X CBSE Study Accelerator E-Notes for Chapter Periodic Classification Of Elements

Classification of elements: the arrangement of elements in such a manner that elements with similar properties are grouped together while elements with dissimilar properties are separated.

EARLY ATTEMPTS OF CLASSIFICATION OF ELEMENTS

1. Lavoisier’s classification of elements: In 1789, Lavoisier first attempted to classify the elements into two divisions namely Metals and Non-metals. However this classification was not satisfactory as there were many exceptions in each category.

2. Dobereiner’s classification of elements: In 1817, Johann Wolfgang Dobereiner grouped three elements into what he termed triads.

In each case, the middle element has an atomic mass almost equal to the average atomic masses of the other two elements in the triad.

For example, elements like lithium, sodium and potassium have atomic masses 7, 23 and 39 respectively. They are grouped together into a triad as,

Li  (7)      Na  (23)     K      (39)

Here the atomic mass of sodium is the average of atomic masses of lithium and  potassium.

Limitation of Dobereiner’s law

Only three triads were identified from the element known at that time. Hence, this classification was not useful.

Newland’s law of octaves:

X Chemistry Test Paper For Ch - 5 Periodic Classification of Elements

1 mark questions:- 
 1. What is the name given to group of three similar elements by Dobereiner? 
 

2. State "Newlands law of Octaves" for classification of elements. 

3. Name the fundamental property used by Mendeleev as the basis of classification. 

 4. How many groups and periods are there in the Modern periodic table? 

 5. What was the prediction of Mendeleev regarding the gaps in his periodic table? 

 6. How is valency of an element determined? 

 7. What will be the valency of an element having atomic number 16? 

 8. How does valency vary in going down a group? 

 9. Why inert gases have zero valency? 

 10. What would be the valency of an atom containing 8 electrons in its outermost shell? 

 11. How does the electronegative character of elements vary along a period of the periodic table? 

 12. The present classification of elements is based on which fundamental property of elements? 

 13. Among first ten elements in the modern periodic table name the metals present. 

 14. Metals are on which side of Modern periodic table? 

 15. State Mendleev’s periodic law. 

 16. Name two elements, other than Gallium, whose existence was predicted by Mendeleev. 

 17. State Modern Periodic law. 

 18. Write the name given to the vertical columns in a periodic table. 

 19. What name is given to the horizontal rows in a periodic table? 

 20. Why do silicon is classified as Metalloid?

X CBSE Practice Work sheet

CBSE Board Questions

1. (a) What are metalloids ? Write any two examples.

 (b) Given below are some of the elements of first group Li, Na, K (Their atomic numbers are 3,11,19 respectively and they belong to 2nd 3rd and 4th period respectively) Arrange these in the decreasing order of metallic character exhibited by them. 

2. What are amphoteric oxides? Choose the amphoteric oxide from among the following

oxides :– Na2O, ZnO, Al2O3, CO2, H2O

3. Study the variation in the atomic radii of first group elements given below and arrange

them in increasing order :–

Group I element          Na       Li       Rb          Cs                  K

Atomic Radii P.M      86      152    244         262                 231

4. An element X has the electroic configuration as 2, 8, 7 :–

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

(b) What will be the formula of its compound formed with Na?

(c) What is the name given to the family of this element?

5. How do you calculate the valence of the element from its electronic configuration? What is the valence of Mg with atomic number 12 and sulphur with atomic number 16? How does the valence vary in going down in a group?

6. Atomic radii of the elements of the period II are as follows:–

Period II elements :        Be         B      O      N     C       Li

Atomic Radius :           111        88     66     74    77     152

(i) Arrange them in decreasing order of their atomic radii.

(ii) How does the atomic size vary on moving from left to right in a period? Explain why?

(iii) How will the tendency to lose electrons will vary on moving from left to right in this period II?

7. Oxygen (O, 8) and sulphur (S, 16) belong to group 16 of the periodic table:-

(i) Write the electronic configuration and valence of these two elements?

(ii) Which among these will be more electronegative? Why?

8. Given below are the atomic radii of some elements of second period.

Element		B	O	N	C
Atomic Radius in pm	88	66	74	77

Arrange these elements in the increasing order of their atomic number . Give reason for your answer.

9. Two elements ‘A’ and ‘B’ belong to group 1 and 2 respectively in the same period.

Compare them with respect to :–

(a) Number of valence electrons.

