Periodic Classification Of Elements (Q&A).pdf

10th Chemistry/Chapter 2 Periodic Classification of Elements Moomin Ahmad Dar/Tr BHS Goripora/9596206562

Introduction:

Identification of similar species and grouping them together

is called classification. e.g. in a shop, a shopkeeper places

soaps at one place, detergents at other, bakery at some other

place and so on. Similarly in our wardrobe, we place shirts,

trousers, socks, under-wears etc at separate places. In the

same way, in nature, we have discovered a number of

elements. All of them have different physical and chemical

properties. Remembering properties of elements one by one

is quite a cumbersome job. So chemists, right from the

beginning tried to group elements resembling in certain

properties together. This led to classification of elements in

tables called Periodic Tables.

EARLIER ATTEMPTS AT THE CLASSIFICATION OF

ELEMENTS:-

In the early part of the 19th century, many chemists noted

that chemical properties of elements showed some

similarities. The elements could be framed into groups. In

1817, Dobereiner showed that elements came in groups of

three, now known as Dobereiner’s triads. . In England, John

A. R. Newlands also noted that chemical groups repeated

every eight elements. He named this the octave rule, and

compared it to a musical scale. Chemists Dmitrii I.

Mendeleev, a Russian, and a German Lothar Meyer, were

working independently in 1868 and 1869 on the

arrangement of elements into seven columns,

corresponding to various chemical and physical properties.

Define classification of elements?

Ans:- Grouping together of elements with similar properties is

called Classification of elements.

Why do we classify elements?

Ans:- In nature, we have discovered a number of elements. All

of them have different physical and chemical properties.

Remembering properties of elements one by one is quite a

cumbersome job. So chemists, right from the beginning tried

to group elements resembling in certain properties together

so that their chemical and physical properties are

remembered. This led to classification of elements in tables

called Periodic Tables.

State Dobereiner’s law of triades?

Ans:- Dobereiner’s law of triads states that, “when elements

are arranged in the order of increasing atomic weights in a

triad, the atomic weight of the middle element is

approximately equal to the mean of the atomic weights of

other two elements in the triad.”

What are Dobereiner’s triads? What were the merits of

this classification?

Ans:- The Groups of three elements with similar properties

as given by Dobereiner are called Dobereiner Triads. These

triads were given by John Wolfgang Dobereiner in 1817. He

arranged elements of a triad in increasing order of their

atomic weights and observed that the atomic weight of the

middle element is approximately equal to the mean of the

atomic weights of other two elements in the triad

Triad I Triad II Triad III

Lithium (Li) 7 Calcium (Ca) 40 Chlorine (Cl) 35.5

Sodium (Na) 23 Strontium (Sr) 87.5 Bromine (Br) 79.9

Potassium (K) 39 Barium (Ba) 137 Iodine (I) 126.9

e.g. Triad I contains Li, Na and K. All of these are metals and

all of them react with water to form basic solutions called

Alkalies. Also, atomic weight of Na (23) is exactly the mean

of the atomic weights of Li (7) and K (39).

The only merit of this classification was that it

opened doors for classification of elements. Also it related

properties of elements to their atomic weights.


What were the main limitations of Dobereiner

classification?

Ans:- The most important drawback of Dobereiner’s

classification were

1. Doberneir could only group few elements in triads. He

could only find three/four triads out of the then known

30 elements. This means that he could not classify all

the then known elements into triads.

2. Although N, P and As have similar chemical properties,

they could not form a triad as Phosphorus has much

lower mass than the average of other two.

State Newland’s Law of Octaves? How did Newland

classify elements?

Ans:- Newland’s law of octaves states that, “when elements

are arranged in horizontal rows in order of their increasing

atomic masses, the properties of every eighth element are

similar to that of first element like the eighth note of

musical scale.”

