ICSE Class 9 Chemistry Chapter 05 The Periodic Table

The Periodic Table

Syllabus

Dobereiner's Triads, Newland's law of Octaves, Mendeleev's contributions; Modern Periodic Law, the Modern Periodic Table. (Groups and periods)

General idea of Dobereiner's triads, Newland's law of Octaves, Mendeleev's periodic law.

Discovery of Atomic Number and its use as a basis for Modern Periodic law.

Modern Periodic Table (Groups 1 to 18 and periods 1 to 7).

Special reference to Alkali metals (Group 1), Alkaline Earth metals (Group 2) Halogens (Group 17) and Zero Group (Group 18)

Introduction

It is a human instinct to classify, i.e. to put things of one kind in one group and of another kind in another group.

Towards the end of the 18th and the beginning of the 19th centuries, more and more elements began to be discovered, and their individual properties and compounds had begun to be studied. Soon, the need arose for classifying them so as to make their comparative study easier.

Reasons For Classification Of Elements

Obviously, scientists were in search of characteristics of an element that would never change. It was William Prout who discovered that the atomic mass of an element never differs and that it could therefore form a truly scientific basis for a satisfactory scheme of classification.

1. It helps in studying the elements in an organized manner.

2. It helps in correlating the properties of elements with the fundamental properties of all states of matter.

3. It helps in defining the relationship of one element with another.

In the beginning, attempts were made by chemists to classify elements on the basis of factors such as density, malleability, ductility, etc., and also to consider whether they were metals or non-metals. But none of these early classifications proved satisfactory because:

(i) the number of elements falling in a particular group were so large that it did not serve the purpose of generalization.

(ii) some of the characteristics being considered varied under differing conditions.

(iii) certain elements showed metallic as well as non-metallic characteristics.

Dobereiner's Triads

J. W. Dobereiner a German chemist observed that certain elements displayed similar properties and that they could be placed in groups of three elements each. These groups of three elements each were called triads.

The three elements of a particular triad had similar chemical properties. In fact, Dobereiner was the first scientist to show the clear relationship between properties of an element and its atomic mass.

The atomic mass of the middle element of a triad was approximately equal to the arithmetic mean (average) of the atomic masses of the other two elements of that triad.

The following examples will make this point clear.

(a) Take three elements, calcium, strontium and barium. These elements have following similarities:

(i) All are metals;

(ii) Each has an oxide that is alkaline in nature;

(iii) Each has valency 2.

Since these elements have similar chemical properties, they were put together in one group (or family) to form a triad called Dobereiner's triad.

The atomic mass of calcium is 40, strontium is 88 and barium is 137. It is noticed that the middle element, strontium, has atomic mass approximately equal to the average i.e., mean value of the atomic masses of calcium and barium.

The mean of the atomic masses of calcium and barium = \[\frac{40 + 137}{2} = \frac{177}{2} = 88.5\]

This is nearly the same as the atomic mass of strontium (88).

(b) Take three elements chlorine (35.5), bromine (80) and iodine (127); they also form a triad because:

(i) all are non-metals;

(ii) all react with water to form acids

(iii) all have valency 1.

The mean of the atomic masses of Cl and I = \[\frac{355 + 127}{2} = \frac{1625}{2} = 81.25\], which is approximately the same as the atomic mass of bromine.

Reasons for discarding the law of triad

(i) Dobereiner failed to arrange all the then known elements in the form of triads.

(ii) The law did not fully apply even within the same family.

For example, taking halogens, viz. the first three members, Fluorine (19), Chlorine (35.5) and Bromine (80), it is observed that the mean of the atomic masses of Fluorine and Bromine is \[\frac{1}{2}\](19 + 80) = 49.5, not 35.5.

So, Dobereiner's scheme of classification of elements was not very successful, though it did contain important insights and principles.