(b) Valency

(c) Metallic character

(d) Size of atom

(e) Formulae of their oxides.

10. (a) Atomic radius of hydrogen is 37pm. Express it in meters. (b) How does atomic size vary in a group and in a period ?

11.  From the part of a periodic table, answer the following questions

1 Hydrogen	2	13	14 Carbon			15	16 Oxygen		17 Fluorine
X				P				Q
Y								R
Z								T

(a) Atomic number of oxygen is 8. What would be the atomic number of, Fluorine ?

(b) Out of ‘X’ and ‘Q’ which element has larger atomic size. Give reason for your answer Fluorine.

(c) Out of ‘Y’ and ‘Z’ which element has smaller atomic size. Give reason for your answer.

12.State Modern Periodic Law. Name the two elements of first period.

13.Two element X, Y and Z belong to 17th group but to 2nd,3rd and 4th period respectively. Number of valence electrons in Y is 7. Find the number of valence electrons in X and Z.

14. How does the metallic character of the elements vary (i) in a group (ii) in a period of the modern periodic table ?

15. Na, Mg, Al and P belong to 3rd period but are placed in first, second, thirteenth and fifteenth group. Number of shells occupied in Mg is three. What is the number of occupied shells in Na, Al and P. Give reason for your answer.

Q. What is the number of elements in first , second and third period of the periodic table? Give reason for your answer. 

X- chemistry Guess Test paper - Ch -5-Periodic-Classification-of-Elements

1. “Elements in Periodic Table show periodicity of properties” List any four such properties.

2. The following table shows the positions of six elements A, B, C,D,E and F in the modern periodic table

Group\Period	1	2		3to 12	13		14	15	16	17	18
2	A							B				C
3		D					E					F

(a). Which element will form only covalent compounds ?  

(b)  Which element is a metal with valency two ?

(c). Out of D and E , which one has a bigger atomic radius ? Give reason for your answers. 

3. (a) Atomic number of Mg and Al are 12 and 13 respectively. Write their electronic configuration. 
(b) Mention the period of the Modern Periodic Table to which the above two elements belong. Give reason for your answer

4. (a) State the Modern periodic law. (b) What is the total number of periods and groups in Modern Periodic Table
5. From the part of a periodic table, answer the following questions

1 Hydrogen	2	13	14 Carbon			15	16 Oxygen		17 Fluorine
X				P				Q
Y								R
Z								T

(a) Atomic number of oxygen is 8. What would be the atomic number of, Fluorine ?

(b) Out of ‘X’ and ‘Q’ which element has larger atomic size. Give reason for your answer Fluorine.

(c) Out of ‘Y’ and ‘Z’ which element has smaller atomic size. Give reason for your answer.

6. State two limitations of Newland's law of Octaves.

7. Name the scientist who proposed modern periodic law? On which fundamental property of elements it is based?
8. Why could no fixed position be given to hyderogen in Mendleev’s Periodic table?

9. What are metalloids? Give two examples.

10. In group 1 of periodic table three elements X, Y and Z have atomic radii 133pm , 95pm and 65pm respectively giving a reason, arrange them in the increasing order of their atomic number in the group.

11. In modern periodic table, the isotopes of Chlorine Cl-35 and Cl-37 having different atomic masses will be kept in different slots or they would be assigned same position on the basis of their chemical properties? Give reason in support of your answer.

12. Nitrogen (At no. 7) and Phosphorus (At no. 15) belong to group 15 of the periodic table:-

13. How and why does the atomic size vary as you go down a group?

14. Why was Dobereiner’s system of classification of elements into triads not found to be useful?

15.(a) “Silicon is classified as a metalloid.” Justify this classification.

(b) Name two more such metalloids.

(c) In which part of the Periodic Table we can look for metalloids. On which side of these we can get non- metals ?

CBSE X Periodic Classification of Elements Solved assignment

Periodic Classification of Elements Assignment Solved

Q1. Explain Dobereiner’s Law of triads along with an example .

Ans. Dobereiner tried to arrange elements with similar properties into groups of 3 elements each. He called these as triads. He showed that when the 3 elements into a triad were arranged in the order of increasing atomic masses, the atomic mass of the middle element was roughly the average of the masses of the other two elements. e.g.           Li                   Na                          K

atomic  mass                                     6.9                 23                          39

                                                         39+6.9/2  =    45.9/2   =    22.9   =  23

Q2. What were the limitations of Dobereiner’s Law of triads?

