2.Periodic Classification Of Elements-Full

15/05/2014

Q.1 Give the need and importance 0f classification of elements?

Ans. The number of elements known so for is 115. These elements combine together to form million of compounds. It is, therefore, very difficult to study each element separately. So, in order to make the study of elements and their compounds easier, it has become necessary to arrange and classify the elements in a systematic manner i.e. in periodic table.

Q.2 What is Dabereiner’s law of Triads?

Ans. Dabereiner’s law of Triads:- In 1829, a German chemist, Dabereiner’s suggested that the properties of elements can be related to their atomic masses. He held the elements in order of their increasing atomic masses and prepared groups of three similar elements known as triads.

According to Dabereiner’s law of Triads, “When elements are arranged in the order of increasing atomic masses, groups of three elements (triads) having similar chemical properties are obtained. The atomic mass of the middle element of the triad being equal to the arithmetic mean of the atomic masses of the other two elements.”

Example: The elements like Lithium, Sodium and Potassium form a triad as:

(a). They have similar chemical properties as:

I. All these elements are metals.

II. These are monovalent i.e they have valency 1.

III. All of them react with water to form alkalis and hydrogen gas.

(b).

Elements of the Triad	Symbols	Atomic Masses
1. Lithium	Li	7
2. Sodium	Na	23
3. Potassium	K	39

Arithmetic mean of atomic masses of Lithium (Li) & Potassium (K) = 7+39

= 46 = 23 = Atomic mass of sodium.

Q.3 What are the limitations of Dabereiner’s classification of elements?

Ans. Dabereiner’s failed to arrange all the then known elements in the form of triads of elements having similar chemical properties and he could only identify three triads from the elements known at that time, so his classification was not much successful.

Q.4 State and explain Newland’s Law Of Octaves?

Ans. Newland’s Law Of Octaves:- In 1864, an English scientist, John Newlands showed that when elements are arranged in the order of increasing atomic masses, there is a repetition of properties in every eighth element. This repetition in the properties of the elements is just like the repetition of note in an octave of music. So it is also known as Law of octaves.

According to Newlands, “When elements are arranged in the order of increasing atomic masses, the properties of the eighth element are the repetition of the properties of first element.”

A scheme of his elements is shown as:

H Li Be B C N O

F Na Mg Al Si P S

Cl K Ca Cr Ti Mn Fe

Co & Ni Cu Zn Y In As Se

Br Rb Sr Ce & La Zr – –

In the above classification, if lithium be the first element, then the eighth element sodium (according to Newland) should have the similar properties to those of lithium and it has been found that these elements have similar chemical properties. Similarly, Sodium and Potassium have similar properties.

Q.5 What are the limitations of Newlands Law of Octaves?

Ans. Limitations of Newlands Law of Octaves:- The main limitations of Newlands Law of Octaves are as:

1. Newlands law was applicable to the classification of elements up to calcium only and after calcium every eighth element did not possess the properties similar to that of the first element.

2. Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future, but later on several new were discovered whose properties did not fit into Newlands law of octaves.

3. Newland put even two elements in one slot and that in too the column 0f unlike elements having different properties. E.g. The two elements Co & Ni were put in one slot and that too in the column of elements like fluorine, chlorine and bromine which have very different properties from these elements.

4. Iron element which resembles Cobalt and nickel elements in properties, was placed far away from these elements.

Q.6 State and explain Mendeleev’s Periodic table?

Ans. Mendeleev’s Periodic Table:- Mendeleev, a Russian Chemist in 1869, put forward his periodic law which states that, “The properties of elements are a periodic function of their atomic masses.” It means that, if the elements are arranged in the increasing order of their atomic masses, the elements with similar properties are repeated at regular intervals.

Based upon this law, Mendeleev prepared a table known as Mendeleev’s periodic table. In Mendeleev’s periodic table, there are 7 horizontal rows known as periods and 8 verticals columns called group. Out of these 8 group, the first seven groups are of normal elements and the eighth group is of transition elements. There was no group of noble gases as these were not known at that time. The similar properties used by Mendeleev to classify elements into groups were similar formulae of their oxides hydrides.

