10th Class Physical Sciences – Classification of Elements – Periodic Table – Synopsis of the Lesson – Part 1

At the time of Robert Boyle (1661) only 13 elements were known.
Towards the end of the eighteenth century, by the time of Lavoisier another 11 elements were discovered.

By 1865 about 63 elements were known.
By 1940, a total of 91 elements from natural sources and another seventeen elements synthetically were obtained.
By now, including synthetic elements, there are more than 115 elements.

See: 10th Physical Sciences – Classification of Elements – Synopsis – Part – 2

There was a need to classify these elements. Chemists started to frame ways to group these elements and compounds on the basis of their physical and chemical properties.

In the beginning of 18th century Joseph Louis Proust stated that hydrogen atom is the building material and atoms of all other elements are simply due to the combination of number of hydrogen atoms.

A German chemist Johann Wolfgang Dobereiner (1829) noted that there were groups of elements with three elements known as triads in each group with similar chemical properties.

According to Dobereiner the atomic weight of the middle element is the average of the atomic weights of the first and third element. This law failed for very low mass and for very high mass elements.

Newlands a British chemist in 1865 found that when elements are arranged in ascending order of their atomic weights, they appeared to fall into a pattern in which their properties repeat at regular intervals.

Every eighth element starting from a given element resembles in its properties to that of the starting element. This is law of octaves.

Newlands periodic table was restricted to only 56 elements and did not leave any room for new elements. Elements that were discovered later could not be fitted into Newlands table in accordance with their properties.

Mendeleev arranged the elements known at that time in a chart in a systematic order in the increasing order of their atomic weights. Mendeleev divided the chart into 8 vertical columns known as groups.

The horizontal rows of Mendeleev’stable are called periods. The Mendeleev’s periodic law states that the physical and chemical properties of the elements are a periodic function of their atomic weights.

Based on the arrangement of the elements in the table Mendeeleev predicted that some elements were missing and left blank spaces at the appropriate places in the table. He predicted the properties of these new additional elements in advance purely depending upon his table.

He named those elements as eka-boron, eka-aluminium and eka-silicon. Later these were discovered as scandium, gallium and germanium respectively. There were limitations of Mendeleev’s periodic table. Anomalous pair of elements: certain element of highest atomic weights precede those with lower atomic weights. Dissimilar elements placed together in the table.

In 1913 H.J. Moseley found that each element emits a characteristic pattern of X- rays when subjected to bombardment by high energy electrons. The periodic law is changed from atomic weight concept to atomic number concept and it is now called the modern periodic law.

The modern periodic law is stated as ‘the properties of elements are the periodic function of their atomic numbers’. The modern periodic table is the extension of the original Mendeleev’s periodic table known as short form of the periodic table.

The modern periodic table is called the long form of the periodic table. The physical and chemical properties of atoms of the elements depend not on the number of protons but on the number of electrons and their arrangement (electronic configurations) in atoms.

The modern periodic law may be stated as ‘the physical and chemical
properties of elements are the periodic function of the electronic
configurations of their atoms’. The modern periodic table has 18 vertical columns known as groups and 7 horizontal rows known as periods. Depending upon to which sub-shell the differentiating electron i.e., the last coming electron enters in the atom of the given element, the elements are classified as ‘s’, ‘p’, ‘d’ and ‘f’ block elements.

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