In this chapter, we learn how elements are classified in a systematic way so that their properties can be studied easily. Instead of studying each element separately, scientists grouped elements with similar properties together. This chapter is important for the SSC board exam as it includes definitions, reasoning questions, and concept-based questions. If you understand the logic behind classification, this chapter becomes very simple.
Table of Contents
Important Concepts of the Chapter
Need for Classification of Elements
All the atoms of an element are of only one type. At present, 118 elements are known to the scientific world, but around the year 1800, only about 30 elements were known. As time passed, many new elements were discovered and more information about their properties was collected.
Studying such a large number of elements individually became difficult. Scientists therefore started searching for patterns in the properties of elements to make their study easier and more systematic.
Early Attempts at Classification
In the initial classification, elements were divided into metals and non-metals. Later, a third group called metalloids was identified. As knowledge about elements increased, different scientists proposed various methods of classification.
These early attempts laid the foundation for the modern periodic table.
Dobereiner’s Triads
In the year 1817, a German scientist Dobereiner suggested that the properties of elements are related to their atomic masses. Dobereiner grouped elements into sets of three called triads.
In each triad,
- The elements were arranged in the increasing order of atomic mass
- The atomic mass of the middle element was approximately the average of the other two
However, this method worked only for a few elements and therefore all the known elements could not be grouped into Dobereiner’s triads.
Newlands’ Law of Octaves
In the year 1866, Newlands arranged elements in increasing order of atomic mass.
He observed that every eighth element had properties similar to the first, just like musical notes.
This was called the Law of Octaves.
Although this law explained similarities among lighter elements, it failed for heavier elements and newly discovered elements.
Mendeleev’s Periodic Table
In the period 1869 to 1872 A.D, Russian scientist Dmitri Mendeleev developed the periodic table of elements. He arranged elements in increasing order of atomic mass and grouped them based on physical and chemical properties of the elements.
Important features of Mendeleev’s periodic table:
- Elements with similar properties were placed in the same group
- He left gaps for undiscovered elements
- He predicted properties of unknown elements accurately
Mendeleev observed that elements with similar chemical and physical properties repeat after a definite interval when arranged in increasing order of atomic mass. Based on this observation, he proposed Mendeleev’s Periodic Law.
Mendeleev’s Periodic Law
The properties of elements are a periodic function of their atomic masses.
However, some elements did not fit properly due to atomic mass anomalies.
Merits of Mendeleev’s periodic table
Dmitri Mendeleev arranged elements with the understanding that scientific knowledge is progressive and can be modified with new discoveries. As a result, his periodic table showed the following merits:
- The atomic masses of some elements were revised to place them correctly according to their properties. For example, the atomic mass of beryllium was corrected from 14.09 to 9.4 and it was placed before boron.
- Mendeleev left vacant places for elements that were not discovered at that time. He predicted the existence and properties of eka-boron, eka-aluminium, and eka-silicon.
- These predicted elements were later discovered as scandium, gallium, and germanium. Their properties matched closely with Mendeleev’s predictions, proving the accuracy and usefulness of his periodic table.
- Initially, there was no place for noble gases. After the discovery of noble gases like helium, neon, and argon, Mendeleev added a zero group without disturbing the existing arrangement, showing the adaptability of his table.
Demerits of Mendeleev’s Periodic Table
- Cobalt and nickel have nearly the same atomic mass, creating confusion about their correct order.
- Isotopes have the same chemical properties but different atomic masses, making it difficult to place them in a table based on atomic mass.
- The increase in atomic mass from one element to the next is not uniform, so it was not possible to predict the number of elements between two heavy elements.
- Hydrogen shows similarities with both alkali metals (Group I) and halogens (Group VII), making it difficult to decide its correct position in the periodic table.
Modern Periodic Law
To overcome limitations of earlier tables, scientists introduced the modern periodic law.
According to this law, the physical and chemical properties of elements are periodic functions of their atomic numbers.
This law forms the basis of the modern periodic table used today.
Modern Periodic Table
The modern periodic table arranges elements in increasing order of atomic number.
Key features:
- 7 horizontal rows called periods
- 18 vertical columns called groups
- Elements in the same group show similar chemical properties
- Apart from the seven periods, two rows are shown separately at the bottom of the periodic table. These rows are called the lanthanide series and the actinide series.
- Including these two series, the modern periodic table contains 118 boxes, which means there are 118 places for 118 elements.
- The modern periodic table is divided into four blocks based on the electronic configuration of elements:
- s-block → Groups 1 and 2
- p-block → Groups 13 to 18
- d-block → Groups 3 to 12
- f-block → Lanthanide and actinide series
- The d-block elements are known as transition elements.
- A zig-zag line in the p-block helps to distinguish between metals, non-metals, and metalloids.
- Metalloids are present along the zig-zag line.
- Metals lie on the left side of the zig-zag line.
- Non-metals lie on the right side of the zig-zag line.
This arrangement removes most of the defects of Mendeleev’s periodic table.
Periods in the Modern Periodic Table
Periods are horizontal rows in the periodic table.
As we move from left to right in a period:
- Atomic number increases by one
- Number of shells remains the same
- Valence electrons increase
Properties of elements change gradually across a period.
Groups in the Modern Periodic Table
Groups are vertical columns in the periodic table.
Elements in the same group:
- Have the same number of valence electrons
- Show similar chemical properties
For example, all alkali metals in Group 1 are highly reactive.
Metals, Non-metals, and Metalloids
Elements are broadly classified into:
- Metals – Good conductors, malleable, ductile
- Non-metals – Poor conductors, brittle
- Metalloids – Show properties of both metals and non-metals
This classification helps in understanding physical and chemical behaviour of elements.
Important Laws / Formulae
Modern Periodic Law
Statement:
The physical and chemical properties of elements are periodic functions of their atomic numbers.
This law is very important for theory questions and reasoning-based questions in SSC exams.
Important Definitions
- Atomic Number – Number of protons in the nucleus of an atom.
- Period – Horizontal row in the periodic table.
- Group – Vertical column in the periodic table.
- Valence Electrons – Electrons present in the outermost shell of atom.
- Metalloids – Elements showing properties of both metals and non-metals.
Processes to Understand (No Numericals)
This chapter does not include numericals, but students should understand the following processes:
- How elements are arranged based on atomic number
- How electronic configuration decides the position of an element
- How valence electrons affect chemical properties
These concepts are often tested through reasoning questions.
Important Points for SSC Board Exam
- Modern periodic law is very important
- Difference between Mendeleev’s table and modern table is frequently asked
- Definitions carry 1–2 marks
- Reasons based on periodic trends are common
- Group and period trends must be understood clearly
- Examples related to metals and non-metals are important
Quick Revision Summary
- Classification helps in systematic study of elements
- Dobereiner proposed triads
- Newlands gave the law of octaves
- Mendeleev arranged elements by atomic mass
- Modern periodic table is based on atomic number
- Periods are horizontal rows
- Groups are vertical columns
- Valence electrons decide chemical properties
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