The Table was built upon the calculations of atomic weights, also often called the relative atomic mass. In the 19th century, chemists measured the mass of equal volumes of various elements and compared those masses relative to the mass of hydrogen, which is the lightest element. Hydrogen was assigned the value of 1. Thus, carbon, which is about 12 times as heavy as hydrogen, has the atomic weight 12. (Carbon is the modern standard for atomic weight.)
Later discoveries would lead to the understanding that most hydrogen atoms consist of just one proton. Other elements have higher atomic weights, because their atoms possess more protons and neutrons.
It was only in the 1930s, after the discovery of the neutron and proton, that elements started being classified according to the number of protons, rather than the atomic mass. Nowadays, the Table uses atomic number — this is the number of protons in each atom.
The atomic number is unique to every element, since the number of protons in the nucleus will be the same. The atomic weight may differ for different isotopes of the same element, since there may be a different number of neutrons. Hydrogen for example, has three naturally known isotopes. These are naturally stable hydrogen (“Protium”) Deuterium and Tritium with the respective atomic weights of 1, 2 and 3. All the isotopes have an atomic number of 1.
Many elements are hard to find in nature due to being radioactive. Radio-active elements emit particles and transform into more stable elements. Many radioactive elements have brief half-lives — within a few minutes or seconds half the mass on average, will be transformed into a more stable element through decay.
The Earth has existed for over 4.6 billion years. Many elements with relatively short half-lives may have been present but have decayed to the point where they can no longer be found. Such highly radio-active elements can be synthesised by pumping protons into the nucleus of more stable elements.
Meitnerium for example, is a synthetic element with a half-life of just 4.5 seconds. It can’t be found in nature. Many isotopes of common elements such as oxygen -12 and hydrogen -7 are also fleetingly seen in labs because they have half-lives of just fractions of a second.
As chemists started to hunt for new elements, the Table was gradually filled. All the elements between Hydrogen (Atomic no 1 ) and Plutonium (94) occur naturally on Earth. But many of these were synthesised first, before being discovered in nature.
From 95 (Americium) onwards, all the elements have to be synthesised. The first element discovered via synthesis was Technetium (At no: 43) in 1937. Mendeleev had predicted its existence and called it “Ekamanganese”. It does occur naturally. The first completely synthetic element was Curium, discovered in 1944 by bombarding plutonium with alpha particles.
The most recently discovered element was Tennessine (At no: 117) which was found in 2010 by American-Russian collaboration. Researchers at the Joint Institute for Nuclear Research in Dubna (Russia) bombarded Berkelium (At Wt 97), itself a synthetic element, with Calcium (At Wt 20) beams to force the Calcium protons to merge into the Berkelium atoms. Similar bombardment led to the earlier discovery of Oganesson (At Wt 118) in 2002.
Very little is known about the properties of the transuranic elements (beyond At Wt 92) because most are synthetics with short half-lives. Studying them in labs to discover their chemical properties is hard. At the same time, researchers are trying to find elements at 119 or beyond. The Table remains an invaluable resource for classification in its 150th year.