this wasn't always so. Boron used to go by names like "bore" and "boracium" among others. The name "boron" didn't stick (in the English-speaking world at least) until early workers recognized its profound chemical similarity to carbon (btw, I found the ultimate authority on chemical name origins, so unless I think of something original to add, I'm just going to link to van der Krogt from now on. Notice that one can scroll up and down by atomic number or alphabetically. It's a totally cool website IMO *jealous*. And it figures that the guy is Dutch too).
Boron sits atop an imaginary line running diagonally down across the periodic chart which divides the metals from the non-metals:
Boron is one of the so-called metalloids, having chemical properties intermediate between metallic and non-metallic, just as the name suggests. What makes some elements metals and other non-metals is pretty well explained by band theory, but a simplistic view is to invoke electronegativity. Electronegativity is like a measure of electron selfishness: some elements cling so selfishly to their own electrons that they're unwilling to share them, not even with their identical neighbors-no conductance! So the diagonal line also demarcates a certain threshold electronegativity, the non-metals being more electronegative, epitomized by fluorine in the upper right (not shown).
Boron's chemistry is dominated by its electron deficiency, a consequence of its possessing fewer valence electrons than it has available electron valence orbitals. Like carbon, boron builds borocentric molecules, typically with up to 4 other atoms surrounding it. Those four neighbors require that boron contribute four electrons to the magic octet, but boron only has three valence electrons to share (neutral boron actually has five electrons to match the +5 charge of its nucleus, but two of the five electrons are permanently locked away in a helium-like configuration: the filled so-called K-shell in the Kos reference). So when forming tetravalent compounds, boron always comes up one electron short and has to borrow one [LOL-boron builds more house than it can afford!].
Boron is used in the semi-conductor industry to introduce electronic "holes" in the atomic structure of chip material like silicon. When a boron atom assumes a position in a silicon structure normally occupied by a silicon atom, there is a bond missing one electron (or in other words, a hole). These holes facilitate the movement of positive charges or "holes" through materials. Other atoms can be doped into chips to do the opposite, i.e., introduce negative charges (i.e., electrons) and when used in conjunction, truly miraculous things happen which actually allow you to read what you're reading here.
This concludes another boring lesson in the chemical elements.