NCERT SOLUTIONS
Chapter 5 : Periodic Classification of Elements
Questions
Page No. 81
- Did Dobereiner’s triads also exist in the columns of Newlands’ octaves ? Compare and find out .
Ans: Dobereiner’s traids did exist in the second column of Newlands’ octaves.
For example: The element of Lithium (Li), potassium (K) and Sodium (Na) constitute a Dobereiner’s traid but are also found in the second column of Newlands’ octaves.
2. What were the limitations of Dobereiner’s classification ?
Ans: limitations of Dobereiner’s classification are
(i) They were not applicable to very low mass or very high mass elements.
(ii) All the elements could not fit into Dobereiner’s traid .
(iii) It failed to arrange all the then known elements in the form of traids of elements having similar chemical properties . Dobereiner could identify only three triads from the elements known that time .
3. What were the limitations of Newlands’ law of octaves ?
Ans: The limitations of Newlands’ law of octaves are
(i) Newlands law was applicable to the classification of elements upto calcium only . After calcium every eight elements did not possess the properties similar to that of the first elements.
(ii) Newland assumed that only 56 elements existed in nature and no more elements would be discovered in the future. But later on several new elements were discovered whose properties did not fit into Newlands’ laws of octaves .
(iii) In order to fit elements into his Table Newlands’ put even two elements together in one slot that too in the column of unlike elements having very different properties .
For example: The two elements cobalt(Co) and Nickel(Ni) were put together in just one slot and that too in the column of element like fluorine, chlorine and bromine which have very different properties from these elements .
(iv) Iron(Fe) which resembles elements like cobalt and nickel in properties was placed far away from these elements.
Questions:
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
Ans: The formulae for the oxides of the following elements are
K, C, Al, Si, Ba
2. Besides gallium, which other elements have since been discovered that were left by Mandeleev in his Periodic Table ? (any two) .
Ans: Scandium and Germanium .
3. What were the criteria used by Mandeleev in creating his Periodic Table ?
Ans: Mandeleev used the relationship between the atomic masses of the elements and their physical and chemical properties . He used similarity in physical properties, similarity in the formation of hydrides and oxides of elements .
4. Why do you think the noble gases are placed in a separate group ?
Ans: Noble gases are chemically inert and are present in atmosphere in extremely low concentrations. Therefore, owing to their similar inert behaviour and similar electronic configuration they are placed in a separate group .
Questions:
Page No. 90
1. How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table ?
Ans: (i) In the Modern periodic table, elements are arranged in the increasing order of their atomic number. This removes the anomaly regarding certain pairs of elements in Mendeleev’s Periodic Table .
(ii) The atomic number of the cobalt is 27 and that of nickel is 28 . Hence cobalt will come before nickel even though its atomic mass is greater.
(iii) All isotopes of the same elements have different atomic masses but the same atomic number .Therefore, they are placed in the same position in the modern periodic table .
(iv) Hydrogen has been alloted special position i.e. it is placed at the top of alkali metals in the first group.
2. Name two elements you would expect to show chemical reactions similar to magnesium . What is the basis for your choice ?
Ans: Calcium and Berillium are similar to magnesium because all the three elements belong to the same group and have two valence electrons in their outer shells .
3. Name
(a) Three elements that have a single election in their outermost shells .
Ans: Lithium(Li), Sodium (Na) and potassium (K) have a single electron in their outer most shells .
(b) Two elements that have two electrons in their outermost shells .
Ans: Magnesium(Mg) and Calcium(Ca) .
(iii) Three elements with filled outermost shell .
Ans: Neon(Ne), Argon(Ar) and Xenon are filled outermost shells .
4. (a) Lithium, sodium and potassium are all metals that react with water to liberate hydrogen gas .Is there any similarity in the atoms of these elements ?
Ans: Lithium, sodium and potassium all belong to the same group . The atoms of lithium, sodium and potassium have only one eletron in their ontermost shells and all of these are metals . All of these react with water to form alkalies .
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common .
Ans: The atoms of helium and neon have their outermost shells completely filled .Helium has its first shell completely filled while neon has its first and second shells (K and L) completely filled .
5. In the Modern Periodic Table, which are the metals among the first ten elements ?
Ans: Lithium and Beryllium are the metals among the first ten elements in the Modern Periodic Table .
6. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristics ?
Ga Ge As Se Be
Ans: Beryllium show has maximum metallic characteristics . In the periodic table, the elements placed on the left side show maximum metallic characteristics. Since beryllium occupies the most left position in comparison to other elements . Hence it shows maximum metallic characteristics .
Exercises:
Page No. 91
1. Which of the following statements 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 electrons increases .
(c) The atoms lose their electrons more easily .
(d) The oxides become more acidic .
Ans: (c) The atoms lose their electrons more easily .
2. Element X forms a chloride with the formula XCl2, which is a solid with a high melting point . X would most likely be in the same group of the Periodic Table as
(a) Na (b) Mg (c) Al (d) Si
Ans: (b) Mg
3. Which element has
(a) Two shells both of which are completely filled with electrons ?
Ans: Neon (Ne) (2,8)
(b) The electronic configuration 2,8,2 ?
Ans: Magnesium (2,8,2)
(c) a total of three shells, with four electrons in its valence shell ?
Ans: Silicon (2,8,4)
(d) a total of two shells, with three electrons in its valence shell ?
Ans: Boron (2,3)
(e) twice as many electrons in its second shell as in its first shell ?
Ans: Carbon (2,4)
4. (a)What property do all elements in the same column of the Periodic Table as boron have in common ?
Ans: Elements in the same column or group as boron have valency of three and have three valence electrons .
(b) What property do all elements in the same column of the periodic table as fluorine have in common ?
Ans: (i) They are all non-metal .
(ii) They have seven electrons in their outermost shells .
(iii) They have valency of one .
(iv) They form acidic oxides .
5. An atom has electronic configuration 2,8,7 ?
(a) What is the atomic number of this element ?
Ans: The atomic number of this element is 17 .
(b) To which of the following elements would it be chemically similar ?
(Atomic numbers are given in parentheses.)
N(7) F(9) P(15) Ar(18)
Ans: F(9)
6. 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-metal .
Ans: Since the valency of group 17 elements is 1 and all these elements accept electrons .Thus A is a non metal .
(b) State whether C is more reactive or less reactive than A .
Ans: C is less reactive than A because as we move down in a group, the reactivity of non metal increases .
(c) Will C be larger or smaller in size than B ?
Ans: C is smaller in size than B because B and C both are related to the same periods and the size decreases as one moves from left to right in a period .
(d) Which type of ion, cation or anion, will be formed by element A ?
Ans: A will form anion because it is a non-metal .
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 ?
Ans: Electronic configuration of Nitrogen is 2, 5 .
Electronic configuration of phosphorus is 2,8,5 .
Nitrogen is more electronegative than phosphorus . On moving down in a group, the number of shell increases .Therefore the valance electrons move away from the nucleus change decreases This causes the decreases in the tendency to attract electron and hence electronegativity decreases .
8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table ?
Ans: Modern Periodic Table is based on the atomic number and atomic number is directly related to the electronic configuration . One can find the group number and period number of an element on the basis of electronic configuration . Hence one can find out the position of an element in the Modern Periodic Table .
9. In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38 . Which of these have physical and chemical properties resembling calcium ?
Ans: The electronic configuration of elements with:
atomic number 12 is 2,8,2 .
atomic number 19 is 2,8,8,1 .
atomic number 20 is 2,8,8 ,2 .
atomic number 21 is 2,8,8,3 .
atomic number 38 is 2,8,18,8,2 .
Therefore elements with atomic number 12 i.e. magnesium (Mg) and 38 i.e. strongtium (Sr) will have simillar chemical properties as element with atomic number 20 i.e. calcium (Ca) .
10. Compare and contrast the arrangement of element in Mendeleev’s Periodic Table and the Modern Periodic Table .
Ans:
Additional Questions:
1. What are Dobereiner’s triads ?
Ans: Dobereiner tried to arrange the elements with similar properties into groups . He identified some groups having three elements each . These groups are called triads or Dobereiner’s triads .
2. What is Newlands’ law of octaves ?
Ans: Newlands arranged the then known elements in the order of increasing atomic masses . He started with the elements having the lowest atomic mass (hydrogen) and ended at thorium which was the 56th element . He found that every eight element had properties similar to that of the first . He compared this to the octaves found in music . Therefore he called it the Law of Octaves . It is known as Newlands’ law of octaves .
3. What is Mandeleev’s periodic law ?
Ans: Mendeleev’s periodic law states that “the properties of elements are the periodic function of their atomic mass” .
