Skip to main content

Periodic Classification of elements (Notes) NCERT

Dobereiner’s traids

When elements are arranged in the. Order of increasing atomic masses, group of three elements known as traids, having similar properties are obtained. The middle element of the traid being equal to the arithmetic mean of the atomic masses of the other two elements. 

Following are the examples.

1. The alkali Metal group

Elements Symbols Atomic masses
Lithium Li 7
Sodium Na 23
Potassium K 39

2. Alkaline Earth metal group

Elements Symbols Atomic masses
Calcium Ca 40
Strontium Sr 88
Barium Ba 137

3. The halogen group Elements

Elements Symbols Atomic masses
Chlorine Cl 35.5
Bromine Br 80
Iodine I 127

Limitations 

Dobereiner failed to place all the known elements in the form of traids of elements having similar properties.

 Newland’s Law of octaves 

When elements are arranged in the order of increasing atomic masses, the properties of the eighth element (starting from a given element) are a repetition of the properties of the first element. 

Limitations 

Following are the limitations of Newland’s law of octaves. 

1. Newland’s law of octaves was applicable till calcium only. 

2. Newland assumed that only 56 elements existed in nature and no more elements will discovered in the future. But later on, several new elements are discovered which doesn’t fit in Newland’s law of octaves. 

3. In order to fit elements into his table, Newlands put two elements together in one slot which have different chemical properties. 

What is a Periodic table?

 The periodic table is a chart in which elements having similar chemical properties lie in the same vertical column. A periodic table contains of horizontal rows, periods and vertical column, groups. 

Mendeleev’s Periodic Table 

A Russian scientist, Mendeleev found that when elements are arranged in the order of atomic masses, the elements with similar properties occur at regular intervals.

 Following are the two factors on which Mendeleev’s Periodic Table work:

 1. Increasing atomic masses 

2. Grouping together of elements having similar properties.

Merits 

Following are the merits:- 

1. Mendeleev’s periodic table predicted the existence of some elements which was not discovered that time (Gallium, Scandium and Germanium) 

2. Mendeleev’s periodic table could predict the properties of several elements on the basis of their properties. 

3. When can place noble gases group in Mendeleev’s periodic table.

 Limitations 

Following are the limitations:- 

1. The positions of isotopes could not explained. 

2. Cobalt and Nickel are not arranged on the basis of increasing atomic masses. 

3. Position of hydrogen remain unsolved. 

Modern periodic table 

Modern periodic table or long form of periodic table was prepared by Neils Bohr. In modern periodic table, elements are arranged on the basis of increasing atomic masses. 

Explanation of Modern periodic table When the elements are arranged on the basis of increasing atomic mass, then the elements having same number of valence electrons occur at regular interval. 

What is periodicity? 

The repetition of elements having similar properties at regular intervals on regular intervals. Periods and groups 

The horizontal rows in periodic table are called periods. There are 7 periods in modern periodic table. 

The vertical columns in a periodic table are called groups. There are 18 groups. 

Characteristics of periods

1. Valence Electrons On moving left to right, the number of valence electrons increases from 1 to 8. 

2. Valency On moving left to right, in each period the valency of elements increases from 1 to 4 then decreases from 4 to 0.

 3. Size of atoms On moving from left to right in a periodic table, the size of atoms decreases.

 4. Metallic Character On moving from left to right in a period, the metallic character of elements decreases.

 5. hemical reactivity On moving from left to right in a period, the chemical reactivity of elements first decreases and then increases. 

6.Nature of oxides On moving left to right, the basic nature of oxides decreases and acidic nature of oxides increases. 

Characteristics of groups 

1.Valence electrons All the elements of a group of the periodic table have the same number of valence electrons. 

2.Size of atoms On going down in a group of the periodic table, the size of the atoms increases.

 3.Valency All the elements in a group have same valency. 

4.Metallic character On going down in a group of the periodic table, the metallic character of elements increases. 

5.Chemical Reactivity All the elements in a group have similar chemical properties. 

6. Nature of oxides On going down, there is no change in the nature of oxides of elements. 

Merits of Modern periodic table

 Following are the merits of the modern periodic table:- 

1. The modern periodic table is based on the atomic numbers of elements which is the most fundamental property of elements. 

2. The position of hydrogen assigned.

 3. It tells us why properties of elements are rrpeated after 2, 8, 18 and 32 elements. 

4. There are no anomalies in the arrangement of elements in the modern periodic table

Download the pdf  

Comments

Post a Comment

Popular posts from this blog

How to improve your handwriting

 When anyone comment on your bad handwriting then you feel hurt.So, we come here to give some tips to you for improving your handwriting. Why good Handwriting? If you have bad handwriting,no one can easily read your written paragraph or written document.The people who had good handwriting can achieve more marks in exams as well as people can easily read their written document. Following are some tips for them how have not good handwriting:- 1. Use a Nice Pen The adjective “nice” is subjective — you’ll have to hunt to find the pen that works for you! Choose your pen on the basis of the stroke width, the grip, and the jet black ink. I also like that it’s so responsive; I don’t have to exert a lot of pressure on the pen to ensure consistent ink flow. You can find thousands of pens on the market, so I encourage you to shop around. Pick up a pen here and there, and give it a try! It doesn’t matter if you pre
Post a Comment
Read more

Rules of money

The author of the book "The Psychology of Money" says that when I work in a hotel,I observe that a millionaire is always come to that hotel in which I work. He always come in his expensive car and always come with greater money for showing how great he was. Once he give a great amount of money to waiter and told him to buy some gold coins and he started throwing them in the pond with her friends,only for fun.  Let's take a look on the biography of that millionaire. He was 20 years old when he make component of a WiFi router and patent it and after that he sell some companies. Here we conclude that how rich and intelligent was that person but he knew the importance of the money that he earn? No, he doesn’t knew the rules and the importance of the money. Let us see what are rules of money that author share with us in his book. Rule #1: Luck and Risk When Bill Gates was studying in a high school in America. He was so lucky because at that t
1 comment
Read more

How to insert images in PowerPoint presentation

Pictures perform a very good role in making our presentation effective, good-looking. In this article, we learn how to insert images in a slide of presentation. Here the steps which help you in inserting images in your presentation. 1. Click on slide where you want to insert a picture. 2. Click on insert tab. 3. In the Illustrations group, select pictures option. Pictures dialog box will open. 4. Select the picture which you want to insert in your slide. 5. Click on Insert button. The picture will inserted in your slide. You can also insert a Clip Art ( A ready made picture) by performing the following steps. 1. Click on Insert menu. 2. Select Clip Art option.   The clip art pane will appear on right hand of the window. 3. A Search box appear, type the keyword related to the picture you are looking for. 4. Press Enter button OR Click on go button. 5. Select the picture which you want to insert in your slide. The selected picture will be inserted in you
Post a Comment
Read more