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Classification of Elements and Periodicity in Properties - Why do we need to Classify Elements?

Grade 11CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The primary reason for classification is the vast number of elements known today. As of now, 118 elements have been discovered, making it nearly impossible to study the physical and chemical properties of each element and its compounds individually.

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In the year 1800, only 31 elements were known. By 1865, this number rose to 63. The rapid discovery of new elements necessitated a systematic method to organize them.

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Classification allows scientists to group elements with similar properties together, which helps in identifying patterns and trends in their behavior.

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It provides a rational basis for the study of inorganic chemistry by relating the electronic configuration of atoms to their position in the periodic table.

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A well-organized classification system enables the prediction of the properties of undiscovered elements. For instance, Mendeleev successfully predicted the properties of 'Eka-aluminum' and 'Eka-silicon' before Gallium and Germanium were discovered.

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The modern basis of classification is the Atomic Number (ZZ), which is more fundamental than atomic mass for determining chemical properties.

📐Formulae

Z=Atomic Number=Number of protons in an atomZ = \text{Atomic Number} = \text{Number of protons in an atom}

A=Z+n (where A is the Mass Number and n is the number of neutrons)A = Z + n \text{ (where } A \text{ is the Mass Number and } n \text{ is the number of neutrons)}

General valence shell configuration for Group 1: ns1\text{General valence shell configuration for Group 1: } ns^1

General valence shell configuration for Group 17: ns2np5\text{General valence shell configuration for Group 17: } ns^2 np^5

💡Examples

Problem 1:

If we know that Lithium (LiLi), Sodium (NaNa), and Potassium (KK) belong to the same group in the periodic table, how does classification help us understand their reactivity with water?

Solution:

Because they are classified in the same group (Group 1), they all have a general valence shell electronic configuration of ns1ns^1. Therefore, they all react with water to form hydroxides and release hydrogen gas: 2M(s)+2H2O(l)→2MOH(aq)+H2(g)2M(s) + 2H_2O(l) \rightarrow 2MOH(aq) + H_2(g) (where M=Li,Na,KM = Li, Na, K).

Explanation:

Classification groups elements with similar chemical behaviors together, allowing us to generalize the reactions of one element to others in the same family.

Problem 2:

Why did Mendeleev leave gaps in his Periodic Table?

Solution:

Mendeleev left gaps for elements that were not yet discovered. He used the concept of periodicity to predict that elements with specific properties would eventually fill those spots.

Explanation:

Classification is not just about organizing known data, but also serves as a predictive tool for future discoveries based on observed periodic trends.