The modern periodic table organizes 118 elements, the simplest substances that make up the universe. It’s organized a bit differently than the first published table, which is 150 years old this month. (Getty Images/iStockphoto)

Your science teacher may pull out a colorful chart when talking about elements, the simplest substances that make up the universe. That chart, called the periodic table, was one of the most important developments in chemistry. An early version of it first appeared 150 years ago this month when a Russian scientist published his method of ordering the elements.

The scientist was Dmitri Mendeleev. Mendeleev grew up in Siberia as the youngest of 14 children. His teenage years were tough. His father died, and the glass factory his mother managed burned down. But his mother arranged for him to go to


Dmitri Mendeleev, a Russian chemist, was the first to publish a periodic table. He did it in 1869. (UniversalImagesGroup/Getty Images)

college in St. Petersburg, then the capital. He became a professor and began writing Russian chemistry textbooks. In one textbook, Mendeleev aimed to include a table to explain patterns in the elements.

Mendeleev’s periodic table of 1869 listed only the 63 elements known at the time. Today’s modern table contains 118, each represented by a little box containing information that students and scientists who study chemistry can use in their work. That includes a symbol or abbreviation of each element’s name, such as Ca for calcium, as well as two numbers.

“The first of these is the atomic number, which measures the number of a certain kind of particle called a proton in the nucleus [that is, the center] of an atom,” Michael Gordin told KidsPost. Gordin, a professor at Princeton University, studies science history and has written a book on Mendeleev. He explained using oxygen with atomic number 8.

“It’s in position 8 on the periodic table because it has eight protons in its nucleus,” Gordin said.

Each atom — the smallest piece of an element that has all its properties — contains an equal number of negatively charged particles called electrons. So the nucleus of an oxygen atom is surrounded by eight electrons.

“Those electrons are the key to another property of the periodic table,” Gordin said. “The number and arrangement of the electrons determine how an atom will combine with other atoms to form compounds [such as water]. It turns out that those chemical properties repeat themselves according to a regular pattern — that is, the properties are periodic.”

It’s that property that gave rise to the term periodic table.

“So if you arrange all the elements by order of increasing atomic number, you’ll notice that certain chemical


Mendeleev’s early table listed only 63 elements but he left room for those that he thought would be discovered. (Science History Images/Alamy Stock Photo)

properties seem to repeat, and you can place elements with similar properties under each other,” said Gordin, referring to the columns or groups numbered 1 through 18 in the modern table.

The second number in the periodic table boxes is called the atomic mass or weight. Hydrogen has an atomic mass of 1 and calcium is 40 (to the nearest whole numbers), which means a calcium atom is 40 times heavier than an atom of hydrogen.

Because protons and electrons were unknown in Mendeleev’s day, he arranged his elements in order of increasing atomic mass. But what was most amazing about his table was that he left spaces for elements he believed would be discovered.

“He used the regularities visible in his arrangement to predict in detail the properties of three yet-undiscovered elements,” said Gordin, referring to the metals gallium, scandium and germanium (discovered in 1875, 1879 and 1886, respectively). “Predictions of this sort were completely unheard of in the chemistry of his day, and they solidified the reputation of his table. That’s why we are still talking about his version of the table today.”

Fun fact

While most elements are metals, they are not all hard solids. Gallium would melt in your hand while indium is so soft you can scratch it with a fingernail.

The elements and
the solar system

The names of several elements are very similar to the names of some of our solar system’s bodies, which were named after ancient mythical gods. Uranium (Uranus) is named after the Greek god of the sky, while neptunium (Neptune) and plutonium (Pluto) got their names from Roman gods for the sea and the underworld, respectively. Mercury is both an element and a planet and named after the winged Roman god of travel. The names of the elements helium and selenium come from Greek names (Helios and Selene) for the sun and moon gods, respectively.