Before 1800 only thirty-one elements were known. By 1840, twenty-seven more were discovered. In 1830, Johann Wolfgang D�bereiner pointed out similarities in the chemical behaviour of Lithium, Sodium and Potassium. The other group was chlorine, bromine and iodine.

In 1860, John Alexander Reina Newlands postulated that all elements could similarly be grouped based on the chemical behaviours. In 1864 he noticed that when arranged in the order of increasing atomic weights, every eighth element showed similar chemical properties. He proposed the "Law of Octaves". By 1865 he was able to arrange the known elements into a table of eight columns. He then proposed that the elements be given a number according to their position in the table. This was the first attempt to assign an "atomic number" to each atom.

D.I. Mendeleev

By 1869, Dmitri Ivanovich Mendeleev, a Russia, extended the table to cover eighteen groups, including the transition transition and rare earth elements. For this he was credited with the discovery of the Periodic Table. Working independent a German physicist, Julius Lothar Meyer, came up with a similar periodic tables a few months later.

Mendeleev observed that many elements had similar properties, and that they occur periodically, hence the name, periodic table. His periodic law states that the chemical and physical properties of the elements vary in a periodic way with their atomic weights.

The most noticeable were the similarities in the chemical properties of lithium, sodium, potassium, rubidium, and cesium. And that of beryllium, magnesium, calcium, strontium, and barium.

Mendeleev arranged the atoms in rows of increasing atomic weight, and columns with similar chemical and physical properties. He referred to the columns as groups.

Tellurium and iodine caused Mendeleev some problems. The atomic mass of tellurium was greater than iodine, yet according to the properties tellurium should be in the group before iodine. The reason for this discrepancy is that the properties of the atom is determined by the atomic number (the number of protons in an atom), but the atomic weight is the average weight of the atom. Atom with the same atomic number need not have the same weight. Some atoms may have one neutron more or one neutron less. Such atoms with the same atomic number but slightly different mass is known as isotopes. The present Periodic Table arranged the atoms in order of atomic number rather than mass.

Uranium, with 92 protons in its nucleus, is the heaviest element that occurs naturally in any quantity. To make atoms heavier than that, physicists must bombard uranium with neutrons, or squash smaller atoms together. Physicists think that nuclei are made up of concentric shells of protons and neutrons. A nucleus is particularly stable if it contains just the right number of protons and neutrons to fill its shells without gaps. Theorists believe that atoms ought to start experiencing stabilising shell effects in the region of the periodic table near element 114. Russian researchers sent beams of light atoms, such as calcium, against very heavy atoms such as plutonium or curium to produce atoms of element 114 and 116 that have survived for a few seconds.