Happy National Periodic Table Day!

The periodic table of elements is one of the most recognizable icons of chemical science:

A simple periodic table of elements with groups 1-18 labeled across the top, periods 1-7 labeled down the side, and different colored blocks indicating different categories of elements. — *An example periodic table of the elements (image by Offnfopt)*

It may not be as widely-known as next week’s February holiday, but National Periodic Table Day, February 7, is a great excuse to talk about this amazing tool that is still relevant over 150 years after it was first laid out in this familiar format! One of the coolest things about it is that, when Mendeleev first laid it out in the form that we all know and love, it had gaps in it that could predict the existence of elements that hadn’t been discovered yet.

Nowadays all those gaps are filled in, but the elegance of the layout is still pretty amazing. All known elements of matter are organized in multiple ways:

Elements are numbered in sequence on how many protons are in their atomic nucleus, starting with 1 (Hydrogen) at the top left and going up to 118 (so far – Oganesson) at the bottom right
The columns (increasing from left to right) are organized by the atom’s number of valence electrons
The rows correspond to atomic radius (increasing from top to bottom, but also along columns from right to left)
Elements with similar characteristics end up grouped together in families like alkali metals, lanthanides, noble gases, etc.

A schematic of the periodic table, labeled with large arrows showing directional trends. Top to bottom: Atomic radius. Bottom to top: electron affinity and ionization energy. Left to right: Ionization energy and electron affinity. Left to right: atomic radius. Diagonal bottom left to top right: Nonmetallic character. Diagonal top right to bottom left: Metallic character. — *Trends across the periodic table (image by Mirek2)*

There’s so much fun to explore about the periodic table! For just a few examples, you can learn more more about how to read the periodic table, a brief history of the periodic table, and the variety of alternative periodic table designs. We even had a Sustainable Nano post back in 2019 all about Celebrating the 150th Anniversary of the Periodic Table: Nano-Style!

The periodic table has inspired all kinds of offshoots, too. Some chemistry-related like the Six Different Periodic Tables by Compound Interest, and others are tools for categorizing all manner of pop culture such as iPad apps, fictional minerals, and superheroes.

Want to learn about the periodic table in song form? Here’s a fun one that goes through the entire table in order, with illustrations: