Sunday, December 23, 2012

CBSE/NCERT Board Solved Questions class X NCERT Ch. Periodic Classification

 Solved Questions Set-01
Q.1. Why does silicon is classified as Metalloid?

Ans: Silicon is gray color solid at room temperature with very high melting point and boiling point that lose or gain 4 electrons 3s2, 3p2] having both metallic and non metallic properties so it is classified as Metalloid eg. Sio2
Q.2.Why inert gases have zero valencies?

Ans: It is because Inert gases have 2 or 8 electrons in valence shell.

Q.3. How is valency of an element determined?

Ans: The valency of an element measures its ability to combine with other elements. The valency is determined by knowing place in periodic table.. All the elements in group VIII have eight electrons in their outer shells, and thus have a valency of zero (highly stable). Elements in group I just have one valence electron in their outer shells and thus have a valency of one.

Q.4. why could no fixed position be given to hydrogen in Mendeleev’s Periodic table?

Ans: Hydrogen placed in the first column above alkali metals. it is because hydrogen and alkali metals have similar properties.
However, hydrogen also resembles halogens. Like halogens, hydrogen is a gas, and exists as a diatomic molecule (H2). It forms covalent compounds like halogens unlike alkali metals. Hence, it can also be placed above the halogen group .Therefore, Mendeleev was not able to explain the position of hydrogen.

Q.5. 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

Ans: It is 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).

Q.6. Why was Dobereiner’s system of classification of elements into triads not found to be useful?

Ans: it is because he could not arrange all elements in triads.

Q.7. Oxygen (O, 8) and sulphur (S, 16) belong to group 16 of the periodic table :-
(i) Write the electronic configuration and valency of these two elements?
(ii) Which among these will be more electronegative? Why?

Ans: (i) Oxygen (O, 8) = 2,6  ; Vacancy =2           Sulphur (S, 16) = 2,8,6 ; Vacancy =2  
(ii) Oxygen will be more electronegative due to electro negativity decreases from top to bottom because atomic size increases.

Q.8. How does electronic configuration of atoms change in a period with increase in atomic number?

Ans. On moving across a period from left to right, the atomic number of the elements increases, therefore, the number of electrons in the valence shell increases from 1 to 8, i.e, the first element in the given period will have one electron in its valence shell and the last element in the same period will have eight electrons.

Q.9. What happens to the melting points and boiling points of elements while moving down in a group?

Ans. The melting points and boiling points decrease while moving down in group of metals.
The melting points and boiling points increases while moving down in group of non-metals.

Q10. Why Chlorine (atomic number 17) is more electronegative than sulphur (atomic number 16)

Ans: The nucleus of chlorine has more tendency to attract an extra electron than the nucleus of sulphur because chlorine needs only one 1 electron to complete its shell. Hence, chlorine is more electronegative than sulphur.

 Solved Questions Set-02

1. How does the electronic configuration of an atom relate to its position in the Modern periodic table?

Ans. The outermost electron in  electronic configuration give idea of group in Modern periodic table and also period.

2. Give examples of some :  a. Metalloids b. Alkali metals c. Halogens d. Alkaline Earth metals e. Nobel gases or inert elements

Ans: a. 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

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 if predicted. 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.

Solved Questions Set-03

VERY SHORT ANSWER TYPE QUESTION (1 MARKS)

1. In the modern periodic table which are the metals among the first ten elements.

Ans. Only lithium, beryllium.

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

Ans. Ga.

3. Which of the following statement is not correct statement about the trends when going from left to right across the periods of periodic table.
A. The element becomes less metallic in nature.
B. The number of valence electron increase.
C. The atoms lose their electron more easily
D. The oxide becomes more acidic

Ans. The atoms lose their electron more easily.

4. Element X forms a chloride with the formula XCl2 which is solid with a high melting point. X would most likely be in the same group of periodic table as (a). Na (b). Mg (c). Al  (d). Si?

Ans. (b). Mg

5. Which element has two shells, both of which are completely filled with electrons?

Ans. Ne (2,8).

6. The electronic configuration 2, 8, 2 ?

Ans. Mg (2,8,2).

7. A total of three shells, with four electrons in its valence shell?

Ans. Si (2,8,4).

8. A total of three shells, with three electron in its valence shell?

Ans. Al(2,8,3).

9. Twice as many electrons in its second shell as in its first shell?

Ans. C (2,4).

