Book cover for Objective Chemistry for Engineering and Medical Entrance Examinations

Objective Chemistry for Engineering and Medical Entrance Examinations

K Rama Rao

ISBN #9789332541771

1st Edition

2,463 Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter outlines the evolution of elemental classification, beginning with early models like Dobereiner's triads and Newlands' octaves, progressing through Mendeleev's formulation of periodic law, and culminating with Moseley's atomic number-based revision and modern electronic configuration insights. The iterative advancement of scientific ideas emphasized in this chapter underscores how empirical observations and theoretical refinements contribute to the accurate organization and prediction of element properties.

Learning Objectives

1

Trace the historical evolution of the periodic classification of elements.

2

Explain the key periodic laws proposed by Dobereiner, Newlands, Mendeleev, and Moseley.

3

Analyze how periodic trends are organized and predicted in the modern periodic table.

4

Understand the iterative nature of scientific progress in refining elemental categorizations.

Key Concepts

CONCEPT

DEFINITION

Dobereiner’s Triads

An early classification where elements were grouped in sets of three based on similar chemical properties, with the middle element's properties being an average of the other two.

Newlands’ Octaves

A periodic concept suggesting that after arranging elements by increasing atomic weight, every eighth element exhibits similar properties, analogous to musical octaves.

Mendeleev’s Periodic Law

The principle stating that the properties of elements are a periodic function of their atomic weights, which allowed for the prediction of undiscovered elements based on gaps in the table.

Moseley’s Atomic Number-Based Periodic Law

An advancement that established atomic number (number of protons) as the organizing principle rather than atomic weight, resulting in more accurate predictions of elemental properties.

Modern Electronic Configuration Insights

The contemporary approach that uses the arrangement of electrons in an atom to explain periodic trends and chemical behavior.

Example Problems

Example 1

Increasing order of atomic weights was violated (anomalous pairs) in the case of (1) Te, I (2) $\Lambda r, K$ (3) Co, Ni (4) $\Lambda \mathrm{ll}$

Example 2

The wrong statement among the following is (1) Mcndclcev arranged the Elements in a tabular form according to increasing atomic weight. (2) Onc of the dcfects in Mcndclcev's periodic table is position of lanthanides. (3) Maximum number of groups in Bohr's periodic table is 18 . (4) Elements with similar chemical properties occur only within the same period.

Example 3

Which of the following statement is false? (1) In modern periodic table the elements are arranged in increasing order of atomic number. (2) The number of periods in the long form periodic table is $7 .$ (3) The long form periodic table is nothing but just a graphical representation of Paulis principle. (4) Elements of III period are called typical elements.

Example 4

In Mendelecv's periodic table Fe, Co, Ni arc placed in (1) same period (2) same group (3) both (4) none

Example 5

What is the atomic number of the element, which is in the same group of periodic table in which the clement with atomic number 15 is present? (1) 5 (2) 7 (3) 11 (4) 17
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Step-by-Step Explanations

QUESTION

How does Mendeleev's periodic law help in predicting the properties of unknown elements?

STEP-BY-STEP ANSWER:

Step 1: Arrange the known elements in order of increasing atomic weight.
Step 2: Observe the recurrence of similar chemical properties at regular intervals or periods.
Step 3: Identify gaps in the sequence where properties suggest the existence of an undiscovered element.
Step 4: Use the pattern of properties from surrounding elements to predict the properties of the unknown element.
Final Answer: Mendeleev’s periodic law provides a systematic method to infer the properties of unknown elements by recognizing periodic patterns based on atomic weights.

Mendeleev’s Periodic Law

QUESTION

What was the significance of Moseley’s discovery in refining the periodic classification?

STEP-BY-STEP ANSWER:

Step 1: Analyze the X-ray spectra of elements to measure their atomic numbers.
Step 2: Recognize that elements arranged by atomic number exhibit more consistent periodic trends compared to the arrangement by atomic weight.
Step 3: Realize that using atomic number corrected inconsistencies in Mendeleev’s table.
Final Answer: Moseley’s work shifted the basis of periodic classification from atomic weight to atomic number, resulting in a more accurate and predictive periodic table.

Moseley’s Atomic Number-Based Periodic Law

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Common Mistakes

  • Confusing the basis of classification (atomic weight vs. atomic number).
  • Overlooking the significance of early periodic models as foundational rather than complete representations.
  • Assuming that modern periodic trends were immediately apparent, rather than the result of gradual scientific refinement.
  • Misinterpreting periodicity as a simple repetition, rather than a complex pattern influenced by electron configurations.