Mendeleev left some gaps in his periodic table for those elements which were not known at that time. He named these elements as “eka-boron”, “eka-aluminum” and “eka-silicon” by prefixing the term “Eka” to name of the preceding element in the group. The term “Eka” is derived from a Sanskrit word which means “first”. So, eka-boron means first come boron and then the unknown element.

A part 0f Mendeleev’s periodic table is shown as:

Group

III

Group

VII

Group

VIII

Oxides→

Hydrides→

R₂O

RH₂

R₂O₃

RH₃

RO₂

RH₄

R₂O₅

RH₃

RO₃

RH₂

R₂O₇

RO₄

–

Periods

↓1

1.0

7.0

9.1

11.0

12.0

14.0

16.0

19.0

23.0

24.3

27.0

28.4

31.0

32.0

35.5

Ist series

2^nd series

39.1

63.5

40.1

65.4

…

48.1

…

51.4

52.1

55.0

79.9

Fe Co Ni

55.8 58.9 58.7

Ist series

2^nd series

85.4

107.9

87.6

112.4

89.0

114.0

90.6

119.0

94.0

120.0

96.0

127.6

126.9

Ru Rh Pd

101.0 102.9 106.4

Ist series

2^nd series

132.9

197.2

137.3

200.0

Q.7 What are the merits of Mendeleev’s periodic Table?

Ans. Merits of Mendeleev’s Periodic Table:– Some of the merits of Mendeleev’s periodic table are as:

I. Mendeleev’s periodic law predicted the existences of some unknown elements and left proper gaps for them. Later on, when these elements were discovered, they were placed in those gaps without disturbing the existing elements.

2. Mendeleev’s periodic table also predicted the properties of several elements like gallium, scandium and germanium and later their properties were found same as that of the actual properties.

3. Mendeleev’s periodic table could accommodate noble gases when they were discovered. These are placed in a separate group because they are chemically unreactive.

Q.8 What are the main limitations of Mendeleev’s periodic table?

Ans. Main Limitations Of Mendeleev’s Periodic Table:– The main defects of Mendeleev’s periodic table are as:

1. Position of Isotopes:- Isotopes of various elements have been put together at one place in the periodic table although they have same properties but different atomic masses, which is against his law. E.g. the two isotopes of chlorine, Cl-35 and Cl-39 are placed in the same group although they have different atomic masses.

2. wrong order atomic masses of some elements:-Some elements having higher atomic masses have been placed before the elements with lower atomic masses.

e.g. Cobalt (Co) with atomic mass 58.9 has been placed before nickel (Ni) with atomic mass 56.6

3. Wrong position of Hydrogen:- A correct position could not be assigned to hydrogen in the periodic table. In Mendeleev’s periodic table, hydrogen has been placed in Group-I with alkalis, because like alkali metals it combines with halogens to form compounds having similar formulae.

On the other hand, hydrogen also like halogens (Cl, F, Br etc) exits in the form of diatomic molecule (H₂). Thus, it resembles also with halogens and thus should be placed in Group-VIII of halogen elements. Hence hydrogen could be placed in Group-I as well as in Group-VIII and thus the correct position of hydrogen could not be assigned by Mendeleev.

Q.9 What is the present bases for periodic classification of elements and explain how the limitations in Mendeleev’s periodic table were removed in Modern Periodic Table?

Ans. Present Bases For Periodic Classification Of Elements:– The present bases for the classification of elements is the atomic number of elements. It was Mosley who showed in 1913 that the atomic number of an element is more fundamental property than atomic mass and hence atomic number is the better for the classification of elements. The significance of atomic number in the classification of elements is that atomic number being equal to the number of electrons, so it helps in arranging the elements according to their electronic configuration. Thus, it is true to say that the present bases for classification of elements is their electronic configuration.

Explanation for the limitations of Mendeleev’s Periodic classification of elements:– When the elements are arranged according to their atomic numbers on the bases of Modern Periodic law, then all the defects of Mendeleev’s periodic table were automatically removed.

E,g. isotopes of an element need not to be given separate place in the periodic table because they have same atomic number and thus can be placed at one group.

Similarly, the misfits like the wrong position of Cobalt & Nickel gets removed as these elements are correctly placed in the increasing order of their atomic numbers.

