Element Elements and Compounds Structure of Atom Compounds A compound is a substance composed of two or more elements, chemically combined with one another

ElementElements and CompoundsStructure of Atom

Compounds

A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.

Water, sugar, salt, carbon dioxide, methane etc are the examples of compounds.

Classification of Elements and Periodicity in Properties

Genesis of Periodic Classification

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.


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Dobereiner’s Triads

In the year 1829, Johann Wolfgang Dobereiner, a German scientist, was the first to classify elements into groups. He grouped the elements with similar chemical properties into clusters of three called 'Triads'. The distinctive feature of a triad was the atomic mass of the middle element.

When elements were arranged in order of their increasing atomic mass, the atomic mass of the middle element was approximately the arithmetic mean of the other two elements of the triad.

Johann Wolfgang Dobereiner


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Examples


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Defects of Triad Classification

A large number of similar elements could not be grouped into triads e.g., iron, manganese, nickel, cobalt, zinc and copper are similar elements but could not be placed in the triads.

Since he failed to arrange the then known elements in the form of triads his attempt at classification was not very successful.

It was possible that quite dissimilar elements could be grouped into triads.


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Newlands’ Octaves

The English chemist, John Alexander Newlands in 1865 profounded the Law of Octaves.

According to the law of octave, when elements are placed in increasing order of their atomic mass, every eighth element shows similarity with the preceding eighth element in their physical and chemical properties.

John Alexander Newlands


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The relationship of Newlands’ Octaves was just like every eighth note that resembles the first in octaves of music.

Newlands’ Octaves


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Newlands’ Octaves

This classification was good with a few elements only, and was not applicable to all elements. It was not valid for elements having atomic masses higher than Ca.

After the discovery of Nobel gases, it became difficult to fit them in Newland’s periodic table.

Limitations of Law of Octaves


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Lothar Meyer Atomic Volume CurveLothar Meyer plotted the physical properties such as atomic volume against atomic weight and obtained a periodically repeated pattern.

Each fall and rise to a peak corresponds to a period.

Lothar Meyer


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Lothar Meyer Atomic Volume CurveHydrogen is a special case and can be considered as making up the first period all by itself.

The second period included seven elements, from lithium to fluorine; the third period too included seven elements, from sodium to chlorine.

The fourth period had seventeen elements, ranging from potassium to bromine.


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Lothar Meyer Atomic Volume CurveOn the basis of his observations, Lothar Meyer proposed that the physical properties of the elements are a periodic function of their atomic masses.

By 1868, Lothar Meyer had developed a table of the elements that closely resembles the Modern Periodic Table. However, his work was not published until after the work of Dmitri Mendeleev.


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Mendeleev’s Periodic TableIn 1869, Mendeleev classified the then known 56 elements on the basis of their physical and chemical properties in the increasing order of the atomic masses, in the form of a table.

The physical and chemical properties of the elements are periodic functions of their atomic masses.

Mendeleev’s Periodic Law

Dmitri Ivanovich Mendeleev


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Mendeleev’s Periodic Table

Mendeleev's periodic table contains eight vertical columns of elements called 'groups' and seven horizontal rows called 'periods', Each group has two sub-groups A and B.


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The properties of elements of a sub-group resemble each other more markedly than the properties of those between the elements of the two sub-groups.


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Based on the positions in the periodic table, Mendeleev could predict the properties of some undiscovered elements. He left three blanks for elements that were not discovered at that time.


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Mendeleev’s Periodic TableHe predicted the elements as eka-boron, eka-aluminium and eka-silicon. He named them so, as they were just below boron, aluminium and silicon in the respective sub-groups.

Eka-boron was later named as scandium, eka-aluminium as gallium and eka-silicon as germanium.


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Limitations of Mendeleev’s Classification

The position of hydrogen was not correctly defined. It was placed in Group I although its properties resembled both the Group I elements and the group VII elements.

In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses, while some dissimilar elements were grouped together. Thus, the position of these elements was not justified.

For example, cobalt (at. mass 58.9) was placed before nickel (at. mass 58.6); copper and mercury are similar in their properties but were placed separately. Copper was placed in group I although it did not resemble the elements of this group.


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Limitations of Mendeleev’s Classification

According to Mendeleev's periodic table isotopes of an element should be placed at separate places. However isotopes have not been given separate places in the periodic table.

Fourteen elements that follow lanthanum called lanthanides and fourteen elements following actinium called actinides were not given proper places in Mendeleev's periodic table.

Mendeleev's table was unable to explain the cause of periodicity among elements.


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Modern Periodic Table

In 1913, Henry Moseley observed regularities in the characteristic X-ray spectra of the elements. A plot of n (where n is frequency of X-rays emitted) against atomic number (Z ) gave a straight line and not the plot of n Vs atomic mass. He suggested that atomic number is a more fundamental property of an element than its atomic mass.

Henry Moseley


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Modern periodic law states that the properties of the elements are periodic functions of their atomic numbers.


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The modern periodic table is also known as the long form of the periodic table or the extended form of the periodic table.



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The elements are arranged in the order of increasing atomic numbers in such away that elements with similar properties fall in the same vertical column.


These vertical columns are referred to as 'Groups'. There are eight groups, which are further sub-divided into eighteen sub-groups. There are 7 horizontal rows in the periodic table. These are called the periods.

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Element Elements and Compounds Structure of Atom Compounds A compound is a substance composed of two or more elements, chemically combined with one another