CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

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CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

Till Now in this whole universe total number of elements known to us is around 118.But upto 1800, only 30 elements were known and their properties were also not known much. With the discovery of more elements, scientists collected more and more  new information about the properties of these elements and looked for ways to arrange them on the basis of similarities in their properties. The earliest attempt to classify the elements resulted in grouping the then known elements as metals and nonmetals. Later on further classifications were given out as our knowledge of elements and their properties increases we start giving different classifications.
 

Döbereiner’s Triads

In year 1817. Johann Wolfgang Döbereiner German chemist, recognise some groups having the elements each which he called triads! Döbereiner showed that when the three element a triad were written in the order of increasing atomic masses, the atomic mass of the middle element nearly the average of the atomic masses of the other two elements.

CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

Example: Consider the triad made up of lithium (L), sodium (Na) and potassium (K) with the respective atomic masses 6.9. 23.0 and 39.0. The atomic mass of sodium (23) is the average of the masses of lithium and potassium.

 Achievements of Döbereiner’s Triads :-

This was a good step in finding atomic mass and properties of middle element. The triads identified by Döbereiner are placed in the same group even in the Modern Periodic Table (Present Periodic Table).

 Example 1. What were the limitations of Döbereiner’s classification?     [NCERT]

Ans. All the elements discovered at that time could not be arranged as grouped into triads as Döbereiner could identify only three triads from amongest the elements known at that time.

For example:- N, P and As, Atomic mass of P (31.0) is not an arithmetic mean of atomic masses of N (14.0) and As (74.9), which is 44.4. 

Newlands’ Law of Octaves 

In 1866, John Newlands, an English scientist, arranged the then known elements in the order of increasing atomic masses. He had started with the element having the lowest atomic mass (hydrogen) and ended at thorium which was the 56 element. He found that every 8th element was having properties similar to that of the first. He compared this to the octaves (group of 8) found in music. Therefore, he called it the ‘Law of Octaves. It is known as ‘Newlands Law of Octaves! In Newlands’ Octaves, the properties of lithium and sodium were found to be the same. Sodium is the eighth element after lithium. Similarly, beryllium and magnesium resemble each other.

CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

Limitations of Newland’s Law of Octaves

(1)We could apply The Law of Octaves, only up to calcium as after calcium every eighth element did not possess properties similar to that of the first.

 (2) Newlands thought that only 56 elements existed in nature and no more elements will be discover in the future. But after some time on, several new elements were discovered, whose properties did not fit into the Law of Octaves.

 (3) In order to fit elements into his Table, Newlands adjusted two elements in the same group, but also put some unlike elements under the same note.

Example: . Cobalt and nickel are in the same slot and these are placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements. . Iron, which resembles cobalt and nickel in properties has been placed for away from these elements 

(4) Newlands’ Law of Octaves worked well with elements having less masses only.

 Example 2. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

 Ans. Yes, Dobereiner’s triads also exists in the columns of Newlands Octaves. For example, Li, Na and K form Döbereiner’s triads and they also exist in Newland’s octaves under the same note or column.

Mendeleev’s Periodic Table

The main credit for classifying elements goes to Dmitri Ivanovich Mendeleev, a Russian chemist. He was the most important contributor to the early development of a Periodic Table of elements wherein the elements were arranged on the basis of their fundamental property, the atomic mass, and also on the similarity of chemical properties.

(1) When Mendeleev started his Periodic Table, 63 elements were known.

(2) He found the relationship between the atomic masses of the elements and their physical and chemical properties.

(3) Among chemical properties, Mendeleev considered only the compounds formed by elements with oxygen and hydrogen.

(4) He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification.

(5) He then took 63 cards and on each card he wrote down the properties of one element.

(6) He arranged the elements with similar properties and pinned the cards together on a wall, 

(7) He observed that most of the elements got a place in a Periodic Table and were arranged in the order of their increasing atomic masses. 

(8) It was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties. 

Mendeleev Periodic Law:

 ‘The properties of elements are the periodic function of their atomic masses.

CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

Features of Mendeléev’s Periodic Table

(1) Mendeleev’s Periodic Table contains vertical columns called groups and horizontal rows called periods.

(2) This table contains 8 groups and 6 periods.

