5 Periodic Classification of Elements

CHAPTER  5.
PERIODIC CLASSIFICATION OF ELEMENTS
PAGE No: 81

1. Did Dobereiner's triads also exist in the columns of Newlands Octave? Compare and findout.

Yes. one of the Dobereiner's triads exist in the columns of Newlands' octave .
Li, Na & K are found in the first column of Dobereiner's triads whereas the same elements in the same order are placed in the second column of Newlands' octave.

2. What were the limitations of Dobereiners classification?

Dobereiner could classify only three triads from the elements known at that time.

It is not found useful since all of the elements known at that time could not be arraigned in triads

3.  What were the limitations of Newlands Law of  Octaves?

1. It was applicable to only lighter elements up to calcium having atomic mass 40u.
     After calcium , the first and eighth element did not show similar properties.
    Eg : Cr ( chromium ) & Y  ( yttrium) placed in the same column, but having different properties

2. It is assumed by Newlands that only 56 elements existed in nature and no new elements would be        discovered in the future. But later several new elements were discovered, whose properties does          not fit into Law of Octaves.

3.  Some similar elements are seperated from one another while some dissimilar elements was placed       in the same column
      Eg: Fe ( iron) is placed away from Co ( cobalt )and Ni ( nickel)
      Co and Ni  do not resembles halogens( F, Cl, Br) , but placed in the same column

 4. When noble gases were discovered, it is the first and the ninth element which has similar                      properties and not the eighth element.

PAGE No: 85

1. Use Mendeleev's Periodic table to predict the formulae for the oxides of the following elements
    K, C, Al, Si, Ba

  Element                      Group             Oxide Formula                     Oxide

 K (Pottasium)            group I                    R2O                                   K2O

C ( Carbon)                 group IV                RO2                                  CO2


Al ( Aluminium)         group III                R2O3                                 AL2O3 

Si (Silocon)                 group IV                RO2                                  SiO2

Ba (Barium)                group II                  RO                                   BaO

2 . Besides Gallium which other element have since been discovered that were left by Mendeleev in  his Periodic Table? ( any two)

Eka - Boron  in Mendeleev 's periodic table  was found out and named as  Scandium
Eka - Aluminium   in Mendeleev 's periodic table  was found out and named as Gallium
Eka - Silicon  in Mendeleev 's periodic table  was found out and named as Germanium

3. What were the criteria used by Mendeleev in creating his Periodic Table

Mendeleev used atomic masses, physical properties  and chemical properties  as the criteria for creating his periodic table. Here the elements where arraigned in the order of increasing atomic masses

Mendeleev's Periodic law states that physical and chemical properties of the elements are the periodic functions of there atomic masses

4. Why do you think the Noble gases are placed in a separate group?

Noble gases were discovered very late because they are inert and present in extremely low concentration in atmosphere.
It includes Helium, Neon, Argon, Krypton and Xenon

But when they were discovered they could be placed in a new group with out disturbing the existing Mendeleev's table

It is named as zero group which is added later by  Ramsay

PAGE No: 90

1. How could the Modern Periodic Table remove various anomalies of Mendeleev's Periodic Table?

1. Placement of Hydrogen with alkali metals is justified on the basis of its electronic configuration.

2. In modern periodic table elements with similar valence electrons fall in the same group.Hence are      arraigned in the gradation of chemical and physical properties.

3. All the isotopes of the same element has a single position on modern periodic table, since the               elements are arraigned in the order of atomic number

4. Prediction of properties of elements could be made with precision when elements are arranged on        the basis of increasing atomic number.

2. Name two elements you would expect to show chemical reaction similar to magnesium. What       is the basis of your choice?

Calcium and Beryllium
It belongs to the II group of Modern periodic table, and having two electrons in there outermost shell.
Since all  these elements have two electrons in the outermost shell, they show similar properties.

They are called as Alkaline earth metals . These include Beryllium, Magnesium, Calcium, Strontium, Barium and Radium

3. Name
(a) Three elements that have a single electron in there outermost shell

Lithium, Sodium, Potassium, Rubidium , Cesium and Francium. These belongs to the first group of the periodic table and called as Alkali Metals

(b)  Two elements that have two electrons in there outer most shell.

Beryllium, Magnesium, Calcium, Strontium , Barium and Radium.  These belongs to the second group of the periodic table and called as Alkaline earth  Metals

(c) Three elements with filled outermost shell

Helium , Neon, Argon, Krypton, Xenon and Radon  These belongs to the eighteenth group of the periodic table and called as Noble gas

4 (a) Lithium , sodium , potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements.

Yes , there are similarity in the atoms of these elements.
Let us examine the balanced equations

2Li(s)   + 2H₂O(l)       à      2LiOH  (aq) + H₂(g)

2Na (s)   +  2H₂O (l)    à     2NaOH (aq)  + H₂(g)

2K (s)  +  2H₂O(l)      à      2KOH (aq) + H₂(g)

From this we can understand that all these elements  has the valency 1. From its electronic configuration, it is evident that all the elements are similar with one electron  in its outermost shell. Hence it belongs to I group. This group is called as alkali metals.

(b) Helium is an unreactive gas  and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?