Newland's Law Of Octaves

John Newland (a scientist and a lover of music) arranged elements in ascending order of atomic mass and found that every eighth element had properties similar to the properties of the first element, just as the eighth note of a musical octave is the same as its first note. Based on this observation, Newland gave his law of octaves for classification of elements.

According to Newland's law of octaves, when elements are arranged by increasing atomic mass, the properties of every eighth element starting from any element are a repetition of the properties of the starting element.

Newland divided the elements into horizontal rows of seven elements each, as shown above [the noble, i.e. inert, gases were not known at that time].

Merits of Newland's classification

1. This system worked quite well for the lighter elements. For example, lithium, sodium and potassium were brought together.

2. It relates the properties of the elements to their atomic masses.

3. For the first time, it was shown that there is a distinct periodicity in the properties of elements.

Newland was honoured in 1887

The Royal Society presented the Davy Medal to Newland in 1887 for his work on classification of elements.

Reasons for discarding the law of octaves

This classification did not work with heavier elements, i.e. those lying beyond calcium. As more and more elements were discovered, they could not be fitted into Newland's Octaves.

Newland adjusted two elements cobalt (Co) and nickel (Ni) in the same slot and these were 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.

Mendeleev's Periodic Table

In 1869, Dmitri Ivanovich Mendeleev, a Russian chemist, arranged all the 63 elements known at that time in increasing order of their atomic mass. Elements with similar properties were put one after the other in the same vertical column (group), with blank spaces where the expected periodicity of the properties was disrupted, i.e., where the properties of an element did not tally with the properties of the element placed above it. He observed that elements with similar properties occurred at regular intervals. This he called as periodicity of properties of elements.

Based on this, he propounded a law that is called Mendeleev's Periodic Law.

Mendeleev's Periodic Law: Physical and chemical properties of elements are a periodic function of their atomic masses.

Periodic table is a chart of elements prepared in such a way that elements with similar properties occur in the same vertical column (or group). It is called periodic because elements with similar properties occur at fixed intervals (or periods); and it is called a table because elements are arranged in tabular form.

Essential features of Mendeleev's periodic table:

Mendeleev's Periodic table contains vertical columns, called groups and horizontal rows called periods.

There are in all eight groups, i.e. Group I to Group VIII. Each of these groups from I to VII is divided into two sub-groups: A and B. Group VIII has no sub-group. Inert gases were not known at that time.

All elements of a sub-group (or of Group VIII, which has no sub-group) have similar properties and show the same valency, which is equal to the group number (for upto Group IV) or group number subtracted from eight (for Groups V-VII).

In a period, elements gradually change from metallic to a non-metallic character or metallic character decreases as one moves from left to right across the horizontal row.

Mendeleev's Periodic Table was published in a German journal in 1872. He used letter 'R' to represent the element of that particular group. Hydride of any element of group IV is written as RH4. For example, hydride of carbon is CH4. Oxide of any element of the same group is RO2 (CO2).

Merits of Mendeleev's table

(1) Grouping of elements. He generalized the study of the elements then known to a study of mere eight groups.

(2) Gaps for undiscovered elements. In order to make sure that elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table. These gaps were left for subsequent inclusion of elements not known at that time. Mendeleev correctly thought that such elements would be discovered later.

(3) Prediction of properties of undiscovered elements. He predicted the properties of the then unknown elements on the basis of properties of elements lying adjacent to the vacant slots. He actually predicted the properties of some undiscovered elements in 1871.

For example:

(i) Eka aluminium (means one place below aluminium in the group); its atomic mass and chemical properties are quite similar to those of the element gallium discovered in 1876.

(ii) Properties of eka-silicon are the properties of germanium.

(4) Incorrect atomic mass corrected. He was able to correct the values of atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.

Defects in Mendeleev's periodic table

(1) Anomalous pairs:

The following pairs of elements did not follow Mendeleev's principles:

(i) Argon with atomic mass 39.9 precedes potassium with atomic mass 39.1.

(ii) Cobalt with atomic mass 58.9 precedes nickel with atomic mass 58.6.