(3) The formula for oxides and hydrides are written at the top of the columns of Mendeleev’s periodic table, where the letter ‘R’ is used to represent any of the elements in the group. For example, the hydride of carbon, CH, is written as RH, and the oxide CO2, as RO,

Achievements of Mendeleev’s Periodic Table

(1) Mendeleev’s periodic law predicted the existence of some elements that had not been discovered at that time. Mendeleev left some gaps in his Periodic Table. Mendeleev named them by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group. For example: Scandium, gallium and germanium, discovered later, have properties similar to Eko-baron. Eka-aluminium and Eka-silicon, respectively. The properties of Eko-Aluminium predicted by Mendeleev and those of the element, gollium which was discovered later and replaced Eka aluminium, are listed as follows.

CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

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

(3) It could accommodate noble gases when these gases were discovered in a new group without disturbing the existing order.

Limitations of Mendeleev’s Classification 

Position of Hydrogen:-

The electronic configuration of hydrogen resembles that of alkali metals Like alkali metals, hydrogen combines with halogens. oxygen and sulphur to form compounds having similar formulae. On the other hand, just like halogens, hydrogen also exists as diatomic molecules and it combines with metals and non-metals to form covalent compounds Thus, a correct position could not be assigned to hydrogen in the periodic table. 

Position of Isotopes 

The position of isotopes could not be explained since the elements are arranged according to their atomic masses and isotopes are atoms of the same element having similar chemical properties but different atomic masses 

CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

Position of isotopes, which were discovered much later, could not be explained as they were placed in the same group.

Wrong Order of Atomic Masses

There were a few instances where Mendeleev had to place an element with a slightly greater atomic mass before an element with a slightly lower atomic mass. The sequence was inverted so that elements with similar properties could be grouped together. For example, cobalt (atomic mass 58.9) appeared before nickel (atomic mass 58.7). Wrong order of atomic masses of some elements could not be explained. 

Non-uniform Variation of Atomic Masses 

Another problem was that the atomic masses do not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements especially when we consider the heavier elements.

Modern Periodic Table

In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass. Accordingly. Mendelev’s Periodic Low was modified and atomic number was adopted as the basis of Modern Periodic Table Modern Periodic Law: ‘Properties of elements are a periodic function of their atomic number: Elements when arranged in order of increasing atomic number. lead us to the classification known as the Modern Periodic Table Prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing atomic number Note: See the table on the next page.

Features of Modern Periodic Table

The Modern Periodic Table has 18 vertical columns known as groups and 7 horizontal rows known as ‘periods.

Position of Elements in the Modern Periodic Table

In order to find the position of an element in the modem periodic table, the group number and period number of the element is to be found out from its electronic configuration

Period

(1) The period number of an element is equal to the number of electron shells in its atom fan atom has two occupied shells, it will belong to the second period.
(2) The number of elements in a period is fixed by the maximum number of electrons which can be accommodated in the different shells of an atom.
(3) The maximum number of electrons that can be accommodated in a shell is given by the formula 2n2 where n is the shell number.
CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

(4) Each period marks a new electronic shell getting filled. 

(5) two or more elements have the same number

of valence shells, then they belong to the same period of the periodic table. 1st period contains 2 elements and is called very short period

2nd period contains 8 elements and is called short period.

3rd period contains 8 elements and is called short period.

4th period contains 18 elements and is called long period.

5th period contains 18 elements and is also called long period.

6th period contains 32 elements and is called very long period.

7th period contains rest of the elements and is incomplete.

Groups

(1) The group number of an element having upto two valence electrons is equal to the number of

valence electrons.

(2) The group number of an element having more than 2 valence electrons is equal to the number of valence electrons + 10.

Elements having 1 valence electron are placed in Group 1.

Elements having 2 valence electrons are placed in Group 2.

Elements having 3 valence electrons are placed in Group 13

Elements having 4 valence electrons are placed in Group 14.

Elements having 5 valence electrons are placed in Group 15.

Elements having 6 valence electrons are placed in Group 16

Elements having 7 valence electrons are placed in Group 17.

Elements having & valence electrons are placed in Group 18.

(3) The elements in a group do not have consecutive atomic numbers.

(4) All the elements in a group have similar electronic configurations and show similar properties. All elements contain the same number of valence electrons. Groups in the Periodic Table signify an

identical outer shell electronic configuration

(5) The number of shells increases as we go down the group.

(6) If two or more elements have the same number of valence electrons, then they belong to the same group of the periodic table

Example 4. Which element has

(A) two shells, both of which are completely filled with electrons?

(B) the electronic configuration 2,8,2 ?

(C) a total of three shells, with four electrons in its

valence shell?

(D) a total of two shells, with three electrons in its valence shell?

(E) twice as many electrons in its second shell as in its first shell? (NCERT)

Ans. (A) The element having two shells, both of which are completely filled will have an electronic configuration of (2,8). Therefore, the element Neon (Atomic number = 10) has two shells.

both of which are completely filled.

