Grade-11 Classification of elements and periodicity in properties

 Classification of elements and periodicity in properties

 

Why do we need to classify elements?

Before the 19^th century only a few elements were known so they could be easily studied but with time many more elements were discovered and their compounds were prepared hands it was felt that these elements should be classified into a few groups to make their studies systematic and easier it is expected that a systematic classification will help us in organizing the knowledge and predicting new elements for undertaking further studies

Dobereiner’s triad

Dobereiner’s triad:  In 1829, a German scientist John Dobereiner classified certain elements in a Group of three called triad which had similar chemical properties. Dobereiner’s triad states that “when the elements in a triad were arranged in the order of increasing atomic mass is the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the other two elements.”

Limitations of Dobereiner’s Triad classification: Dobereiner’s method of classification could arrange only a limited number of elements the idea of trades could not be applied to all elements

Newlands law of octaves

In 1864 John Newlands an English chemist and a musician arranged the elements known at that time in order of increasing atomic mass which state that the properties of each element resembled those of the 8^th element before it and off the 8^th element following it in other words he found that the properties of the elements were repeated at every 8^th element like the death note of an octave in music Newlands called this as the law of octaves

The drawbacks of Newland's classification are he failed badly with dealing with heavier elements beyond calcium. The idea of octaves could not be held for noble gases.

Lothar Meyer's atomic volume curve

In 1869 a German chemist Julius Lothar Meyer plotted a graph of atomic volume versus the atomic mass of various elements he noticed that the elements with similar properties occupied similar positions on the curve for example lithium, sodium, potassium, rubidium, and cesium occupied the big position. It is the first classification of elements which had a definite pattern and periodicity in properties of elements

 

Mendeleev’s classification

The Russian chemist Dimitri Mendeleev arrange the elements in the increasing order of atomic mass. He observed that the elements with similar properties appeared at regular intervals the Mendeleev spirit law states that, “the properties of the elements are a periodic function of their atomic mass.”

In other words, according to this law when the elements are arranged in increasing order of their atomic masses the elements with similar properties are repeated at regular intervals

Mendeleev periodic table:

The elements were known at that time and are arranged in increasing order after atomic mass in a table known as the periodic table. Elements having similar properties were placed in vertical columns called groups and horizontal rows were called periods. When the order of increasing atomic mass was not rigidly followed then he left gaps in his periodic table and even predicted the properties of such elements.

For example, Mendeleev had predicted the properties of elements with an atomic mass of 72 which he named eka-silicon.

Modern Periodic Table

Modern Period Table was designed by Henry Moseley in 1913 A.D. Modern Periodic Table states that “the properties of the elements are the periodic function of their atomic number.”

In Modern Period table: The regular repetition in elements occurs due to periodicity in the outer electronic configuration of elements. Isotope does not need a separate position to the classification of an element based on the classification of elements. The left portion of the periodic table includes highly reactive metals, the right includes highly reactive non-metals and the middle-portion includes transition metals. Elements were divided into s, p, d, and f blocks.