1869: Mendeleev’s Periodic Table of Elements

The periodic table of the elements is a cornerstone of modern chemistry and an iconic visual representation almost all school children are familiar with.  Its origins trace back to 1869, when Russian chemist Dmitri Mendeleev, the inventor of the periodic table in 1869, published his groundbreaking system. Aside from a few minor changes we still use Mendeleev’s system of organization for our modern periodic table.

The Development of the Periodic Table of Elements

Title Page to Volume I of Dmitri Mendeleev's Principles of Chemistry
Title Page to Volume I of Dmitri Mendeleev’s Principles of Chemistry

By the middle of the 19th century scientists had discovered dozens of elements, but there was no way or organizing them. By 1869 there had been a total of 63 elements discovered and isolated.  As the number of known elements was increasing, several scientists began to notice relationships among some of elements and patterns in how they combined with each other.  Scientists had been trying to develop a classification system of elements for decades for the known elements for decades but no agree upon system had been reached.  For one example, an English scientist named John Newlands proposed the Law of Octaves in 1865.  He noticed that every eighth element shared similar characteristics when arranged by atomic weight, much like the notes in a musical octave.  In music, an octave consists of eight notes, with every eighth notes a repetition of the previous note just at a higher pitch.

There were however limitations to his law. It has limited applicability, working reasonably well for the lighter elements but breaking down for heavier elements. It also had limited predictive power as it made no room for undiscovered elements. For these and other reasons it was not accepted by all scientists of the time, although it did have a role in the evolution of the periodic table by emphasizing the importance of looking for patterns in the elements of nature. 

The Inventor of the Periodic Table in 1869: A Visionary Approach

Enter Dmitri Mendeleev, a chemistry professor at the University of St. Petersburg and the true inventor of the periodic table. In 1861, at age 27, he earned his doctorate after studying abroad in Heidelberg, where he worked with leading chemists like Robert Bunsen, sharpening his analytical skills. A few years later, in 1867, he began working on publishing a chemistry textbook titled Principles of Chemistry. It was his research for this textbook that led him develop this relationship between the chemical properties of the elements to their atomic weights.

Mendeleev’s journey to becoming the inventor of the periodic table in 1869 was not without its quirks and obstacles. Legend has it that Mendeleev conceived the periodic table’s structure in a dream after struggling with the problem for days, arranging element cards on his desk like a game of solitaire.

In any case, his key breakthrough came when he organized his table in order of increasing atomic weight.  He placed elements with similar properties underneath each other.  In a few instances where it made sense, he swapped some elements out of order of increasing atomic weight to better line up the chemical properties. In doing this he inadvertently set up his table by increasing atomic number rather than atomic weight. By the time his table was finished he had discovered what is now called the Periodic Law, which means that the physical and chemical properties of the elements repeat in a periodic manner.  

Mendeleev's Periodic Table of Elements
Mendeleev’s Periodic Table of Elements

Mendeleev’s true genius came in the fact that he left spaces on his table, correctly predicting the existence of elements that had yet been discovered.  He even predicted the properties of these missing elements based on their position in his table.  For instance, he correctly predicted the existence of elements that would later be known as gallium, scandium, and germanium.  Although his table was initially met with skepticism, when these elements were eventually discovered and their properties closely matched Mendeleev’s predictions it provided a strong validation of his table, quickly leading to its acceptance.

The Modern Periodic Table

Mendeleev’s table has continued to evolve as new elements were discovered and the understanding of the atomic structure increased. It is a product of the collaborative efforts of many scientists involving a few important changes and additions.

Most notably, thanks to the work of Henry Moseley, we now organize the table by atomic number, which is the total number of protons in the nucleus.  This change happened as a result of the discovery of isotopes and lead to the realization that atomic number is the fundamental basis for the organization of the elements.  In addition to that change the modern periodic table now includes over 100 elements, up from the 63 known to Mendeleev when his first table was published.

The Modern Periodic Table of Elements
The Modern Periodic Table of Elements: The table today contains 118 known chemical elements
(Credit: American Chemical Society)

The are a few other changes and additions to make note of.

  • Noble gases: the original table did not include the specific group of noble gases as these elements were not yet discovered.
  • Electron structure: the modern periodic table is often presented with electron configurations for each element.
  • Improved measurements: significant advances in technology have allowed for more precise measurements of atomic weight and structure.
  • Filling of d- and f- blocks: these blocks were not fully understood during Mendeleev’s time and have been refined to reflect their electronic structures and chemical properties.
  • More comprehensive periodic trends:  the modern periodic table provides more information about periodic trends such as atomic radius, ionization energy, and electron affinity.