Ans. Dobereiner could identify only 3 triads from the element known at that time. Hence, this system is not useful.

Q3. State newland’s law of Octaves.

Ans. Newland arranged the elements in the increasing order of the atomic masses and found that every eight element had properties similar to that of the first like the notes of music.

Q4. What were the limitations of newland’s Law of Octaves?

Ans. 1. It was only applicable only up to Calcium as after Calcium every eighth element did  not posses , properties similar to that of first.

2. At a few places unlike elements were put together like cobalt and nickel along with F, Cl and Br.

Q5. Which 2 criteria did Mendeleev used to classify elements in his periodic table ?

Ans. Atomic mass and similarity in chemical property.

Q6. State Mendeleev’s periodic law.

Ans. Properties of elements are a periodic function of their atomic masses.

Q7. Why did Mendeleev leave gap in his periodic table?

Ans. Mendeleev place elements with similar properties one below the other leaving gap  for yet undiscovered elements.

Q8. Name the elements which have since been discovered that were left by Mendeleev  in his Periodic Table?

Ans. Eka Boron – Scandium             Eka   Al - Gallium         Eka   Si - Germanium

Q9. What are the limitations of Mendeleev's periodic table?

Ans. 
1. Position of Isotopes was not clear in his table.       

2. Wrong Order of atomic mass of certain elements  – Elements with higher atomic  mass were placed before elements having lower atomic mass.    e.g. Cobalt was placed before nickel.

3. Position of hydrogen was not clear in his periodic table.

Q10. What is Modern periodic Law ?

Ans. The Properties of elements are a periodic function of their atomic number.

Q11. How were the drawbacks of Mendeleev's periodic table resolved in the modern  periodic table?

Ans. As the modern periodic table was based on the atomic numbers so all the isotopes could be  placed together in one slot.  Secondly, the wrong order of atomic masses was also sorted as Cobalt's atomic number  was lower than Nickel even though it had a higher atomic mass.

Q12. How are the position of elements related to their atomic number in the Modern periodic table?

Ans. The number of valence electrons present in an atom of an element is equivalent to  the group number, while the total number of shells give the period number in which the element to present.

e.g 1. Atomic no. of Mg=12 ;  Electronic configuration=2,8,2     therefore, Gp. no.=2 or IIA   Period no.=3

e.g 2. Atomic no. of Cl=17;  Electronic Configuration=2,8,7       therefore, Gp. no.=17^th  or VIIA

Q13. Explain the gradation in properties of elements in the modern periodic table.

Ans. a. Valency: 

 Valency of elements remain the same  while going down a group 

 Moving across a   period, the valency of  elements increases   from 1 to 4 and then decreases till 0.

b. Atomic size or radius:

  It increases on going down a group

  It decreases in   Moving from left to right along a period.

c. Metallic and non-metallic  properties: :   Moving down a group of metals, metallic nature increases due to increasing size. However, in a group of non-metals the non-metallic character decreases on going    down

Going across a period, the metallic character decreases while Non-metallic character increases

Q14. Give examples of somea. Metalloids – Boron , Silicon , Germanium

b. Alkali metals – Sodium , lithium , Potassium

c. Halogens – Fluorine , Chlorine , Bromine

d. Alkaline Earth metals – Magnesium , Calcium

e. Nobel gases or Inert elements – Helium , Neon , Krypton

X Mendeleev’s Periodic table and its properties 10th Periodic Classification of Elements)

Dmitry Mendeleev a Russian chemist while trying to classify elements discovered that on arranging in the increasing order of atomic mass, elements with similar chemical properties occurred periodically.

In1869, he stated this observation in the following form which is known as Mendeleev’s Periodic Law.

A periodic function is the one which repeats itself after a certain interval. According to the periodic law : The chemical and physical properties of elements are a periodic function of their atomic masses.

A tabular arrangement of the elements based on the periodic law is called periodic table. 

Mendeleev believed that atomic mass of elements was the most fundamental property  and arranged them in its increasing order in horizontal rows till he encountered an element which had properties similar to the first element. 

He placed this element below the first element and thus started the second row of elements. Proceeding in this manner he could arrange all the known elements according to their properties and thus created the first periodic table.