John Alexandra Newlands, an English scientist in

1866 arranged all the then known elements in order of

increasing atomic weights. He started with the element

having the lowest atomic mass (hydrogen). He noticed that

the property of every eighth element was similar to the

properties first element, just like the eighth note in musical

scale - Western as well as Indian. Western Indian Groups of the 8th elements

Do Sa Lithium, Sodium, Potassium

Re Re Beryllium, Magnesium, Calcium

Me Ga Boron, Aluminium

Fa Ma Carbon, Silicon

So Pa Nitrogen, Phosphorus

La Da Oxygen, Sulphur

Ti Ni Fluorine, Chlorine

Do Sa - -

Newlands’ gave the following Periodic table based on his

law of octaves.

Newlands Octaves (Periodic Table of Newland of 1866) H Li Ga 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 Di,Mo Ro, Ru Pd Ag Cd U Sn Sb Te I Cs Ba, V Ta W Nb Au

Pt, Ir Tl Pb Th Hg Bi Th What were the advantages or achievements of

Newland’s periodic table?

Ans:- The main achievements of Newlands Classification

were;

a. The law of octaves was the first logical attempt to

classify elements on the basis of atomic weights.

b. Periodicity of properties elements was recognized for

the first time.

What were the drawbacks or limitations of Newland’s

periodic table?

Ans:- The main draw backs of Newland’s classification

were;

i. This law could be best applied, only up to the element

calcium.

j. Newlands suggested the existence of only 56 elements.

k. Newly discovered elements could not fit into the octave

structure.

l. The feature of resemblance of the 8th element when

arranged in increasing order of their atomic mass was not

successful with heavier elements.

State Mendeleev’s periodic law?


Ans:- Mendeleev’s periodic law states that, “the physical

and chemical properties of elements are a periodic function of

their atomic weights (masses).”

What is meant by Periodic function?

Ans:- Periodic function means that properties get repeated

after regular intervals (periods).

Define periodicity of properties?

Ans:- The repetition of properties of elements after regular

intervals is called Periodicity of properties.

What is Mendeleev’s periodic table? What are its main

characteristics?

Ans:- Dimitri Ivanovich Mendeleev in 1869 put forward a

periodic table known as Mendeleev’s Periodic Table.

Mendeleev framed his periodic table on the basis of the

chemical formulae of the hydrides and oxides formed by

various elements as shown in the table below.

MENDELEEV”S PERIODIC TABLE

I II III IV V VI VII VIII

R2O RH

RO RH2

R2O3

RH3

RO2 RH4

R2O5 RH3

RO3 RH2

R2O7 RH

RO4

A B A B A B A B A B A B A B A B C 1 H

2 Li

Be

B C

N

O

F

3 Na

Mg

Al Si

P

S

Cl

4 K

Cu

Ca

Zn …

…

…

Ti

As

V

Se

Cr

Br

Mn F

e

Co

Ni

5 Rb

Ag

Sr

Cd In

Y

Sn

Zr

Sb

Nb

Te

Mo

I

Te

Ru

Rh

Pd

6 Cs

Au

Ba

Hg Tl

La

Pb

Hf

Bi

Ta

..

W

..

..

Os

Ir

Pt

7

Th

U

The main characteristics of Mendeleev’s periodic table are:-

a. All the elements are arranged in vertical columns

(groups)and horizontal rows (periods).

b. There are Nine columns and Seven periods in the table.

c. The columns are numbered in Roman Numerals from I

to VIII and the ninth Column group is called Zero Group.

d. The periods are numbers from 1 to 7.

e. The periods contain different number of elements.

f. Many gaps were left blank by Mendeleev for the then

unknown elements.

What were the main achievements or merits or

advantages of Mendeleev’s Periodic table?

Ans:- the main merits of Mendeleev’s Periodic table are;

a. Through this table, it was very easy to study the physical and chemical properties of various elements.

b. He classified elements on the basis of their atomic masses which was the most fundamental property of atoms in his time.

c. His table corrected errors atomic masses of elements based on their position in the table.

d. He correctly left spaces for the then unknown elements and predicted their physical and chemical properties.

e. His table easily adjusted noble gases as Zero group at extreme right when discovered.