Q.10 Discuss in detail : Modern Periodic Table

Ans. Modern Periodic Table:– The modern periodic table was prepared by Bohr. It is also known as Long form of periodic table. It is based on the modern periodic law which states that, “ The properties of elements are the periodic functions of their atomic numbers.” Thus, the elements are arranged in the increasing order of their atomic numbers. The horizontal rows of elements in a periodic table are called Periods. There are seven periods in the long form of periodic table. First period has 2 elements while 2^nd & 3^rd periods have 8 elements each. These are called short periods. Fourth and fifth periods contain 18 elements each. These are called long periods. Sixth period contains 32 elements. It is called very long period and the seventh period is incomplete period and contains the remaining elements. The elements in a period have consecutive atomic numbers.

The vertical columns in a periodic table are called groups. There are 18 groups in the long form of periodic table which are numbered as 1 to 18. The elements in a group do not have consecutive atomic numbers. The groups 1 & 2 and 13 to 17 contains normal elements.

The group 3 to 12 elements are called transition elements. In these elements, the outermost as well as the next to outer most (penultimate shell) are incomplete. Group 18 elements are known as noble gases 0r inert gases and valence shells of these elements are completely filled.

The elements with atomic numbers 51 to 71 are called Lanthanide series (because their first element is lanthanum) and the elements with atomic numbers 89 to 103 are called actinide series (because their first element is actinium) and are placed in two rows at the bottom of the periodic table.

In the modern periodic table, the elements have been divided into metals and non-metals. The elements on the left side of the periodic table are metals where as those on the right side are non-metals. Metals and non-metals are separated by another type of elements called metalloids (like B, Si, Ge, Ar etc.) which are placed diagonally in the periodic table.

Q,11 Describe the main characteristics of periods in the long form of periodic table.

Ans. Characteristics Of Periods In The Long Form Of Periodic Table:– The horizontal rows of elements in a periodic table are called periods. There are seven periods in the long form of periodic table.

The main characteristics of periods are as:

1. Valence Electrons:– The number of electrons present in the outermost shell of an atom are called valence electrons. On moving from left to right in a period, the of valence electrons in elements increases from 1 to 8 (except in the first period where it increases from 1 to 2 ). The first element in every period has 1 valence electron while the last element in every period has 8 valence electrons (except He.)

Electronic configuration: 2 8 1 2 8 2 2 8 3 2 8 4 2 8 5 2 8 6 2 8 7 2 8 8

No.of valence electrons: 1 2 3 4 5 6 7 8

2.Valency:- The number of electrons lost or gained by an atom of an element to achieve nearest inert gas electronic configuration is known as its valency.

On moving from left to right in each period (except first period), the valency of electrons first increases from 1 to 4 and then decreases to 0.

Electronic configuration: 2 8 1 2 8 2 2 8 3 2 8 4 2 8 5 2 8 6 2 8 7 2 8 8

No.of valence electrons: 1 2 3 4 5 6 7 8

Valency: 1 2 3 4 3 2 1 0

3. Size of atoms:- The size of atoms is also known as atomic size or atomic radius. As we move from left to right in a periodic table, the atomic number of elements increases which means that number of protons and electrons in the atoms increases. Due to large positive on the nucleus, the electrons are pulled more close to the nucleus and thus, the size of atoms decreases. Hence, on moving from left to right in a period of the periodic table, the size of atoms decreases.

Atomic radius: 186 160 143 118 110 104 99

4. Metallic character:- On moving from left to right in a period, the metallic character of elements decreases while the non-metallic character increases. Metals are in the left side while the non-metals are in the right side in the period and in between them lies metalloids.

Nature of elements: ⌠______Metals_____⌡ ↓ ⌠__Non-metals__⌡

Metalloid

5. Chemical Reactivity:– On moving from left to right, the chemical reactivity of elements first decreases and then increases.

Chemical Reactivity: ↓ ↓ ↓

Very reactive Least reactive Very reactive

6. Nature of oxides:- On moving from left to right in a period, the basic nature of oxides decreases and the acidic nature of oxides increases. E.g. In the third period of elements, sodium oxide is highly basic in nature and magnesium oxide is comparatively less basic. The aluminum and silicon oxides are amphoteric in nature. Phosphorus oxides are acidic, sulphur oxides are more acidic and chlorine oxides are highly in nature.