Main features of Mendeleev’s periodic table

1. The horizontal rows present in the periodic table are called periods. You can see that there are seven periods in the periodic table. These are numbered from 1 to 7 

2. Properties of elements in a particular period show regular gradation (i.e. increase or decrease) from left to right.

3. The vertical columns present in it are called groups. You must have noticed that these are nine in number and are numbered from I to VIII and Zero


4. Groups I to VII are subdivided into A and B subgroups. Groups Zero and VIII don’t have any subgroups.

5. All the elements in a particular group are chemically similar in nature. They show regular gradation in their physical properties and chemical reactivities.

After learning about the main features we shall now learn about the main merits of Mendeleev’s periodic table. Merits of Mendeleev’s periodic classification

1. Classification of all elements : Mendeleev’s was the first classification which successfully included all the elements.

2. Prediction of new elements: Mendeleev’s periodic table had some blank spaces in it. These vacant spaces were for elements that were yet to be discovered. 

 For instance, scandium, gallium and germanium, discovered later, have properties similar to Eka–boron,Eka–aluminium and Eka–silicon, respectively. The properties of Eka–Aluminium predicted by Mendeléev and those of the element, gallium which was discovered later and replaced Eka aluminium, are listed as follows

Although it was a very successful attempt but it also had some defects in it. Now we shall discuss thedefects in this classification. 
Defects in Mendeleev’s periodic classification: In spite of being a historic achievement Mendeleev’s periodic table had some defects in it.  The following were the main defects in it:

1. Position of hydrogen: Hydrogen resembles alkali metals (forms H+ ion just like Na+ ions) as well as halogens ( forms H- ion similar to Cl- ion).Therefore, it could neither be placed with alkali metals(group I ) nor with halogens (group VII ).

2. Position of isotopes: Different isotopes of same elements have different atomic masses, therefore, each one of them should be given a different position in the periodic table. On the other hand, because they are chemically similar, they had to be given same position.

3. Anomalous pairs of elements: At certain places, an element of higher atomic mass has been placed before an element of lower atomic mass. For example, Argon (39.91) is placed before potassium (39.1)

CHECK YOUR PROGRESS IN LEARNING

1. Elements A, B and C constitute a Dobereiner’s triad. What is the relationship in their atomic masses?

2. How many elements were included in the arrangement given by Newland?

3. Which property of atoms was used by Mendeleev to classify the elements?

4. How many groups were originally proposed by Mendeleev in his periodic table?

5. Where in the periodic table are chemically similar elements placed, in a group or in a period?

6. Mendeleev’s periodic table had some blank spaces in it. What do they signify?

7. What name was given to the element whose properties were similar to the element eka-aluminium
predicted by Mendeleev?

10th Periodic Properties of Elements in modern periodic Table (Solved problems)

1. Define valency and describe its variation in a period and a group.

Ans: Valency of an element is its combining capacity.Valency of an element = number of valence electrons (if equal to or less than 4)Valency of an element = 8 — number of valence electrons (if more than 4)

2. What happens to the metallic character of the elements as we move in a period from left to right in the periodic table? Give reason.

Ans: Metallic character of the element decreases along a period due to decrease in atomic size along a period or outermost electrons are closer to nucleus. So they cannot be easily lost.

3. How were the positions of different isotopes decided in modern periodic table? 

Ans: All the isotopes of an element have same atomic numbers. In modern periodic table, the elements have been allotted places based on their atomic numbers. Therefore, all the isotopes of an element have been assigned the same position in the modern periodic table

4. Where do you think hydrogen should be placed in the modern periodic table?

Ans: The modern periodic table is based on the atomic numbers of the elements and also upon their electronic distribution. Alkali metals as well as hydrogen have one electron in their valence shell. Therefore, hydrogen should be placed in the group of alkali metals. However, it has been assigned a separate position and is not a member of the group of alkali metals since it is a non-metal.

5. How were the positions of the elements Co and Ni resolved in the modern periodic table?

Ans: On the basis of atomic masses, Ni (58.7) should be placed before Co (58.9). This was defect in Mendeleev’s table because the element Co was placed before Ni. However, the atomic number of (Co = 27) is less than that of Ni ( 28). Therefore, they have been correctly arranged in the modern periodic table.

6. The two isotopes of chlorine have atomic masses 35 u and 37 u. Should they be placed in same slot in the periodic table?

Ans: Yes, they should be placed in the same slot because periodic table is based on the atomic numbers of the element and both the isotopes of chlorine have the same atomic number (Z = 17).

