How many elements are there? How many can you name? Elements like hydrogen, helium, and carbon readily come to mind, but what about hassium or flerovium?
Some elements, like gold, silver, lead, and iron have been known to us since ancient times. Others, such as nihonium, moscovium, tennessine, and oganesson, were first synthesized in the early 2000’s, but only recently added to the periodic table in 2016.
So what is the periodic table, and how did it come to be?
An Early History of the Periodic Table
By the mid-1800s, over fifty elements had been discovered, and scientists began noticing patterns in some of their properties. In 1817, German chemist Johann Wolfgang Döbereiner was among the first to group elements with similar characteristics. He organized these elements into triplets, which he called “triads.”
In 1862, French geologist Alexandre-Emile Béguyer de Chancourtois took a different approach and arranged elements by atomic weight instead. He noticed that elements displayed periodic properties when arranged this way. Unfortunately, this remarkable find was overlooked because Chancourtois was a geologist and not a chemist.
John Newlands, an English chemist, combined Döbereiner’s and Chancourtois’ ideas to create one of the first periodic tables in 1864. Using his “Law of Octaves,” Newlands ordered the then 63 known elements by atomic weight and grouped them into eight rows according to their chemical similarities. Although Newlands’ table was promising, his Law of Octaves didn’t apply to larger elements. He also sometimes grouped elements that weren’t chemically similar at all (for instance, he grouped iron, a metal, with oxygen and sulfur, both non-metals). Nevertheless, Newlands was on to something. Five years later, Russian chemist Dmitri Mendeleev refined Newlands’ idea to develop the periodic table we know today.
Mendeleev’s Periodic Table
Like those before him, Mendeleev grouped elements by atomic weight and physical and chemical similarities. But unlike his predecessors, who tried to fit elements into spaces on their tables where they didn’t belong, Mendeleev intentionally left gaps in his table as a placeholder for elements that hadn’t been discovered yet. Then, he used the unknown elements’ positions on his table to predict their chemical and physical properties…before they were even proven to exist!
For instance, Mendeleev left a gap on his table beneath the element aluminum. He temporarily named this element “eka-aluminum.” “Eka” is Sanskrit for “one,” and “eka-aluminum” indicated that the unknown element was one space beneath aluminum on his periodic table. Using his table, Mendeleev predicted that this element would have an atomic mass of 68, a density of 6.0 g/cm3, anda low melting point.
Many doubted Mendeleev’s table until 1875, when French chemist Paul-Émile Lecoq de Boisbaudran discovered gallium. This new element had properties almost identical to Mendeleev’s predicted eka-aluminum. Mendeleev also predicted elements that he called “eka-boron,” “eka-silicon,” and “eka-manganese.” Today, we know these elements as scandium, germanium, and technetium respectively. These findings—some which occurred after Mendeleev’s death—paved the way for the widespread acceptance of his periodic table.
Today’s Periodic Table
Mendeleev’s table has since been modified to include elements and element series that were discovered after his death. Today’s periodic table contains 118 elements that are grouped by:
- Groups. These are the 18 vertical columns on the periodic table. Elements within the same group usually have the same electron configurations in their valence shells (the outermost shell of an atom). Elements within the same group tend to show patterns in ionization energy, electronegativity, and atomic radius.
- Periods. These are the 7 horizontal rows in the periodic table. Elements within the same period share the same number of electron shells. Elements within the same period also show trends in atomic radius, ionization energy, electron affinity, and electronegativity, but the trends between elements within a period are not as strong as those within a group. Groups and periods of elements are like families: elements within the same group are more closely related, like immediate family members. Elements within the same period are more distantly related, like extended family members.
- Blocks. These sets of elements are grouped according to which atomic orbital contains the element’s highest energy electrons, called the characteristic orbital. Blocks are named after their characteristic orbital, which are the s, p, d, and f orbitals.
- Element families. All elements are either metals or nonmetals, but are further subdivided on the periodic table. The universally accepted families are: alkali metals, alkaline-earth metals, lanthanides, actinides, transitional metals, post-transition metals, metalloids, nonmetals, halogens, and Noble gases.
Each element on the periodic table is assigned an element square that contains a lot of information about the element. Most element squares contain the element’s:
- Name (think hydrogen, helium…).
- Chemical symbols. This is a one- or two-letter abbreviation for the element. Unlike an element’s name, which varies between languages, an element’s symbol does not. Unsynthesized, yet predicted elements have temporary three-letter designations on most periodic tables.
- Atomic number. This is the number of protons found in a nucleus of the atom. Elements are arranged in order of increasing atomic number on the periodic table.
- Atomic mass. This is the mass of an element’s atom. This is typically expressed in units called atomic mass units on most periodic tables.
Element squares on some periodic tables include information such as the electron configuration, oxidation state, and density.
Will We Discover More Elements?
The most recent additions to the periodic table occurred in 2016. Will we add more elements to the periodic table? Elements heavier than plutonium are synthetic, meaning that they do not naturally occur on Earth and must be produced in a lab. Theoretically, heavier elements are possible, but they are very unstable and difficult to synthesize. Scientists can’t agree as to whether there’s a limit to how many elements can be created. However, if successfully synthesized, we do know that element 119 will start a new row on the periodic table.
The periodic table is a product of hundreds of years of elemental discovery and classification. Today, it is one of the most important tools in chemistry, providing useful information on the characteristics of known and unknown elements.