Periodic Classification of Elements - Classification of Elements, Mendeleev’s Periodic Classification, Long Form, Practice Problems, FAQs

You must have been to a library where you may have seen a lot of book racks. A sizable library has thousands of volumes on a range of subjects. Despite this, the library staff can quickly find and provide you with any specific book or piece of content you want.

How is it possible?

The books in a library are arranged into different groups and subgroups and given special identifying numbers. Both the books and the racks will have these reference numbers by which the location of the books and the general content of the books in the particular rack can be guessed. They are arranged on shelves accordingly. Consequently, the identification codes make it simple to locate books.

Reverting to chemistry now. Compounds make up the majority of the matter that humans can see, touch, and feel. These chemicals are currently present in millions. You might be shocked to learn that only roughly 110 fundamental unique substances known as elements are used to create compounds through numerous permutations and combinations.

Each of these elements or their atoms and molecules has unique physical and chemical properties. But it will be a Herculian task to study all these elemental properties. Fortunately, some of the elements share some common properties and also show a gradual change in properties, 

By grouping the elements into a few categories due to their common features, understanding the elements was made easier. We only analyse the properties of groups rather than every single element or compound. Various scientists have made an effort to categorise elements according to their qualities.Let’s discuss it in detail.

Table Of Contents

• Classification of Elements
• Mendeleev’s Periodic Classification
• The Long Form of the Modern Periodic Table
• Practice Problems
• FAQs

Classification of Elements

1. Necessity for classification of elements

At present 118 elements are known. Some of them are man-made in laboratories of very low life. With such a large number of elements, it is very difficult to individually study the chemistry of all these elements and their innumerable compounds.

The categorisation of elements is done for the reasons listed below.

1. The categorisation might make them easier to study.

2. The classification can result in a relationship between an element's characteristics and a general feature that applies to all elements.

3. The classification might also show a relationship between the different elements.

2. Origins of the Periodic Table

If chemists had a method for connecting the information they know about the more than 100 known elements, they would be overwhelmed with separate bits of knowledge. In order to organise information such that relationships between elements may be clearly seen and understood, elements are grouped.

Scientists started looking for correlations between elements even before there were so many identified elements. The German scientist Johann Döbereiner, the English chemist John Newland, and the English physician William Prout all made three significant attempts to identify these links during the nineteenth century. The atomic model put forth by English scientist John Dalton in 1803 served as the foundation for the research of all three of these researchers. But all of them were able to relate only a few known elements and could not account for new elements being discovered later. 

Mendeleev’s Periodic Classification

The term ‘Periodic’ is used to indicate that on arranging all elements in some order. elements with similar properties are seen repeated at regular intervals . Mendeleev. Dmitri Ivanovich, a Russian chemist, was the first to put forward the successful arrangement of most of the elements known at his time. In 1869, he published a periodic table of elements.

Mendeleev’s Periodic Law

This law states that:

When the elements are organised in ascending order of their atomic weights, the elements having similar properties are repeated after regular and specific intervals because the “physical and chemical properties of the elements are a periodic function of their atomic weights”.

Characteristics of Mendeleev’s Periodic Table

1. Have elements arranged in horizontal rows called periods and grouped in vertical columns known as a group?

2. There are in all eight groups denoted by Roman numerals from I to VIII.

3. Groups I to VII are further subdivided into A and B. 

4. Elements under the same subgroup exhibit similar qualities.

5. Elements of Subgroups on each row have different properties except for the same valency.

6. Elements of subgroups are differentiated from each other, by placing subgroup elements in the top left corner and B subgroup elements in the bottom right-hand corner of each period. For example, Potassium (K) and copper (Cu) are both placed in groups 1A and 1B respectively. As they have totally different properties, K is placed at the top left corner and Cu at the bottom right corner of group1 and period 4th of the table.

7. Group VIII is unique in the sense, that there is no subgroup but each group in each period has 3 elements of similar properties.