(B) Electronic configuration (2,, 2) means that the atomic number of the element is 12,

which is that of Magnesium (Mg).

(C) Element having three shells, with four electrons in its valence shell of M shell we have the electronic configuration 2,8,4 Therefore the atomic number of the element Si 14 and the element is silicon which belongs to group 14

(D) Element having two shells with three electrons in its valence shell or shell will have the electronic configuration 23 Therefore the atomic number of the element is 5 and the element is born which belong. to group 13

(E) Element having twice as many electrons in its second shell -L shell as in its first shell shell) will have the electronic configuration 2,4 Therefore, the atomic number of the element is 6 and the element is carbon which belongs to group 14.

 Arrangement of elements in the modern periodic table on the basis of the electronic configurations of their atoms

CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

Position of Hydrogen:

There is an anomaly when it comes to the position of hydrogen because it can be placed either in group 1 or group 17 in the first period.

    Hydrogen has been placed at the top of Group above the alkali metals because the electronic configuration of hydrogen is similar to those of alkali metals Both have 1 valence electron each. Since the size of hydrogen atom is much smaller than that of alkali metals, many properties of hydrogen are different from those of alkali metals.

Explanation of the Anomalies of Mendeleev’s Classification of Elements

(1) Explanation of the position of hydrogen:

A unique position has been given to hydrogen. It has been placed of the top left corner in group 1. period 1 because of its unique characteristics.

(2) Explanation for the position of isotopes:

    Since all the isotopes of an element have the same atomic number, they are put at one place in the same group of the periodic table.

(3) Explanation for the Position of Cobalt and Nickel:   

        Since the elements are arranged according to their atomic number, and the atomic number of cobalt is 27 while that of nickel is 28, so cobalt with lower atomic number should come first and nickel with higher atomic number should come later.

Comparison of Mendeleev’s Periodic table and Modern periodic table

CBSE Class 10 Chapter 5 Periodic Classification of Elements Notes

Example 5. How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?           ( NCERT )

Ans. The Modern Periodic Table is based on grouping elements with similar properties on the basis of their atomic numbers, whereas Mendeleev’s periodic table was based on atomic masses. This led to certain anomalies, such as position of hydrogen, position of isotopes and the fact that atomic masses do not increase in a regular manner on moving from one element to the next Hydrogen has been allotted a unique position, ie, it is placed at the top of alkali metals in the first group based on similarity in electronic configuration with alkali metals.

2) The anomaly regarding position of isotopes of an element have been resolved as Isotopes of an element have the same atomic number and so they have been allotted the same position in modern periodic table.

(3) The position of Cobalt and nickel is now Justified as atomic number of Cobalt is less than atomic number of nickel and hence it is placed at group whereas Nickel is placed at 10 group.

(4) As the elements have been arranged in the order of increasing atomic number, the number of elements lying between any two elements can easily be predicted.

Trends in the Modern Periodic Table
 
Valency:-
         The valency of an element is its combining capacity with other atoms in order to obtain the nearest inert gas configuration. It is related to the number of valence electrons present in the atom of an element. Variation along a period: On moving from left to right, the valency increases from 1 to 4 and then decreases to from group 15 to 18 as the valency is determined by the number of valence electrons. Example: Valency of Na (Z = 11) is 1 as its electronic configuration is 2, 8, 1 and it attains its nearest inert gas configuration by losing one electron
Valency of O (Z = 8) is 2, as its electronic configuration is (2.6) and it requires two electrons to attain its nearest inert gas configuration.

Variation along a group: All elements in a group have the same valency as elements in the same group have the same number of valence electrons.

Valency of group 1 elements = Number of valence electrons = 1

Valency of group 2 elements = Number of valence electrons = 2

Valency of group 13 elements – Number of valence electrons = 3

Valency of group 14 elements = Number of volence electrons = 4

Valency of group 15 elements = 8 – Number of valence electrons – 3

Valency of group 16 elements – 9 – Nurnber of valence electrons = 2

Valency of group 17 elements = 3 – Number of valence electrons = 1

Valency of group 18 elements = 8 – Number of valence electrons = 0

Example: Elements Li, Na and K belong to group 1 as they all have one valence electron and hence have valency 1.

Li 2,1

Na: 2,8,1

K 2,8,8,1

Elements F, Cl and Br belong to group 17 as they as they all have seven valence electrons and hence have valency 1 (8-7).

F: 2,7

Cl: 2,8,7

Br: 2,8,8,7

Atomic size

The size of an atom refers to the radius of atom and is the distance between the centre of the nucleus and the outermost shell of an isolated atom. It is expressed in picometer.

