The Element Boron
Boron, B, has an atomic number of 5 and is the only non-metal belonging to Group 13 of the periodic table. One of the defining features of boron is that it contains an empty p-orbital that allows for acceptance of an electron pair causing it to behave as a strong Lewis acid. Because of this, boron has found widespread applications in sealants, epoxies, cardboard, and synthesis of slime (which you can learn to make here)! Humans tend to enjoy boron, ants not so much.
“Non-boron” Boron Facts
- The name boron comes from the Arabic word “buraq” and the Persian word “Burah”, which means borax.
- Countries including the U.S., Tibet, Chile, and Turkey mine about two million tons of borate annually.
- Boron is not abundant in its pure form and only makes up less than 0.001% of the earth’s crust. In nature, compounds of oxygen contain boron.
- Pure boron is hard and abrasive, yet brittle. It is unlikely for sturdy tools to contain this metalloid; however, ceramics containing boron are commonly used to make cutting tools for use in machining hard metals.
- Borosilicate glass is much more high quality and durable than normal glass. It is ideal for scientific glassware because of its resistance to temperature changes and most chemicals.
- To achieve pure crystalline boron is a difficult process: Boron tribromide (BBr3) or boron trichloride (BCl3) has to get reduced with hydrogen on an electrically heated tantalum filament.
- Boron may have been an important component of the origin of life on Earth. The RNA World Hypothesis states that DNA became the genetic material as a result of evolution and that RNA came first as an unstable molecule. Borates are thought to stabilize the ribose- the building blocks of RNA- meaning that they could have been vital to the formation of RNA and therefore life on Earth.
- Boric oxide has no definite melting point but will begin to soften at a temperature of about 325˚C (617˚F).
- Crystalline boron is rare, expensive but quite beautiful
Boron and the Periodic Table
Boron has atomic symbol B and atomic number 5. It lies to the left of carbon and above aluminum in group 13 of the periodic table. It is a metalloid with crystalline and amorphous forms.
The boron atom has an electron configuration of 1s22s22p1 . It’s 3 valence electrons allow to form a wide variety of boranes and borates. It has much less reactive than aluminum, and has a different aqueous chemistry than aluminum does.
Boron’s Applications in Today’s World
Boron has many uses. It presents in several forms, with the most common being amorphous boron. This is a dark powder that does not react with oxygen, water, acids, or alkalis. When in contact with metals, amorphous boron turns into boride. Amorphous boron produces the green color seen in pyrotechnics and flares. Also, rockets utilize this compound for their ignition source.
Boron filaments are both high-strength and lightweight, making them ideal for aerospace structures.
Boron-10 is an isotope of the element boron. It is fairly unique because of its ability to readily absorb neutrons of low energy. Its functions include nuclear magnetic resonance spectroscopy and the utilization of control rods of nuclear reactors and neutron detectors.
Boric acid is commonly used an an ant and roach killer. It kills them by interfering with their digestive and nervous systems. Luckily, boric acid is not too toxic for humans, with a toxicity similar to sodium chloride (table salt).
History of Boron
Joseph-Louis Gay Lussac and Louis-Jaques Thénard, and Sir Humphry Davy falsely discovered Boron in 1808. They combined boric acid with potassium to isolate the element. While these chemists thought that they had discovered pure boron, it was not until 1909, when Ezekiel Weintraub actually obtained it, falsifying the 1808 discovery.
Boron is nonreactive towards water nor hydrochloric and hydrofluoric acid. The element is picky, and will only react with nitric or sulfuric acid in heated conditions. Under this reaction, boron will oxidize to boric oxide.
At room temperature, boron is chemically inert; It is only reactive with fluorine and oxygen, which will react to form boron trifluoride and boric oxide. However, at high temperatures, the element reacts with almost all metals (excluding tellurium and the noble gases).
Pure boron is not present in nature. Instead, it exists as compounds, such as borax, boric acid, and borates.
Boron forms permanent covalent compounds, like how carbon and silicon does. However, it distinguishes itself with its one less valance electron in its orbital. It exhibits an oxidation state of +3, making it able to form other oxidation states. Electron donor-acceptor complexes form with compounds that have lone electron pairs.
There are different compounds of borons, called borates. This includes borax, boric oxide, and boric acid.
Common compounds of Boron
Synthesis of Silly Putty
One of the most interesting and fun applications of boric acid is in the synthesis of Silly Putty. Silly Putty can be stretched, molded, rolled, you name it, into various shapes and pieces. The main component in Silly Putty that is responsible for this viscoelastic behavior is polydimethylsiloxane (PDMS). The repeating unit is represented by n with the filled atoms as hydrogen in Figure 1.
PDMS does not solely contribute to the viscoelasticity of Silly Putty. When boric acid is added to PDMS, it reacts with the Si-OH groups to form crosslinks as seen in Figure 2. The physical observation seen when Silly Putty is slowly stretched is actually the breaking and reforming of the crosslinks. However, when Silly Putty is forcefully and quickly pulled apart, the elastic behavior is observed and it will snap cleanly into two.
Despite how “silly” Silly Putty is, it has serious applications as well. During the Apollo space missions, Silly Putty was used to secure tools owing to its mild adhesive properties.
Synthesis of Boron
Today, boron is obtained by heating borax with carbon. However, to achieve pure crystalline boron is a difficult process: Boron tribromide (BBr3) or boron trichloride (BCl3) has to get reduced with hydrogen on an electrically heated tantalum filament. Furthermore, external conditions must be strictly regulated to prevent an accidental reaction between boron and another substance.
Boron Oxidation States
Boron’s exists in the oxidation states of +3 and +1, with +3 being more favorable.
Properties of Boron
Melting point: 2350 K; 2077°C; 3771°F
Boiling point: 4273 K; 4000°C; 7232°F
Density: 2.34 g/cm³
Atomic weight: 10.81
Atomic number: 5
Natural abundance in the Earth’s crust: 0.001%
Electron shell configuration: 1s22s22p1, or [He] 2s22p1
Isotopes: Boron-10, boron-11
Found naturally in the minerals: Colemanite, ulexite, tincal, kernite
Toxicity: Toxic at high doses
Where Can I Buy Boron?
With some borax and liquid glue, you can learn how to easily make slime here!