What were the main demerits of Mendeleev’s Periodic

table?

Ans:- The main demerits of Mendeleev’s Periodic table

were;

a. He could not assign a correct position to hydrogen in the periodic table.

b. In certain pairs of elements, the increasing order of atomic masses was not obeyed like in VIII group Co is placed before Ni in spite of higher atomic mass.


c. Isotopes could not be assigned any certain position in the table.

d. He could not explain cause of periodicity among the elements.

e. It could not explain the grouping of elements with different properties in same subgroups.

Besides Gallium, which two elements have been discovered that fill the gaps left by Mendeleev in his periodic table? Ans:- Scandium and Germanium have been discovered for the Eka-Boron and Eka-Silicon gaps.

What was the criteria used by Mendeleev in creating his periodic table? Ans:- The relation between atomic weights and chemical formulae for oxides and hydrides of elements were used by Mendeleev in creating his periodic table.

Why do you think that the noble gases are placed in a separate group? Ans:- Mendeleev created his periodic table by observing a relation between atomic weights and formulae of oxides and hydrides of various elements. Since the noble gases do not react with either of the two, hydrogen or the oxygen, they do not have any such formula. Hence, they are placed in a separate group at the extreme right of the periodic table named as Zero Group.

State Modern Periodic law? Ans:- Modern periodic law was put forth by J H Moseley in 1913AD. It states that, “the physical and chemical properties of elements are a periodic function of their atomic numbers.”

What is Modern periodic table? How was it created? Ans:- The periodic table based on the modern periodic law is called the Modern Periodic Table. It was framed by Neils Bohr. Elements in the periodic table are arranged in increasing order of their atomic numbers because atomic number is a unique and most fundamental property of an element. Since number of electrons in a neutral atom is called Atomic number, therefore, the basis of classification

of elements according to Modern Periodic Law is the electron configuration of atoms(based on electronic configuration). The Periodic Table is a table of all known elements arranged in ascending order of their atomic numbers in rows and columns. There are 7 rows and 18 columns in the periodic table. In total, there are 120 elements in the current periodic table, and more are discovered each year.

What are the main characteristics of the Modern Periodic Table or Long form of Periodic Table? Ans:- The main characteristic of Modern periodic table are as under; a. Arrangement of elements in the modern periodic table

is carried out by taking their electronic configuration into consideration.

b. The chemical properties of the elements are decided on the basis of their valence electrons.

c. It consists of 7 horizontal rows called periods numbered from 1 to 7.

d. In each period a new shell starts filling up. The period number is also the number of shell which starts filling up in it.

e. The first period is the shortest period of all periods and contains only 2 elements, H and He.

f. The second and third periods are called short periods and contain 8 elements each.

g. Fourth and fifth periods are long periods and contain 18 elements each.

h. Sixth and seventh periods are very long periods containing 32 elements each.

i. It has 18 vertical columns called groups numbered as 1 to18.

j. All elements present in a group have similar electronic configurations.

k. Elements in a group have same number of valence electrons.


l. Elements in a group have similar physical and chemical properties.

What are the achievements of Modern Periodic Table? Ans:- The main advantages of modern periodic table are as under; a. The table is based on atomic numbers of elements. This

is a more fundamental quantity than the atomic mass concept used earlier.

b. The table shows why elements in the same group display same properties. The table also shows how and why the properties of elements differ in the same period.

c. The periodicity is related to electronic configuration. This is displayed very clearly by the periodic table.

d. All chemical and physical properties are a manifestation of the electronic configuration of the elements.

e. The position of isotopes is fully explained. f. The distinction of elements as metals, transition metals

and nonmetals is clearly shown. g. It is easy to remember and reproduce Periodic table.

Hence the periodic table is very systematic and follows the fundamental electronic structure of the elements to classify them.

