As visual representations of scientific concepts go, the periodic table is hard to beat. It’s easily recognizable, relatively simple to understand and for the last 150 years has acted as a reliable roadmap to help chemists uncover new elements and expand the sum of human knowledge. The American Chemical Society (ACS) described it as “perhaps as foundational to chemistry as the discovery of DNA has been to biology.”

But Dmitri Mendeleev’s elemental Excel spreadsheet has also become a kind of cultural touchstone. Tables of every size and shape are easily available. Want a colored-coded representation of key elemental groups? No problem. Need a dog-themed periodic table of the EleMUTTS? Of course! Want to learn the periodic table through a song set to the tune of Jacques Offenbach’s “Infernal Galop”? Of course you do.

It’s no surprise, then, that the United Nations decided the table’s sesquicentennial was worth recognizing: According to the official UNESCO website, 2019 is the International Year of the Periodic Table. So let’s dive in — how did Mendeleev develop his table? How well does it stand up under pressure? And what happens when the table’s not big enough for all the elements at the party?

Let’s Table This Discussion

As noted by the Royal Society of Chemistry, Dmitri Mendeleev wasn’t the first scientist who attempted to group the elements into some type of easily recognizable framework. In 1789, Antoine Lavoisier classified elements by their properties (gases, metals, non-metals and earths). In 1865, John Newlands noticed similarities in elements of atomic weights that differed by seven. He called it the “Law of Octaves,” later known as the “Periodic Law.”

Mendeleev noticed that certain types of elements regularly reoccurred when elements were arranged according to atomic weight — reactive non-metals were directly followed by light reactive metals and less reactive light metals. He wasn’t afraid to move elements around the table based on their behavior, says the Royal Society of Chemistry.

This ran counter to the notion that elements should be ordered by atomic weight, but it produced better far results. For example, nickel is lighter than cobalt but better aligns with palladium — located in the next row down — than rhodium. Choosing action over atomic weight made Mendeleev’s table much more accurate, especially after protons were discovered; when classified according the number of protons in the nucleus, Mendeleev’s table was spot-on, according to the Royal Society of Chemistry.

The table’s biggest benefit was predictability. Elements directly above or below each other in columns — or “groups” — share similar characteristics. For example, the alkali metals of the first column “usually carry a +1 charge in reactions, react vigorously with water, and combine readily with nonmetals,” as noted by ThoughtCo. Those in the same row — or “period” — share the highest unexcited electron energy level. The table helped paved the way for a better understanding of chemical reactions work, predict outcomes when combining elements of differing groups or periods, and helped pave the way to discover new elements.

Filling In the Gaps

Along with his Law of Octaves, John Newlands also created a version of the periodic table. The problem? Keeping the pattern meant cramming multiple elements into the same space and hoping for the best. Needless to say, the Chemical Society was not impressed.

Mendeleev, meanwhile, knew he didn’t have all the answers. So whenever he came across a potential gap in his table, he left it alone. What’s more, he predicted that new elements would be found to close these gaps — and sure enough, they were. He knew there should be an element after aluminum (which he called “eka-aluminium”) but since no known elements had the right properties, he left the space blank. He also predicted the properties of eka-aluminium, which included an atomic weight of around 68, a solid density of 6.0 grams per centimeter cubed and a valency of three. In 1875, Paul-Émile Lecoq de Boisbaudran discovered gallium, which had an atomic weight of 69.72, a density of 5.9 grams per centimeter cubed and a valency of three — just as Mendeleev predicted, says the Royal Society of Chemistry.

Also of note was the discovery of the noble gases by William Ramsay. While some saw them as contradictions to Mendeleev’s work, further research found their natural home as the final group on the periodic table.

What’s Next for the Nature of the Universe?

Chemists have been regularly adding new elements to the table. As noted by JSTOR Daily, the table currently caps out at element 118 — oganesson — a noble gas that has a half-life of just 0.89 milliseconds. Work is already underway to find elements 119 and 120 but as oganesson’s short-lived existence makes clear, this isn’t an easy task.

Starting with element 105, rutherfordium, artificial preparation is required and researchers have very little time to observe any inherent properties before decay occurs. There are predictions that an “island of stability” will be found — a group of elements with exceptionally stable nuclei because their nuclear shells are filled with the “magic number” of protons or neutrons. According to the JSTOR piece, 184 neutrons is the next predicted magic number.

New elements may also be found using technologies from other disciplines, such as the soon-to-launch James Webb telescope, which will observe and help analyze distant gas clouds. Just because we can’t produce super-heavy, super-stable elements here on Earth, that doesn’t mean some ancient gas cloud isn’t out there doing great work.

And while we’re at it, let’s go really far out on a scientific limb: According to Phys.org, a new paper suggests that once elements chunk their way up over an atomic weight of 300, the rules of normal matter may break down. Here’s the idea: Normal, or baryonic, matter is made of protons and neutrons that contain bound quark triplets. Past 300, there’s speculation that elements may be composed of “freely flowing ‘up’ and ‘down’ quarks” that aren’t bound into triplets. This up down quark matter (udQM) would be stable for super-heavy elements beyond the end of the periodic table and could exist on a “continent” — not just an island! — of stability.

Don’t Mendeleev Me Here

Mendeleev’s work was a critical jumping-off point for the advancement of chemistry, technology and scientific speculation. As our brief history of the periodic table shows, however, this isn’t a find-and-forget situation: There’s always more room for elements left off the first draft (and even some that might be too big for our current table).

Along with recognition through 2019’s International Year of the Periodic Table, Mendeleev himself was immortalized in Element 101: a metallic radioactive transuranic element in the actinide series that was discovered in 1955 and named mendelevium in honor of his work.