Everything you wanted to know about the periodic table but were afraid to ask
Science | by Gregory Beatty
The UN has declared 2019 the International Year of the Periodic Table of Chemical Elements. The declaration coincides with the 150th anniversary of the table’s publication by Russian scientist Dmitri Mendeleev in 1869.
But the story goes back much further than that — to the universe’s very beginning 13.8 billion years ago, when the Big Bang occurred and quarks, gluons and other elementary particles that are the building blocks of so-called “baryonic” matter were created.
“Energy was so high then that atoms didn’t exist,” says University of Regina chemist Brian Sterenberg. “As things cooled down, the particles started coalescing into protons, neutrons and electrons.
“At that point, hydrogen and helium atoms formed,” he adds. “As time progressed, the energy continues to go down, and gravity began to act on all this mass to form stars. Then nuclear fusion began, which basically converts hydrogen nuclei into helium nuclei and releases a bunch of energy.”
Stars come in seven main classes. Our Sun (class G) is in the mid-range, both in size and lifespan. It’s about halfway through its projected 10 billion year life. Class O and B stars are much larger, and burn through their fuel in 10 to 40 million years.
As stars age, the range of fusion reactions expands beyond hydrogen and helium, enabling them to make heavier elements such as lithium, carbon, nitrogen, oxygen and so on.
“Iron, element 26, is a tipping point,” says Sterenberg. “Up to then, the fusion reactions are exothermic. So they actually give off energy. Once you get past iron, you run into endothermic reactions, so the elements actually require energy to make them.”
The heavier the element, the more energy required to make it. That makes elements such as silver (47), gold (79) and mercury (80) relatively rare, as they only form in special circumstances such as when stars go nova/supernova at the end of their life.
The two oddballs are beryllium (4) and boron (5). Being so light, you’d think they would be easy to create through fusion. Instead, they’re made through cosmic ray spallation, which breaks down heavier elements. That makes them rare, too.
Groups, Periods & Blocks
If you recall your high school chemistry, the number of protons in an atom determines its atomic number. That’s matched by the same number of electrons, while the protons and neutrons, which are in the nucleus, determine the atomic weight.
A French chemist proposed the first periodic table in 1789. Other attempts followed, with chemists trying to order the elements based on atomic weight.
“Mendeleev started that way too, but then he also started looking at chemical properties, and he realized two things,” says Sterenberg. “One is that atomic weights don’t always go in the correct order. So he actually switched some, putting heavier elements first and lighter ones later, based on their chemical properties.
“The other thing Mendeleev realized that his predecessors didn’t is that there were missing elements that hadn’t been discovered yet. That’s where the table became really important, because it was making predictions. One example is germanium. On the table, it’s atomic number 32. It hadn’t been discovered yet, but he predicted its discovery, and also predicted quite well some of its properties, just based on his table.”
Mendeleev’s table organizes the elements in three ways (groups/columns, periods/rows and blocks) all of which are electron dependant.
“Say you look at column 17 which are the halogens: fluorine (9), chlorine (17), bromine (35) and iodine (53),” says Sterenberg. “The reason they’re all in the same group is their outer electrons have the same configuration. They have what we call seven ‘valence’ electrons which mean there’s great similarity in their chemistry.”
As atoms grow larger, their electrons get clustered into shells around the nucleus. That’s what the rows represent, says Sterenberg. “If you look at period one, which contains hydrogen (1) and helium (2), the outer electrons are in the first layer. Then with period two, lithium (3), beryllium, boron, carbon (6), nitrogen (7), oxygen (8), fluorine and neon (10), the outer electrons are in the second layer.
“Then you go to the third row, sodium (11), magnesium (12), aluminum (13) and so on, the outer electrons are in the third shell. That’s what determines the rows.”
Finally, elements are organized into four blocks: S, P, D and F. “What that signifies is the type of orbital the outermost electrons occupy,” says Sterenberg. “‘Orbital’ sounds like orbit, which is what planets do around the Sun. That’s where it comes from, because in the early days that’s what electrons were thought to do.
“But as they dug deeper with the birth of quantum mechanics, they realized that was a poor picture as electrons don’t behave like planets. So what an orbital is is a mathematical description of how an electron behaves in an atom.”
With S-orbital atoms, says Sterenberg, the space electrons move within is spherical. “But if it’s P-orbital, the space is shaped like a dumbbell with two lobes. D-orbital is a more complicated shape with four lobes, while an F-orbital is even more complicated with six or eight lobes.”
F-block, by the way, is that section of the table set apart at the bottom — you know, like a footnote (ha, ha! Good one scientists). Looking at the arrangement, one might be forgiven for thinking it has scientific significance.
But the reason is much more prosaic.
“It’s a printing convention,” says Sterenberg. “What they did is move F-block out so the table fits conveniently on one page. You can tell by looking at the numbers in period six. Barium is 56, then you jump to lutetium which is 71. So there’s a gap, which is where lanthanum to ytterbium (57 to 70) actually fit. But it makes it harder to print on one page if you put them there.”
The same principle applies for actinium (89) through lawrencium (103) in period seven.
Sterenberg specializes in phosphorus chemistry, where he seeks new ways to bond phosphorus and carbon to create potentially useful compounds. When chemists do work like that, he says, they’re guided by the periodic table and what it tells them about how reactive different elements might be.
“We are always trying new reactions in the lab,” says Sterenberg. “But there are some we don’t bother trying because we know they won’t react. Then there are other things we know will be too reactive. Fluorine is an example. We don’t do any reactions with it because it’s so reactive it’s dangerous. It takes a crazy person to do fluorine chemistry. People do do it, but you have to take precautions because things tend to explode.”
Yet right next to fluorine on the periodic table is neon, which won’t react with anything. Same with its column-mates helium, argon (18), krypton (36) and xenon (54), which are known collectively as the Noble gases.
Another area of research in chemistry involves using nuclear reactors and particle accelerators to create elements heavier than uranium on the periodic table. Uranium, at atomic number 92, is the heaviest natural element.
Scientists have been doing this since the 1940s, and they’re up to 118 now (that would be oganesson, confirmed in 2015). While the synthetic elements are a remarkable achievement, most are of limited practical value, says Sterenberg.
“The reason they don’t occur naturally is because they’re unstable. Depending on the element, it could have a lifetime of a few milliseconds, microseconds, or even nanoseconds. Then they fall apart through a nuclear fission process to lighter elements that are more stable.”
Plutonium (94) and americium (95) are two exceptions. On the lower end of the synthetic scale, they can be made in a nuclear reactor, and have greater stability. Americium is used in smoke detectors, while plutonium is used in nuclear reactors, including to power space probes, and also (sadly) in nuclear weapons.
Scientists speculate that there’s a theoretical limit to how heavy they can go in making artificial elements. What that limit is, they’re not sure. But when they create new elements, they’re engaging in a process that’s as old as the universe itself.
The ultimate irony, of course, is that through their study of the universe, astrophysicists have determined that only about 15 per cent is made up of actual chemical matter. The rest is what’s called dark matter, which includes black holes and neutron/dwarf stars, along with mysterious “non-baryonic” types of matter that we don’t know much about yet.
“One thing I encounter [as a chemist] is a general fear of chemicals and the idea of them being toxic,” says Sterenberg. “But even if the universe has a lot of dark matter, we live in a chemical world made up of the elements.
“Everything you can see and touch is a chemical — including yourself.”