The History of the Modern Periodic Table During

The History of the Modern Periodic Table

During the nineteenth century, chemists began to categorize the elements according to similarities in their physical and chemical properties. The end result of these studies was our modern periodic table.

Johann Dobereiner In 1817, he classified some elements into groups of three, which he called triads. The elements in a triad had similar chemical properties and orderly physical properties. (ex. Cl, Br, I and Ca, Sr, Ba) Model of triads 1780 - 1849

DOBERENIER’S TRIAD

Let us take the example of three elements lithium, sodium and potassium. They form a Dobereiner’s triad. Element Atomic mass Lithium (Li) 7 Sodium (Na) 23 Potassium(K) 39 Elements Atomicmass calcium(Ca) 40 barium(Ba) 88 Strontium(K) 137 Mean of the atomic masses of the first (Li) and the third (K) elements: 7+39/ 2 = 23 u The atomic mass of the middle element, sodium, Na =23 u Elements Atomic mass Chlorine 35. 5 Bromine 80 Iodine 127

Döbereiner’s triads. • Döbereiner is responsible for the law of triads. He stated that “Nature contains triads of elements where the middle element has properties that are an average of the other two members of the triad when arranged by the atomic weight”. These are now known as Döbereiner’s triads.

Demerits: • Dobereneir’s idea of classification of elements into triads did not receive wide acceptance as he could arrange only a few elements in this manner.

EXAMPLE Nitrogen 14. 0 Phosphorus 31. 0 Arsenic 74. 9 This triad could not explain Dobereiner’s triad.

John Newlands In 1863, he suggested that elements be arranged in “octaves” because he noticed (after arranging the elements in order of increasing atomic mass) that certain properties repeated every 8 th element. Law of Octaves 1838 - 1898

LAW OF OCTAVE • Newlands devised a Law of Octaves which states that: “Any given element will exhibit analogous behaviour to the eighth element following it in the table”

1838 - 1898 Law of Octaves

law of octaves failed … WHY? 1. It was found that the Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first. 2. It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future. But, later on, several new elements were discovered, whose properties did not fit into the Law of Octaves. 3. In order to fit elements into his Table, Newlands adjusted two elements in the same slot, but also put some unlike elements under the same note. Can you find examples of these from Table 5. 3? Note that cobalt and nickel are in the same slot and these are placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements. Iron, which resembles cobalt and nickel in properties, has been placed far away from these elements. Thus, Newlands’ Law of Octaves worked well with lighter elements only.

Dmitri Mendeleev FATHER OF PERIODIC TABLE. 1834 - 1907

1. Dmitri Mendeleev In 1869, the Russian scientist Dmitri Mendeleev discovered a set of patterns in the properties of the elements. 2. He noticed that a pattern of properties appeared when he arranged the elements in order of increasing atomic mass. 1834 - 1907

• 63 elements were that time. • Mendeleev concentrated on the compounds formed by elements with Hydrogen and Oxygen. • So formula of hydrides and oxides formed by an element were treated as one of the basic properties for its classification.

• He pinned all the 63 elements in the order of increasing atomic masses such a way that most of the elements got a place in a periodic table. • It was observed that there occurs a periodic recurrence of elements with similar physical and chemical properties.

Mendeleev’s Periodic Law The law states that: “The properties of elements are the periodic function of their atomic masses. ”

Characteristic of Mendeleev’s periodic table: Mendele ev’s Periodic Table contains 8 vertical columns called ‘groups’ and 6 horizontal rows called ‘periods’ 1 -7 groups are divided into sub groups A & B, and 8 th group is transition series having three sets of elements in 4, 5, & 6 periods.

Achievements/Merits of Mendeleev’s periodic table ² Classification was done on more fundamental basis i. e. atomic mass. ² He left some gaps in the periodic table and boldly predicted that these elements would be discovered with the similar properties of boron, silicon and aluminum. Hence, he named these elements as Eka Boron-Scandium, Eka-aluminium-Germanium, Eka-silicon�gallium. ² He made the correction in atomic masses of some elements, e. g. Beryllium (before-13 and after correction- 9) ² He kept the heavier elements before the lighter elements because of their similarity in properties with the elements of the subgroups they are arranged in. ² Noble gas elements lwhich were discovered later got the place in the table without disturbing the main table.

LIMITATIONS/DEMERITS OF MENDELEEV’S PERIODIC TABLE ² Position of hydrogen cannot be decided and given a anomalous position. ² The position of Isotopes which are having different atomic masses are placed in same slot because of their similarity in properties. ² Some elements of higher atomic masses are placed before element with lower atomic masses. e. g. : Co (59. 9) placed before Ni (58. 6) Te (127. 9) placed before Iodine (126. 9) ² Dissimilar elements are placed in same group.

Henry Moseley In 1913, through his work with X-rays, he determined the actual nuclear charge (atomic number) of the elements*. He rearranged the elements in order of increasing atomic number. *“There is in the atom a fundamental quantity which increases by regular steps as we pass from each element to the next. This quantity can only be the charge on the central positive nucleus. ” 1887 - 1915

Periodic Table Geography

The horizontal rows of the periodic table are called PERIODS.

The elements in any group of the periodic table have similar physical and chemical properties! The vertical columns of the periodic table are called GROUPS, or FAMILIES.

Periodic Law When elements are arranged in order of increasing atomic number, there is a periodic pattern in their physical and chemical properties.

Alkali Metals

Alkaline Earth Metals

Transition Metals

These elements are also called the rare-earth elements. Inner. Transition Metals

Halogens

Noble Gases

The s and p block elements are called REPRESENTATIVE ELEMENTS.

The periodic table is the most important tool in the chemist’s toolbox!