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

Ans. Electronic configuration of N (7) = 2, 5 Electronic configuration of P (15) = 2, 8,5 
Nitrogen will be more electronegative due to electro negativity decreases from top to bottom because atomic size increases.

11. In the modern periodic table Ca (At no. = 20) is surrounded by elements with atomic number 12, 19, 21, and 38. Which of these have physical and chemical properties resembling Ca.

Ans. 38,12

VERY SHORT ANSWER TYPE QUESTION (2-marks each)

Q.1. Did Doberenier‘s triads also exist in the column of Newland octaves. Compare and find out.

Ans. Yes, Li, Na, K

Q.2. What are the limitation of Dobernier‘s classification?

Ans. Only few elements are classified into triads. So, this classification is rejected.

Q.3. What are the limitations of Newlands law of Octaves

Ans. (a) It was applicable only upto atomic mass 40 of Ca. (b) Two elements Ni and Co are placed in column as F, Cl, Br which have different properties.

Q.4. Use Mendeleev‘s periodic table to predict the formula for the oxide of following element K, C, Ba, Al

Ans. K –I A –K2O C -IV A –CO2 Al –III A-Al2O3 Ba –II A –BaO

Q.5.Besides Ga, which other elements have been discovered that were left by Mendeleev in his periodic table (Any two)?

Ans. Eka-Boron-Scandium Eka-Silicon-Germanium

Q.6. What were the criteria used by Mendeleev in creating his periodic table?

Ans. Two criteria used by Mendeleev (i) Atomic mass   (ii) Similar chemical properties

Q.7. Why do you think the noble gases are placed in a separate group?

 Ans. These gases were discovered very late and placed in a separate group because they were inert.

Q.8. Name two elements you would expect to show chemical reaction, similar to magnesium. What is the  basis for your choice?

Ans. Ca, Be On the basis of the elements belong to same group-2. So valance electrons are same.

Q.9. (a) What property do all elements in the same column of the periodic table as Boron have in common
        b) What property do all elements in the same column of the periodic table as Fluorine have in common?

Ans. (a)  All elements in the same column as Boron have 3e- in valence shell and form oxide X2O3.
       (b)  All elements in the same column as Fluorine have 7e- in valence shell so their valency is one.

Q.10. An atom has electronic configuration 2,8,7 (a) What is the atomic no. of this element? (b) To which of the following elements would it be chemically similar N(7) F(9) P(15) Ar(18)

Ans. (a) Atomic no=17 (b) F(9).

Short answer type question (3-marks each)

Q.1-How could the modern periodic table remove various anomalies of Mendeleev’s periodic table?

Ans-Various anomalies are removed in following ways-

(a) The position for all isotopes of an element justified since they have same atomic number.
(b) The position of certain elements which are earlier misfit like Co-58.9 is placed before Ni-58.7 are now justified because Co has lower atomic number than Ni.
(c) Cause of periodicity explained due to same electronic configuration repeated after certain gap.

Q.2- Name
(a) three elements that have a single electron in their outermost shells.
(b) two elements that have two electrons in their outermost shells.
(c) three elements with filled outermost shell.

Ans- (a) Li ,Na ,K(Group-1)
        (b Be ,Mg (Group-2) 
       (3) Ne ,Ar, Kr (Group-18)

Q.3- (a) Li,Na,K are all metals that react with water to liberate H2 gas .Is there any similarity in the atoms of these elements.
(b) Helium is an unreactive gas and Neon gas is a gas of extremely low reactivity, what do their atoms have in common.

Ans-(a) there are following similarities.
(1) these atoms have same number of electrons in outermost shell   3Li -2,1 11Na-2,8,1 19K-2,8,8,1
(2) Li,Na,K react with oxygen to form oxide which are basic in nature.  
(b) both He and Ne have completely filled shell

Q.4-The position 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 a metal or non-metals. 
 (b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size then B.       
(d) Which type of ion, cat ion or anion will be formed by element A

Ans-(a)-Non-metal
(b) A is more reactive due to small size, have more tendency to accept outside electron
(c) C is smaller in size than B due to atomic size decreases from left to right in a period
(d) Anion formed due to tendency to accept electron

Q5. How are the positions 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, Group. no.=2 or IIA   Period no.=3 e.g. 2. Atomic no. of Cl=17; Electronic Configuration=2,8,7       therefore, Groupp. no.=17th  or VIIA

Solved Questions Set-04

Q1. What is Modern periodic Law?

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

Q2. What were the limitations of Dobereiner 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 Newlands’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 Newlands’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. 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.

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