Short Notes and Board Exam Questions: Periodic Classification of Elements

• Matter around us is present in the form of elements, compounds and mixtures.

• Elements are substances containing atoms of only one type. E.g., Na, Mg, Au, etc.
• There are 118 elements known to us. All these have different properties.

Need for Periodic Classification

• To make the study of these elements easy, these elements have been divided into few groups in such a way that elements in the same group have similar properties. Now study of a large number of elements is reduced to a few groups of elements.
• Dobereiner’s Traids : When elements were arranged in the order of increasing atomic masses, groups of three elements (known as traids), having similar chemical properties are obtained.

The atomic mass of the middle element of the triad was roughly the average of the atomic masses of the other two elements.

Limitations : Only three traids were recognized from the elements known at that time.

Li             Ca                 Cl
Na           Sr                  Br
K             Ba                   I

• Newland’s Law of Octaves :
Newland arranged the then known elements in the order of increasing atomic masses and found that the properties of every 8th element is similar to that of the 1st element.

He compared this to the octaves found in music and called it the ‘Law of Octaves’.

For example, the properties of lithium (Li) and sodium (Na) were found to be the same.

 

Newland’s Octave

Limitations :

• It was applicable upto calcium (for lighter elements only).

• Properties of new discovered elements did not fit into the law of octave.

• To fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties.

Mendeleev’s Periodic Table : When elements are arranged in the order of increasing atomic masses, the element with similar properties occur at regular intervals. The properties of elements are a periodic function of their atomic masses.
Mendeleev’s periodic table is based on the chemical properties of elements. It contains 7 periods (horizontal rows) and 8 groups (vertical columns).

Table. Mendeleev’s Periodic Table

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Modern Periodic Table

• Atomic number of an element is a more fundamental property than its atomic mass.
• According to the Modern Periodic law : The properties of elements are a periodic function of their atomic number.
• All the anomalies of Mendeleev’s classification disappear.

Merits of Mendeleev’s Periodic Table

(i) Some gaps were left for the undiscovered elements like gallium (Ga), Scandium (Sc) and Germanium (Ge).
(ii) Predict properties of elements on the basis of their positions in the periodic table.
(iii) Accommodate noble gases when they were discovered without disturbing the original arrangement.

Limitations of Mendeleev’s Classification

(i) Position of isotopes could not be explained.

(ii) No fixed position for hydrogen.

(iii) Wrong order of atomic masses of some elements could not be explained.

Explanation of the Anomalies :

(i) Explanation for the position of isotopes (Same atomic number put at one place in the same group).

(ii) Cobalt with atomic number 27 came first and nickel (28) should come later.

(iii) Unlike atomic masses, atomic number is always a whole number, so there is no element between hydrogen and helium.

 

• Atomic Number : It is denoted by Z and equal to the number of protons in the nucleus of an atom.

• Modern Periodic table has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’.

• Elements with same number of valence electrons are placed in the same group. For example, Li : 2, 1 Na : 2, 8, 1 K : 2, 8, 8, 1

Outermost or valence shell in all the three contains 1 electron. These elements have been placed in the same group.

• Number of shells increases as we go down the group.

• Elements with same number of occupied shells are placed in same period.
For example, Li (2, 1); Be (2, 2); B (2, 3), C (2, 4), N(2, 5). These elements have same number of shells (two).

• Each period marks a new electronic shell getting filled.

• Number of elements placed in a particular period depends upon the fact that how electrons are filled into various shell.

• Maximum number of electrons that can be filled in a shell is given by 2n2

where n is shell number.

 E.g., K shell n = 1 or 2n2 = 2(1)2 = 2             First period has 2 elements.
 L shell n = 2 or 2n2 = 2(2)2 = 8                            Second period has 8 elements

• Position of an element in the periodic table tells us its chemical reactivity.
• Valence electron determine the kind and number of bonds formed by the element.

 

Trends in the Modern Periodic Table

Valency : No. of valence electrons present in the outermost shell of its atom. On moving from left to right in each period, the valency of elements increases
from 1 to 4 and then decreases to 0.

Valency remains the same down in a group.


Atomic size : Atomic size refers to the radius of an atom. It may be visualized as the distance between the centre of the nucleus and the outermost shell.