8. There are seven periods, represented by Arabic numerals 1 to 7. 

9. A period starts with an element having similar properties to an element before. So the group has elements of similar properties repeated at regular intervals for the next period.

9. In a period. from left to right, the characteristics of the elements gradually show a gradual change in their physical and chemical properties. For example, on left we have metals which change to metalloids and then nonmetals on the right..

10. Gaps in the arrangement of elements are deliberately made with a strong conviction that there are elements to be discovered, which is exactly what happened.

Drawbacks of Mendeleev’s Periodic Table

i) Hydrogen position:

Hydrogen resembles both first and seventh group elements in some respects. Hence hydrogen can be placed in either group I A or group VII A. Placement of Hydrogen in group 1A is disputable.

ii) Violation of periodicity:

Some elements' placement though following the similarity in properties are not in increasing atomic weight order. Examples are Ar (40) and K (39), Co (58,9 and Ni (58,6), Te (127,6) and I (126,9). Higher atomic weight Argon is placed before lower atomic weight potassium. The periodic law is not followed in these pairs.

iii) Position for Isotopes:

Atoms of the same element that have various atomic weights but the same atomic number are known as isotopes. Almost all elements have one or more isotopes. For instance, hydrogen has three isotopes with atomic masses of 1, 2, and 3. But, there is no mention or recognition of isotopes in the table. Isotopes do not have their own positions in the periodic table.

iv) Grouping of dissimilar Elements:

Some elements that share no chemical similarities are placed in the same group. Similar character Lithium, sodium, and potassium are in group IA and share the same group( even if in a different subgroup B) with unrelated elements like copper, silver, and gold.

v) Cause of Periodicity

Though the law is valid in most of the elements, an explanation for such behaviour was not given.

vi) Position of Lanthanides and Actinides

Elements of lanthanides and actinides are not accommodated in the periodic table.

The Long Form of the Modern Periodic Table

The periodic law of Mendeleev was modified with the more basic property of an atom.

“The properties of the elements are periodic functions of their atomic numbers”

The term "modern periodic table" can also refer to the extended or long forms of the periodic table.

Main Characteristics of Modern Periodic Table

Modern periodic table is an extended form of Mendeleev’s table and is referred to as the Long Form of the periodic table.

1. Arrangement: The elements are arranged in the order of increasing atomic numbers.

2. Periods: Each period starts with the new orbit and hence has a new principal quantum number.

Periods in the Modern Periodic Table

3. Groups:

There is a total of 18 groups in the modern periodic table numbered from 1 to 18. The groups can be visualised as s, p, d and f blocks as per the suborbital electron filling of these elements. Groups 1 and are s -block elements. Groups 3 to 12 are d-block elements. Groups 13 to 18 are called p-block elements.

Groups in the Modern Periodic Table

4. Grouping based on electronic configuration: The elements of group 1 and group 2 and the elements from group 13 to group 18 of the periodic table are known as representative elements. In other words, s -block and p-block elements are collectively known as representative elements.

5. Transitional elements: The elements of groups 3 to group 12 are called transitional elements.

6. Metals and nonmetals: The metallic character decreases from the left to right of the elements in a period. Elements in groups 1 and 2 are very strong metallic elements. The nonmetallic character increase and elements on the right side are nonmetallic.

7. Inert elements: The inert gases found in group 18 at the right-end (He, Ne, ….) and represent the filling up of the p-sub orbitals..

9. Lanthanides and Actinides: Due to their distinct features, lanthanides and actinides are separated from the table and placed at the bottom of the periodic table.

Periodicity and its Significance 

• When the elements are placed in the periodic table in increasing order of atomic number, a pattern known as periodicity emerges in their physical and chemical characteristics. It reflects the current periodic law.

• The melting and boiling points, atomic radii, ionisation energy, electron gain enthalpy, electronegativity, etc. are a few examples of periodic characteristics.