The elements helium and neon belongs to noble gas that is  18th group of the periodic table.

The elements of this group have completely filled valence shell.
Now we can examine their electronic configuration

He :  2
Neon : 2, 8

Now it is evident that the similarity in their atom is the completely filled valence shell.

5. In the modern periodic table, which are the metals among the first ten elements?

Li(Lithium) , Be( Beryllium)

6. By considering the positions in the periodic table, which one of the following elements would you expect to have maximum metallic characteristic?
Ga  ,   Ge  ,  As,  Se  , Be

Be 

Metallic character decreases from left to right. Here beryllium is on the most left among the elements given,

EXERCISE 

PAGE No: 91

1. Which of the following is not a correct statement about the trends when going from left to right across the periods of periodic table?

(a) The elements become less metallic in nature  ✅
(b) The number of valence electron increases✅
(c) The atoms loose their electrons more easily❌
(d) The oxides become more acidic✅

2. Element X forms a chloride with the formula XCl₂, which is  solid with high melting point . X would most likely to be in the same group of the periodic table as

(b)Mg

3. Which element has

(a) Two shells both of which are completely filled with electrons

Neon.  Electronic configuration  2,8

(b)  The electronic configuration 2,8,2

Mg

(c) A total of three shells with four electron in its valence shell

Silicon   Electronic configuration 2,8,4   

(d) A total of two shells with three electrons in its valence shell

Boron      Electronic configuration 2,3

(e) Twice as many electrons in its second shell as in its first shell

Carbon    Electronic configuration 2,4 

4.(a) What property do all elements in the same column of the periodic table as boron have in common?

All the elements of boron family has a valency of  three and are known as boron family. It includes  Boron, Aluminium, Gallium, Indium and Thallium .
Here Boron is a metalloid where us all others are metals.

(b) What property do all elements in the same column of the periodic table as fluorine have in common?

All the elements of fluorine family has a valency of one and are known as halogens. It includes  Fluorine, Chlorine, Bromine, Iodine and  Astatine
All the elements in this group are gases.
They are highly reactive.
The most reactive element fluorine belongs to this family.

5. An atom has electronic configuration 2,8,7

(a) What is the atomic number of the element?

17 and the element is  Chlorine

(b) To which of the following elements would it be chemically similar ( Atomic number is given in parenthesis)
N(7)         F(9)          P(15)          Ar(18)

The electronic configuration of the given elements are as a follows

N(7)     :  2,5
F(9)      : 2,  7
P(15)     :  2, 8, 5
Ar(18)   : 2, 8, 8

Here F(9) only has seven electrons in its valence shell as that of the given element chlorine

6. The positions of three elements A, B and C  in the periodic table are shown below

Group  16                      Group  17
 -----                                  ------
------                                  A
------                                -----
  B                                     C

(a) State whether A is ametal or non-metal

Non metal

(b) State whether C is more reactive or less reactive than A

Less reactive than A

(c)  Will C be larger or smaller in size than B?

C is smaller than B

(d) Which type of ion, cation or anion, will be formed by element A?

Anion, 17th group is halogens and having a valence of 1 and form ionic bonds

7. Nitrogen ( atomic number 7 ) and phosphorus ( atomic number 15) belong to group 15 of the periodic table . Write the electronic configuration of these two elements.Which of these will be more electronegative? Why?

Electronic configuration

Nitrogen : 2,5

Phosphorus : 2,8,5

Nitrogen is more electronegative than phosphorous.

In a periodic table electronegativity decreases from top to bottom. Here nitrogen is above phosphorous in the group 15 of the periodic table

8. How does the electronic configuration of an atom relates to its position in the modern periodic table?

Groups / Column :  Valence electron determines the group number of the element

Periods / Rows  :  Number of shells determines in which period the elements belongs.

The group number of the elements having more than two valence electrons is the number of valance electrons plus ten 

9. In the modern periodic table , calcium ( atomic number 20) is surrounded with elements with atomic number 12, 19,  21  and 38. Which of these have physical and chemical properties resembling calcium?

Electronic configuration  of calcium is 2,8,8,2

Electronic configuration of the given elements are 

Magnesium ( 12)  : 2, 8, 2

Potassium   ( 19)    : 2,8,8,1

Scandium  ( 21)    :  2,8,8,3

Strontium (32)    : 2, 8, 18,2

Magnesium and strontium shows same physical and chemical properties as calcium.

These three elements belongs to the same group with valence electrons two

10. Compare and contrast the arrangement of elements in Mendeleevs periodic table and Modern periodic table

Mendeleev’s Periodic table	Modern Periodic Table
Based on Atomic mass	Based on Atomic Number
In a group all elements does not have same number of valence electrons	In a group all elements have same number of valence electrons
Law states that the physical and chemical properties of elements are periodic functions of their atomic mass	Law states that the physical and chemical properties of elements are periodic functions of their atomic number
The elements having same physical and chemical properties may not fall in a group	The elements having same physical and chemical properties fall in a group
The real elements and its isotopes may not be placed in the same position	The real elements and its isotopes are placed in the same position
A precise prediction of properties of elements is not possible	A precise prediction of properties of elements is possible