• Atomic size or radius of an atom decreases as we move from left to right in a period because due to large +ve charge on the nucleus, the electrons are pulled in more close to the nucleus and size decreases. E.g.,

• Atomic size increases as we move down the group because new shells are being added and this increases the distance between nucleus and outermost electron.

 

Metallic Character

• Metallic character means the tendency of an atom to lose electron.

• Metals occupy the left hand side of the periodic table.

• On moving left to right in a period, the metallic character of an element decreases because the effective nuclear charge increases. It means tendency to lose electron decreases.

• Metals are electropositive as they tend to lose electrons while forming bonds.

• Metallic character increases as we go down a group as the effective nuclear charge is decreasing.

Non-metallic Character

• Non-metals are electronegative as they tend to form bonds by gaining electrons.

• Non-metals occupies the right side of the periodic table.

• Non-metallic character increases across a period because due to increase in effective nuclear charge that means tendency to gain electron increase.

• Non-metallic character decreases as we move down a group due to decrease in effective nuclear charge experienced by the valence electron thus the tendency to gain electron decreases.

• In the middle of periodic table we have semi-metals or metalloid because they exhibit some properties of metals and non-metals.

• Oxides of metals are basic in nature while oxides of non-metals are acidic in nature.

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                                             QUESTIONS

VERY SHORT ANSWER TYPE QUESTIONS (1 Mark)

1. Write down three elements that show Dobereiner’s triad.

2. Write down two drawbacks of Newland’s law of octaves.

3. What was the need for classification of elements ?

4. Which important property did Mendeleev used to classify the elements in his periodic table ?

5. What do you mean by valency ?

6. How many elements are known till date ?

7. State Modern Periodic law.

8. Name the elements and its valency having electronic configuration 2, 8, 3.

9. How many rows and columns are there in modern periodic table ?

10. Why properties of elements are different of same period ?

 

SHORT ANSWER TYPE QUESTIONS (2 Marks)
1. How does the tendency to lose electrons change in a group and why ?

2. Why He, Ne and Ar are called inert gases ?

3. Write two limitations of Mendeleev’s Periodic Table.

4. Why is the position assigning to hydrogen in the periodic table considered anomalous ?

5. What do you mean by metallic character of an element ? How does it vary as we go down a group ? Give reason for this variation.

6. Why metallic oxides are basic in nature whereas non-metallic oxides are acidic in nature ?

7. How does the atomic size vary as we go down a group and move left to right in a period ? Write the reason behind it.
SHORT ANSWER TYPE QUESTIONS (3 Marks)
1. Four elements P, Q, R and S have atomic number 12, 13, 14 and 15 respectively. Answer the following :
(a) What is the valency of Q ?
(b) Classify these elements as metals and non-metals.
(c) Which of these elements will form the most basic oxide ?

2. (a) How do we calculate the valency of an element from its electronic configuration ?
     (b) How does the valency vary in a period ?

3. Study the variation in the atomic radii of elements given below and arrange them in increasing order :
                                                                 Na             Li              Rb           Cs           K
                                                                186           152          246        262       231

(a) Name the element which has the smallest and the largest atomic size.
(b) How does the atomic size vary as we go down a group ?

4. What are metalloids ? Write two examples.

 

LONG ANSWER TYPE QUESTIONS (5 Marks)
1. Write down five major differences between Mendeleev’s periodic table and Modern periodic table.

2. Element A has atomic no. 16.
(a) Name of the element
(b) Physical state
(c) Compound with hydrogen
(d) Metal or non-metal
(e) Nature and formula with oxides

 

VALUE BASED QUESTION

Ria and Reena are the students of Class X. Ria is very much organized and maintained whereas Reena is an unorganised student and always faces a lot of problems in handling life situations.
(a) How organization helps in daily life ?
(b) How can you relate the above fact with the chapter ‘Classification of Elements’. How classification of elements help us studying them properly ?

 

Hints to Long Answer Type Questions

1. Medeleev’s Periodic Table Modern Periodic Table
(a) Elements have been arranged in Elements have been arranged in increasing increasing order of atomic masses. order of their atomic number.
(b) It consist 8 groups. It consist 18 groups.
(c) All the groups from I to VIII are No sub-groups. divided into two sub-groups.

2. Element A(16) = 2, 8, 6.
(a) Sulphur (S)
(b) Solid
(c) H2S
(d) Non-metal
(e) Acidic in nature; oxide – SO2

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