• The periodicity results from the elements' comparable outer electronic configurations occurring at regular intervals.

• Example: Elements of group 1 i.e., alkali metals, have a similar outer electronic configuration i.e.. The outermost shell's principal quantum number, "n," is used here. Similarly, elements of group 17 (halogens) have similar outer electronic configurations, . Hence, having seven valence electrons means that they have many of the same features.

Variation of Metallic Character in Periodic Table

• As we move down the group, the atomic size, and the ability to lose electrons increase. Therefore, the metallic character increases as moves from top to bottom.

• As we move across the period, the atomic size, and the ability to lose electrons decrease. Therefore, the metallic character decreases as we move from left to right.

Variation of Atomic Radius in a Period and Group

The number of orbits in a period stays constant. When going from left to right over a period of time, the atomic number increases by one. As a result, the electron is more strongly drawn to the nucleus. As a result, the atomic radius decreases from left to right in a period.

The nuclear charge rises while the atomic radius falls in a period from left to right.

Over a specific period, the atomic radius reduces up to halogens and grows up to inert gases as one moves from left to right.

Alkali metals are the biggest and halogens are the smallest in size at any given time.

Only the van der Waal radius is relevant for inert gas atoms.

The number of orbits in a group causes the atomic radius to increase steadily from top to bottom, outweighing the effect of the increased nuclear charge.

Practice Problems

Q1. The number of groups in Mendeleev’s periodic table is-?

A) 10
B) 8
C) 7
D)26

Answer: (B)

The total number of groups in Mendeleev's periodic table is 8 numbered from I to VIII.

Q2. The period number of any element corresponds to its highest:

A) Azimuthal Quantum number
B) Principal Quantum Number
C) Magnetic Quantum Number
D) Spin Quantum Number

Answer: Principal quantum number corresponds to the number of shells of an atom and it is represented by and the highest value for an element corresponds to the period number of an element.

Q3. How many elements does the shortest period consist of?
 
A) 2
B) 4
C) 8
D) 6

Answer: A. Shortest period is the first period having two elements i.e., Hydrogen and Helium.

Q4. __________ gave the idea for the first time to classify elements as per their properties.

A) Mendeleev
B) Dobereiner
C) Newland
D) John

Answer:( B) 

It was German chemist named Dobereiner proposed that elements have similarities in properties and hence can be classified based on that. Since the number of elements known at his time was small, he found a few groups of three elements called triads to have similar properties and grouped them together for the first time.

Frequently Asked Questions - FAQs 

Q1. Who gave the modern periodic law?
Answer: In 1869, Dmitri Mendeleev and Lothar Meyer established the periodic law independently, but both defined periodicity in terms of atomic mass. It was Moseley, who observed regularities in the characteristic X-ray spectra of the elements when their frequencies were plotted against their respective atomic number. So the modifications done in the periodic law in modern times (i.e, the physical and chemical properties of the elements are periodic functions of their atomic numbers) can well be attributed to Moseley.

Q2. How do periodic trends relate to periodic law?
Answer: Periodic trends give similar patterns in the periodic table showing us the various aspects of an element such as electronegativity, atomic radius, or ionising power. The periodic law tells us that when grouped by atomic number, certain properties of elements occur periodically.

Q3. In the current periodic table, how many groups and periods are there?
Answer: According to the modern periodic table, the elements are organised in increasing order of atomic number, and there are a total of 18 groups and 7 periods. There are 2 elements in the 1st period, 8 elements each in the 2nd and 3rd periods, 18 elements each in the 4th and 5th periods, and 32 elements in the 6th period and 7th periods.

Q4. Name the still alive discoverer of an element having an atomic number 118.
Answer: 118th element was named after a Russian-Americal nuclear physicist Yuri Oganessian who is the pioneer in the discovery of synthetic elements and was born on 14 April 1933 (age 89 years). The name of this element was given on his name i.e., Oganesson (Og).

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