Development Of Modern Periodic Table

The Periodic Table is a fundamental framework in chemistry, systematically organizing elements based on their atomic structure and recurring chemical properties. In 1869, Russian chemist Dmitri Mendeleev introduced the first widely recognized periodic table, arranging elements by increasing atomic mass. This arrangement revealed that elements with similar chemical properties occurred at regular intervals, a pattern he termed the "periodic law" .

This Story also Contains

1. Introduction of Periodic Table: Basic to Advanced
2. Dobereiner triad rule
3. Newland's Octave Law:
4. Solved Examples Based On Introduction to Periodic Table
5. Conclusion

Mendeleev's table not only grouped elements with analogous properties but also predicted the existence and properties of undiscovered elements, such as gallium, scandium, and germanium, which were later confirmed .

In the early 20th century, British physicist Henry Moseley demonstrated that the elements are more accurately arranged by atomic number—the number of protons in an atom's nucleus—rather than atomic mass.

Understanding the periodic table is essential for students, especially those in Class 11 chemistry, as it forms a fundamental part of the Classification of Elements and Periodic Table chapter. This knowledge is not only crucial for board examinations but also for competitive entrance exams like JEE Main, NEET, SRMJEE, BITSAT, WBJEE, and BCECE.

Introduction of Periodic Table: Basic to Advanced

The Significance of the Periodic Table

The Periodic Table is a fundamental framework in chemistry, systematically organizing elements based on their atomic numbers and recurring chemical properties. Without this systematic classification, understanding the relationships and behaviors of elements would be exceedingly challenging. By organizing elements into periods and groups, the table facilitates comparative studies of elements and their compounds.

Development of the Periodic Table

Prout's Hypothesis

In the early 19th century, English chemist William Prout proposed that all elements are composed of hydrogen atoms, suggesting that the atomic weight of each element is an integer multiple of hydrogen's atomic weight. This idea, known as Prout's hypothesis, posited that hydrogen was the fundamental building block of all matter. However, this hypothesis faced challenges as more accurate measurements of atomic weights revealed discrepancies.

For instance, chlorine's atomic weight is approximately 35.5, which is not an integer multiple of hydrogen's atomic weight. Such inconsistencies led to the hypothesis being reconsidered and eventually set aside.

Limitations of Prout's Hypothesis

Prout's hypothesis had several limitations:

• Inability to Account for All Elements: Not all elements could be explained as combinations of hydrogen atoms.
  
  

• Non-Integer Atomic Weights: The atomic weights of certain elements, such as chlorine (35.5) and strontium (87.5), are not whole numbers, challenging the premise of the hypothesis.
  
  

These limitations highlighted the need for a more nuanced understanding of atomic structure, leading to the development of the modern atomic theory and the periodic table as we know it today.

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Dobereiner triad rule

J.W. Dorbereiner pointed out that within a group of three elements having similar chemical and physical properties, the atomic weight of the middle element is the mean of the other two. Some examples of such triads are given below. He also pointed out the triad - iron, cobalt, and nickel in which the atomic weights of the elements are almost the same.

Some representative triads of Dobereiner

Other examples. (K, Rb, Cs), (P, As, Sb) (H, F, Cl) (Sc, Y, La).

Though it was the first successful attempt to rationalize the problem, it could not be generalized or extended.

Drawback or Limitation: All the known elements could not be arranged as triads.

Newland's Octave Law:

John Alexander Reina Newland in England made the first attempt to correlate the chemical properties of the elements with their atomic weight. According to him -

1. If the elements are arranged according to their increasing atomic weights, every eighth element has similar properties to the first one like the first and eighth note in music. For example

2. Inert gases were not discovered till then.
3. All the elements could not be classified on this basis.

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Solved Examples Based On Introduction to Periodic Table

Example 1: Which is the correct statement?

1) (correct) The law of triads was proposed by Dobereiner

2) The law of octaves was proposed by Einstein

3) Both

4) None

Solution:

As we have learnt

Introduction of Periodic Table - Proust's Hypothesis

He simply assumed that all the elements are made up of hydrogen, so we can say that

Atomic weight of elements = n (Atomic weight of one hydrogen atom

The atomic weight of H = 1, where n = several hydrogen atoms = 1, 2, 3,

Drawback or Limitation:

1. Ex. Every element can not be formed by Hydrogen.
2. The atomic weights of all elements were not found as whole numbers.

Ex. Chlorine (atomic weight 35.5) and Strontium (atomic weight 87.5)

The law of triads was proposed by Johann Dobereiner. The law of octaves was proposed by Newlands.

Hence, the answer is the option (1).

Example 2: According to Newlands' octave law, the periodicity in elements is found when:

1) (correct) Elements are found in increasing order of their atomic masses.

2) Elements are found in increasing order of their atomic number.

3) Elements are found in decreasing order of their atomic masses.

4) Elements are found in decreasing order of their atomic number.

Solution:

Periodicity is the repetition of similar physical and chemical properties after a fixed interval. The periodicity in elements is observed when they are arranged in increasing order of their atomic masses.

Hence, the answer is the option (1).

Example 3: Newland’s octave law was successful in arranging:

1) Heavier elements

2) (correct) Lighter elements

3) Both

4) None

Solution:

Newland’s octave law was successful in arranging lighter elements. After calcium, this law did not work accordingly.

Hence, the answer is the option (2).

Example 4: Newland's octaves law was found true up to:

1) Magnesium

2) gallium

3) potassium

4) (correct) calcium

Solution:

Newlands octave's law was valid up to calcium.

Hence, the answer is the option (4).

Related Topics -

• Mendeleev’s Periodic table
• Electron Gain Enthalpy
• Nomenclature Of Elements With Atomic Number
• Physical and chemical properties of elements

Conclusion

One significant paradigm is left out in this regard – the Periodic Table as the representation of humanity’s efforts to investigate the world of nature. Its very creation is one of the most profound areas in the development of the history of chemistry as a result of the shift that occurred in the outlook of the elements that existed in the world. It evolved from a simple arrangement as done by Mendeleev to the present system based on atomic number; still subjected to changes over the periods depending on the new facts and theories on chemistry.

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