7. Which element is bigger in size, lithium (Z = 3) or Sodium (Z = 11) and why?

Ans: Lithium (Li) atom has only two shells K and L in it whereas a sodium (Na) atom has three shells K L and M. Since a sodium atom has one mote shell than a lithium atom, therefore sodium atom is bigger in size.

8. What were the criteria used by Mendeleev in creating his periodic table?

Ans: Ans. Mendeleev’s periodic table was based on the observation that properties of the elements are a periodic function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then elements with similar properties get their place in the same group in a successive increasing order of atomic masses.

9. Why did Mendeleev leave some gaps in his periodic table?

Ans. Mendeleev left gaps in his periodic table to keep the elements with similar properties together. He predicted that new elements would be discovered later and they would occupy those gaps.

10. Why did Mendeleev treated the hydrides and oxides of element as the basic properties of elements for their classification?

Ans. During the period when Mendeleev gave his periodic table atomic numbers and electronic configurations were not known. Comparison of properties of elements could be best done by comparing their compounds. He selected oxides and hydrides because oxygen and hydrogen form compounds with most of the elements due to their high reactivity.

11. Why are the elements in the same group of the periodic table show close resemblance in their chemical behavior?

Ans: The chemical properties depend upon the number of valence electrons of an element. The elements in the same group of the periodic table show close resemblance in their chemical behavior as they have same number of valence electrons.

12. What happens to atomic radii in a group and period and why?

Ans: Atomic radius is the distance between the centre of atom and the outermost shell.

In a period, atomic radius generally decreases from left to right.

In a period there is a gradual increase in the nuclear charge. Since valence electrons are added in the same shell, they are more and more strongly attracted towards nucleus. This gradually decreases atomic radii.

Atomic radii increase in a group from top to bottom.

As we go down a group the number of shells increases and valence electrons are present in higher shell and the distance of valence electrons from nucleus increases. Also, the number of filled shells between valence electrons and nucleus increases Both the factors decrease the force of attraction between nucleus and valence electron. Therefore, atomic size increases on moving down a group.

13. Why are the ionization energy of elements increases in a period from left to right.

Ans: The minimum amount of energy required to remove an electron from a gaseous atom in its ground state to form a gaseous ion is called ionization energy.

The ionization energy increases in a period from left to right. This is because the force of attraction between valence electron and nucleus increases in a period from left to right.

14. What do you mean by Electron affinity: How it is vary in group and periods in modern periodic table?

Ans: The tendency of element to gain an additional electron. This ability is measured by electron affinity. It is the energy change when an electron is accepted by an atom in the gaseous state. Electron affinity is assigned a positive value when energy is released during the process. Greater the value of electron affinity, more energy is released during the process and greater is the tendency of the atom to gain electron.

In a group, the electron affinity decreases on moving from top to bottom as less and less amount of energy is released.

In a period, the electron affinity increases from left to right, as more and more amount of energy is released

15. What do you mean by Electro negativity?

Ans: Electro negativity is relative tendency of a bonded atom to attract the bond-electrons towards itself. It just compares the tendency of various elements to attract the bond-electrons towards themselves.

Electro negativity decreases in a group from top to bottom. Electro negativity increases in a period from left to right. Electro negativity is related to ionization energy. Electrons with low ionization energies have low electro negativity because their nuclei do not exert a strong attractive force on electrons. Elements with high ionization energies have high electro negativity due to the strong pull exerted on electrons by the nucleus.

Check your understanding:

1) What is the atomic radius of an atom?

2) What is the trend in atomic radius across a period?

3) The trend in atomic radius across a period is caused by _____.

4) What generally happens to atomic radii as one goes down a group or a family?

5) Write the equation for the ionization of an atom.

6) Why inert gases have zero valency?

7) What is ionization energy?

8) Which group or family has the lowest ionization energy?

9) Group 18 or 8A, the noble gases, have the highest ionization energy (True or False).

10) Elements with a high ionization energy lose electrons easily (True or False).

11) The increase in ionization energy across a period is caused by _____.

12) Why does ionization energy generally decrease going down a group or family?

13) What is meant by electron shielding?

14) What is the second ionization energy of an atom?

15) What is electron affinity?

16) Write the equation for electron affinity for an exothermic process.

17) Write the equation for electron affinity for an endothermic process.

18) Which group or family gains electrons most easily?

19) Name two elements, other than Gallium, whose existence was predicted by Mendeleev

20) How does the electrronegetive character of elements vary along a period of the periodic table? 

Periodic Properties of Elements in modern periodic tables Class X (10th)
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