The Importance of the Periodic Table

The periodic table is an indispensable tool for chemists and educators worldwide.  Its organization captures the complexity of the natural world in a simple framework that easily shows the relationships, properties, and reactivity between the chemical elements.

The periodic table also provides a wealth of information about the chemical elements.  It contains information about the atomic structure and weight, electron configuration, valence electrons, and chemical reactivity.  All of this information provides insights into studying the elements and for manipulating matter at the molecular level.  One area of study where this information is particularly useful is in the field of material science.  By understanding the periodic trends, scientists have been able to design and engineer new materials with specific characteristics for a wide range of applications such as in electronics, energy production, agriculture, and medicine.  

The periodic table of the elements was a triumph in scientific reasoning due to its predictive power. It stands as one of the most celebrated foresights in the history of science, serving as a prime example of the power of science to uncover hidden order in the universe.

Continue reading more about the exciting history of science!

Alessandro Volta

Alessandro Volta portrait
Alessandro Volta

Alessandro Volta (1745 – 1827) was an Italian physicist and chemist who was a pioneer in electricity.  He is best known for inventing the battery, which was the first artificial source of continuous electrical current.

Volta was born in Como, Italy, was educated first at home and then at a Jesuit school.  He became interested in electricity at an early age and began publishing papers on the topic in his early twenties.  By 1774 he had  became a professor of physics at the Royal School in Como. One of his first major accomplishments was the discovery and isolation of methane gas. 

In 1791 his friend Luigi Galvani published his views of animal electricity, a form of electricity Galvani believed to be generated in the bodies of animals which would flow through the nerves, causing muscles to move.  Galvani experiments showed that when two different metals came into contact with a dead frog, the frogs muscles would twitch.  Volta disagreed with Galvani on his animal electricity hypothesis as he believed that it was the through the contact of two dissimilar metals that caused the electric current to flow.  In an attempt to prove this Volta built what he called the Voltaic pile, the first ever electric battery. 

The Voltaic pile provided a powerful too for other scientists to made additional observations and discoveries in the field of electricity.  It was easy enough for anyone to make.  Within weeks it was used to dissolve water into hydrogen and oxygen and within a few years newer and more powerful batteries were being created and used to isolate new chemical substances.

In 1791 Volta was already deemed an expert in electricity and he was elected to be a Fellow of the Royal Society.  Impressed by his battery and work on electricity, Napoleon Bonaparte made him a Count in 1809.  Volta died in 1827 but his legacy would live on.  In 1881 in recognition of his fantastic contributions to electrical science the term volt would be the official SI unit of electric potential.

Luigi Galvani

Luigi Galvani portrait
Luigi Galvani

Luigi Galvani (1737 – 1798) was a pioneer of bioelectricity who made important contributions in physics, chemistry, and biology.  He was the first person to demonstrate the electrical basis for nerve impulses when he made a dead frogs muscle move when he jolted it with electricity.

Galvani was born in Bologna, Italy and obtained a degree in medicine at the University of Bologna in 1759.  After graduation he applied for a position at the university and became a lecturer of anatomist of the university.  In the 1770s Galvani had started to become interested in the relationship between electricity and life.

During experiments he conducted, Galvani realized that he could use electricity to make the dissected legs of a frog contract.  For example, when Galvani used a scalpel made of steel to cut through the leg of a frog hanging from a brass hook, the leg visibly twitched.  Based on his observations Galvani concluded that the body contained a type of electrical fluid which he dubbed animal electricity.  He knew that his conclusions would be controversial and he delayed publishing his work until 1791 when he published Commentary on the Effect of Electricity on Muscular Motion.

Some of his scientific colleagues accepted his views, but he received opposition from the Italian physicist Alessandro Volta.  Volta believed that it was through the contact of two dissimilar metals, such as the steel in the scalpel and the brass of the hook, that caused the electric current to flow.  In response to Galvani Volta invented the Voltaic Pile, the first battery.  He realized that the frog’s leg served as a conductor of electricity (electrolyte) and he replaced the leg with brine-soaked paper placed in between to pieces of metal.  Volta’s conclusion ultimately proved correct however Galvani was still correct in attributing the muscular contractions to the electrical stimuli.  Where Galvani was wrong was in the idea of an inherent animal electricity operating within the body.

Galvani continued to investigate animal electricity until the end of his life.  It is his work that inspired Mary Shelly to write her famous work Frankenstein in 1818.  Galvani died at his brothers home in December 1798.