How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic table? Ans:- Modern periodic table is based on the fundamental property of elements called Atomic Number. Due to this reason most of the short comings of Mendeleev’s periodic table have been removed, e.g. a. Position of Hydrogen was solved as it has one valence

electron and one valency, so it is kept in the first period but movable from group 1 to group 17.

b. It explained that cobalt was rightly placed before nickel as it has a lower atomic number.

c. It gave right places to the isotopes as isotopes are the atoms of same element with same atomic numbers and different atomic masses.

How does the electronic configuration of an atom relate its position in the Modern Periodic Table? Ans:- The modern periodic table is based upon atomic numbers of the elements. Since electronic configuration of elements depends upon their atomic numbers, this means that the periodic table is based upon the electronic configuration of the elements. e.g. All the group 1 elements have one valence electron and group 2 elements have 2 valence electrons. The element with valency of 1 is in the 17th group and the element with valency of 2 is in the 16th group. Element which has second shell as its valence shell is in the second period and that element that has its third shell as valence is in the third period. From these facts it can be inferred that number of valence electrons or valency tells about the group number of the element while as the valence shell number gives the period number to which element belongs.

What is meant by trends in periodic Table? Ans:- The gradual and continuous changes in physical and chemical properties of elements across a period and down a group is called Trend.

What are different trends in properties of elements in Modern periodic table? Ans:- Some of the properties which show a trend in modern periodic table are as under:-

a. Valency:- The number of electrons in outermost shell of an atom are called valence electrons. The number of electrons an atom of an element requires to complete its octet is called VALENCY. On moving from left to right in a period, the number of valence electrons increases from 1 to 8 while valency first increases from 1 to 4 and then decreases from 4 to 0. However moving down the group makes no change in number of valence electrons and valencey.

b. Atomic Size:- The distance from the centre of the nucleus to the outermost shell of an atom is called atomic radius or atomic size. Atomic radius or size decreases on moving

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from left to right across a period. However atomic size increases as we move down a group.

c. Metallic and Non-Metallic Character:- The tendency of the atoms of the elements to lose electrons and form positive ions is called Metallic Character. Similarly the tendency of atoms to gain electrons and become negative ions is called Non-Metallic Character. As we move from left to right in a period, the Metallic Character Decreases while Non-Metallic Character increases. However, on moving down a group the metallic character increases and Non-Metallic Character Decreases.

What are the important characters of elements in a period? Ans:- The important characters of the elements in a period are as under; a. The atomic numbers are consecutive. b. The number of valence electrons in the elements

increases incrementally from left to right. c. The elements of the same period have different

valences. d. The atomic radii decrease while going from left to right

in a period. e. Metallic character reduces while going from left to right

in a period. What are the important characters of elements in a

group? Ans:- The main important characters of elements in a group are as under; a. The atomic numbers are not consecutive. b. The number of valence electrons in the elements is

same in a group. c. The elements of the same group have the same

valencies. d. The atomic radii increase while going from top to

bottom in a group. e. Metallic character increases while going from top to

bottom in a group for metallic groups. For non-metallic

groups, the non-metallic nature decreases while going from top to bottom.

f. Chemical reactivity increases while going from top to bottom in a group for metallic groups. For non-metallic groups, the chemical reactivity decreases while going from top to bottom.

Compare and contrast the arrangement of elements in Mendeleev’s and Modern periodic Table? Ans:- The differences between Mendeleev’s and Modern Periodic tables are as under;

Mendeleev’s Periodic Table Modern Periodic Table

1. Elements are arranged in increasing order of their atomic masses.

1. Elements are arranged in increasing order of their atomic numbers.

2. There are no separate positions for isotopes of an element as their atomic masses are different.

2. Separate positions are not required as they have same atomic numbers.

3. Some elements of higher atomic masses have been placed before elements of lower atomic masses.

3. No such problems occur in this table. All elements are in proper order.

4. Some dissimilar elements are grouped together while some similar elements are not grouped together.

4. All similar elements are grouped together.

5. Electronic configuration of an element cannot be calculated from its position.

5. Electronic configuration of an element can be easily calculated from its position.

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Periodic Classification Of Elements (Q&A).pdf

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