(iii) Tellurium with atomic mass 127.6 precedes iodine with atomic mass 126.9.

(2) Position of isotopes:

Isotopes of an element are atoms of that element having similar chemical properties but different atomic masses.

According to Mendeleev's periodic law, isotopes of an element must be given separate places in the periodic table since they have different atomic masses. But they were not assigned separate places.

(3) Grouping of chemically dissimilar elements:

Elements such as copper and silver bear no resemblance to alkali metals (lithium, sodium, etc.) but they have been placed together in the first group.

(4) Separation of chemically similar elements:

Elements that are chemically similar, such as gold and platinum have been placed in separate groups.

(5) Electron arrangement:

It does not explain the electron arrangement of elements.

(6) Position of hydrogen:

Hydrogen was not given a fixed position. It was considered in Group IA as well as in Group VIIA because it forms both a positive ion, viz. in HCl, and a negative ion, viz. in NaH.

Atomic Number As Basis For Modern Periodic Law

The magnitude of positive charge present in the nucleus of an atom was determined by Henry Moseley, an English physicist.

In 1913. Moseley used anodes of different metals in a discharge tube and subjected them to attack by cathode rays. He found, that when cathode rays struck anodes of different metals, the wavelength of the rays produced change. The wavelength of these rays was found to decrease in a regular manner on changing the metal of the anode in order of their position in the periodic table. By this, he concluded that the number of positive charge present in the nucleus of an atom is the most fundamental property of an atom.

The number of unit positive charge present in the nucleus of an atom of a particular element is called the atomic number of that element.

Some of the positive charge present in the nucleus is due to protons, the number of protons is equal to the atomic number of that element.

Thus, Henry Moseley found that atomic number is a better fundamental property of elements compared to atomic mass. This led to the modern periodic law.

Modern periodic law: Physical and chemical properties of elements are a periodic function of their atomic numbers.

If elements are arranged in order of their increasing atomic number, those with similar properties are repeated after regular intervals, i.e. periodicity in the periodic table occurs based on the atomic numbers (number of protons)

Teacher's Note

Elements are organized similarly to how we organize items in a grocery store - related products grouped together for easy finding and comparison.

Exercise 5(a)

1. What is the need for classification of elements?

2. What was the basis of the earliest attempts made for classification and grouping of elements?

3. (a) A, B and C are the elements of a Dobereiner's triad. If the atomic mass of A is 7 and that of C is 39, what should be the atomic mass of B?

(b) Why was Dobereiner's triad discarded?

4. Explain 'Newland's law of Octaves.' Why was the law discarded?

5. Did Dobereiner's triads also exist in the columns of Newland's Octaves? Compare and find out.

6. (a) Lithium, sodium and potassium elements were put in one group on the basis of their similar properties. What are those similar properties?

(b) The elements calcium, strontium and barium were put in one group or family on the basis of their similar properties. What were those similar properties?

7. (a) What was Mendeleev's basis for classification of elements?

(b) Mendeleev's contributions to the concept of periodic table laid the foundation for the Modern Periodic Table. Give reasons.

8. State Mendeleev's periodic law.

9. Use Mendeleev's Periodic Table to predict the formula of (a) hydrides of carbon and silicon (b) oxides of potassium, aluminium and barium.

10. Which group of elements was missing from Mendeleev's original periodic table?

11. State the merits of Mendeleev's classification of elements.

12. Why did Mendeleev leave some gaps in his periodic table of elements? Explain your answer with an example.

13. The atomic number of an element is more important to the chemist than its relative atomic mass. Why?

14. Consider the following elements: Be, Li, Na, Ca, K. Name the elements of (a) same group (b) same period.

15. (a) Name an element whose properties were predicted on the basis of its position in Mendeleev's periodic table.

(b) Name two elements whose atomic weights were corrected on the basis of their positions in Mendeleev's periodic table.

(c) How many elements were known at the time of Mendeleev's classification of elements?

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