Nature of Oxides: ↓ ⌠________⌡ ↓

Highly basic Amphoteric Highly acidic

Q.12 Describe the main characteristics of groups in the long form of periodic table.

Ans. Characteristics Of groups In The Long Form Of Periodic Table:– The verticals columns of elements in a periodic table are called groups. There are eighteen groups in the long form of periodic table.

The main characteristics of groups are as:

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

e.g. All the elements of group-I of the periodic table have 1 valence electron in their atoms.

Group-I Electronic config. No.of valence electrons

2 1 1

2 8 1 1

2 8 8 1 1

2. Valency:- All the elements of a group have the same valency because the number of valence electrons in a group is same.

E.g. All the elements of group-I have 1 valency each as they have 1 valence electron each.

3. Size of atoms:-On going down in a group 0f the periodic table, the size of atoms increases. This is due the fact that on moving from top to bottom in a group a new shell of electrons is added at every step and thus, this increases the size of atoms.

e.g. In group-I of elements lithium is at the top of the group, so it is the smallest atom while francium atom is at the bottom of the group so it is the biggest atom.

Group-I Atomic Radius(pm)

152

186

231

244

262

270

4. Metallic Character:- On going down in a group of the periodic table, the metallic character of atoms increases. This is due to the fact that when moving down in group one more electron shell is increased at every stage and size of atoms increases. The valence electrons becomes more and more away from the nucleus and the of the nucleus on valence electrons decreases. Due to this the atom c lose the valence electrons more easily to form positive ions and hence the electropositive character increases.

E.g. In group-I, lithium is the least metallic element while as francium is the most metallic element.

5.Chemical Reactivity:- The chemical reactivity of metals increases on going down in a group of the periodic table as the tendency of their atoms to lose electrons increases.

E.g. In the group of alkali metals, the chemical reactivity increases from lithium to francium.

The chemical reactivity of non-metals decreases on going down in a group of the periodic table.

E.g. In group-17, of halogen elements, the chemical reactivity decreases from chlorine to iodine.

Group-I Group-17

Least Reactive

6.Nature of oxides:- By going down in a group of the periodic table, there is no change in the oxides of elements.

E.g. All the elements of group-I form basic oxides while as all the elements of group-17 form basic oxides.

Q.13 What is the position of hydrogen in the modern periodic table?

Ans. Position of hydrogen in Modern Periodic Table:- Hydrogen has been placed at the top of group-I above the alkali metals because the electronic configuration of hydrogen is similar to those of alkali metals. Both hydrogen as well as alkali metals have 1 valence electron each. As hydrogen atom is very small in size, its many properties are different those of alkali metals and thus, it is not included in alkali metals.

Q.14 What are the advantages of Modern Periodic Table?

Ans. Advantages of Modern Periodic Table:- Following are the advantages of modern periodic table:

I. It has made the study of the properties of elements and their compounds simple and systematic. It has reduced the chemistry of 115 elements to a few groups.

II. It helps us to predict which compound is formed by an element by knowing its position in the periodic table.

e.g. If an element is on left side of the periodic table, it will be a metal and hence form only ionic compounds and if an element is on the right side, it will be a non-metal and can form ionic as well as co-valent compounds.

III. It helps us to remember the properties of an element if its position is known in the periodic table.

IV. A periodic table chart can be used as a teaching aid in the chemistry in schools and colleges.

Q.15 What are the merits of the modern periodic table?

Ans. Merits of Modern Periodic Table:– Some merits of modern periodic table are as:

1. The modern periodic table is based on the atomic numbers of elements which is the most fundamental property 0f elements.

2. It helps in understanding why elements in a group show similar properties but elements in different groups show different properties.

3. The modern periodic table explains the reasons for the periodicity in properties of elements. It says that the electronic configurations of the elements are repeated at regular intervals, so the properties of elements are also repeated at regular intervals.

4. The modern periodic table tells us why the properties are repeated after 2, 8, 18 and 32 elements.

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