With the advancement in technologies, the number of elements discovered increased. So, for their better understanding scientists started making attempts to classify elements according to their properties and obtain an orderly arrangement. In the year 1817, Johann Wolfgang Döbereiner, a German chemist, tried to arrange the elements with similar properties into groups. Döbereiner identified some groups having three elements each. Döbereiner called these groups as Triads. The limitation of Döbereiner triads is all the elements could not be grouped into triads. The attempts of Döbereiner encouraged other chemists to correlate the properties of elements with their atomic masses. In 1866, John Newlands, an English scientist, arranged the then known elements in the order of increasing atomic masses. According to Newlands, when elements are arranged in the order of increasing atomic masses, the properties of the eighth element are a repetition of the properties of the first element. This repetition in the properties of elements is just like the repetition of eighth note in an octave of music. Therefore, he called it the ‘Law of Octaves’. It is known as ‘Newlands’ Law of Octaves’. It was found that the Law of Octaves is applicable only up to calcium. Even after the rejection of Newlands’ Law of Octaves, many scientists continued to search for a pattern that correlated the properties of elements with their atomic masses. The main credit for classifying elements goes to Dmitri Ivanovich Mendeléev, a Russian chemist. Mendeléev arranged the elements in increasing order of their atomic masses and according to their chemical properties. Mendeléev even predicted the existence of some yet to be discovered elements on the basis of gaps in his Periodic Table. Mendeleev's Periodic table has merits and demerits.

Anomalies in arrangement of elements based on increasing atomic mass could be removed when the elements were arranged in order of increasing atomic number, a fundamental property of the element discovered by Henry Moseley. Henry Moseley gave the modern periodic law, which states that, the physical and chemical properties of elements are a periodic function of their atomic numbers. If the elements are arranged in the order of their increasing atomic numbers, the elements with similar properties are repeated at regular intervals. Elements in the Modern Periodic Table are arranged in 18 vertical columns called groups and 7 horizontal rows called periods. Elements thus arranged show periodicity of properties including atomic size, valency or combining capacity and metallic and non-metallic character.

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