1 picometer = 10-12 m
 
Variation along a period:
On moving from left to right in a period, the size of atoms decreases
Reason: When we move from left to right in a period, the number of electrons and protons increases. Due to the large positive charge on the nucleus, electrons are pulled more strongly towards the nucleus.
 
Variation along a group:
The size of an atom (radius) increases as we go down in a group
Reason: When going from top to bottom in a group, a new shell is added to the atoms which increases the distance between the valence electrons and the nucleus. So, the effective nuclear charge experienced by the valence electrons decreases.
 

Metallic and Non-metallic Character

Metals are the elements that have 1.2 or 3 electrons in their valence shell and lose electrons easily to form positive ions or cations. They are present on the left side and centre of the periodic table. They are called electropositive elements as they have a tendency of losing electrons.
                            Non-Metals are the elements that have 4. 5. 6 or 7 electrons in their valence shell and gain electrons to form negative ions or anions. They are present on the right side of the periodic table. They are called electronegative elements as they have a tendency of gaining electrons.
 
Metalloids:
There are some elements known as metalloids that exhibit properties of both metals and non-metals in the Modern Periodic Table, a zig-zag line separates metals from non-metals. The border line elements – Boron silicon, germanium, arsenic, antimonu, talurium and nolonium-are intermediate in properties and are called metalloids or semi-metals.

Variation along a period: The metallic character decreases and non-metallic character increases as we move from left to right in a period.

Reason: When we move from left to right in u period, the electropositive character of elements decreases but the electronegative character increases. Due to the large positive charge on the nucleus, electrons are pulled more strongly towards the nucleus.

Example: Consider the elements of 3rd period. The variation of metallic character is shown below:

Variation along a group:

The metallic character increases and non-metallic character decreases as we move from top to bottom in a group

Reason: When we go down a group, the electropositive character of elements increases as the tendency of an atom to lose electrons increases as the effective nuclear charge experienced by the valence electrons decrease. The electronegative character decreases down a group as the tendency to gain electrons decreases.

Chemical Reactivity

The chemical reactivity of an element depends upon the electronic configuration of the atom of the element.

Variation along a period: The chemical reactivity of elements first decreases and then increases when we move from left to right in a period.

Variation along a group: The chemical reactivity of metals increases on going down in a group The chemical reactivity of non-metals decreases on going down in a group.

Nature of Oxides
Variation along a period:
    The basic nature of oxides decreases and the acidic nature increases as we move from left to right as metal oxides are basic in nature and oxides of non-metals are acidic in nature.
 
Variation along a group:
    There is no change in the nature of oxides as we go down in a group as all elements of the same group have similar chemical nature.
 

Merits of the Modern Periodic Table

(1) It is based on the atomic number of elements which is the most fundamental property of elements.

(2) It helps us understand why elements in a group show similar properties but elements in different

groups show different properties.

(3) It explains the reasons for the periodicity in properties of elements.

(4) The modem periodic table tells us why the properties of elements are repeated after 2.8.18 and 32 elements.

(5) There are no anomalies in the arrangement of elements in the modern periodic table.

Advantages of the Periodic Table

(1) The periodic table has made the study of chemistry systematic and easy.

(2) It is easier to remember the properties of on element if its position in the periodic tables known.

(3) The type of compounds formed by an element can be predicted by knowing its position in the periodic table.

(4) A periodic table chart is used as a teaching aid in chemistry in schools and colleges.

All other Chapter Notes:

Chapter 1 Chemical Reactions and Equations – Notes
Chapter 2 Acids,Bases and Salts – Notes
Chapter 3 Metals and Non-metals -Notes
Chapter 4 Carbon and Its Compounds -Notes
Chapter 5 Periodic Classification of Elements -Notes
Chapterwise Notes for Class 10 Science Physics
Chapter 10 Light Reflection and Refraction -Notes
Chapter 11 Human Eye and Colourful World -Notes
Chapter 12 Electricity -Notes
Chapter 13 Magnetic Effects of Electric Current -Notes
Chapter 14 Sources of Energy -Notes
Chapterwise Notes for Class 10 Science Biology
Chapter 6 Life Processes -Notes
Chapter 7 Control and Coordination -Notes
Chapter 8 How do Organisms Reproduce? -Notes
Chapter 9 Heredity and Evolution -Notes
Chapter 15 Our Environment -Notes
Chapter 16 Management of Natural Resources -Notes

We hope the given -Notes for Class 10 Science will help you. If you have any query drop a comment below and we will get back to you at the earliest.

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