4. What are groups and periods in Mandeleev’s periodic table ?
Ans: In Mendeleev’s periodic table, vertical columns are called groups and horizontal rows are called periods .
There are 8 groups and 6 periods .
5. What are the achievements of Mendeleev’s Periodic Table ?
Ans: The achievements of Mendeleev’s periodic table are given below:
(i) Prediction of new elements: Mandeleev’s periodic table could predict the existences of some elements that had not been discovered at that time . He left these gaps in the name of Eka – boron, Eka – aluminium and Eka – silicon .
(ii) Correction of atomic masses: It corrected the atomic mass of some of the elements .
(iii) It could classify all the elements discovered at that time .
(iv) When noble gases were discovered they could be placed in a new group without disturbing
the order .
6. What are the limitations of Mendeleev’s Periodic Table ?
Ans: The limitations of Mendeleev’s Periodic Table are
(i) No proper position could be given to the element hydrogen .
(ii) Position of isotopes: There is no separate position given to the isotopes of an element .
(iii) Cause of periodicity: Mendeleev’s periodic table does not explain successfully the cause of periodicity . Some dissimilar elements are placed together .
(iv) Some elements having higher atomic mass are placed before the elements with lower atomic mass .
(v) The atomic masses do not increase in a regular manner in going from one element to the next . So it was not possible to predict how many elements could be discovered between two elements especially when we consider the heavier elements .
7. What is Moderen Periodic Table ?
Ans: When elements arranged in order of increasing atomic number then we get a periodic table or classification is called Modern Periodic Table .
8. What is Modern periodic law ?
Ans: Modern periodic law states that properties of elements are periodic functions of their atomic number .
9. What are metalloids or semimetals ? Give example .
Ans: Metalloids or semimetals are the elements that show the properties of both metals and non-metals .
Example: Silicon(Si), Boron(B), Germanium(Ge) etc .
10. How do valency and number of shells change with respect to groups and periods in Modern Periodic Table ?
Ans: Valency :
(i) Group: In a particular group as we go down valency remains same .
(ii) Period: In a particular period, the valency of elements increase from 1 to 4 and then decreases to zero .
Number of Shells:
(i) Group: In a particular group as we go down the number of shells increases .
(ii) Period: In a particular period the number of shells remains same .
11. How do atomic size of elements change in Modern Perodic Table ?
Ans: Atomic size:
(i) Group: In a particular group as we go down the atomic size of elements increases .
(ii) Period: In a particular period, the atomic size decreases in moving from left to right .
12. Why do atomic size or atomic radius decreases in a definite period in moving from left to right ?
Ans: In a definite period as we go from left to right, the number of shells remains same but the atomic number gradually increases . Due to which nuclear charge increases . Due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom . That is why atomic size or atomic radius decreases in a definite period in moving from left to right .
13. How does atomic size or atomic radius increases down in a group in periodic table ?
Ans: In periodic table as we go down in a particular group, the number of shells gradually increases . This increases the distance between the outermost electrons and the nucleus and so the atomic size increases in spite of the increases in nuclear charge .That is why atomic size or atomic radius increases down in a group in periodic table .
14. How do the tendency to lose electrons change in a group ?
Ans: In periodic Table , as we go down in a particular group, the number of shells gradually increases . This increases the distance between the outermost electrons and nucleus and so the atomic size increases in spite of the increase in nuclear charge . As a result the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are farther away from the nucleus . Therefore, these can be lost easily . Hence, metallic character increases down in a group .
15. How would the tendency to gain electron change as you go from left to right across a period ?
Ans: In a definite period as we go from left to right, the number of shells remains same but the atomic number gradually increases . Due to which nuclear charge increases . Due to increase in nuclear charge which tends to pull the electrons closer to the nucleus . That is why the tendency to gain electron increases as we go from left to right across a period ?
16. How would the tendency to gain electrons change as you go down in a group ?
Ans: In a Periodic Table, as we go down in a particular group, the number of shells gradually increases . This increases the distance between the outermost electrons and nucleus . As a result the nuclear attraction decreases and the outermost electrons can be lost easily . That is the tendency to gain electrons decrease as we go down in a group .
17. What is electronegativity ?
Ans: A chemical property that describes the tendency of an atom to attract electrons towards itself .
18. What is electropositivity ?
Ans: The tendency of an element or an atom to lose electrons from its outermost shell is called electropositivity .
19. How would the tendency to lose electrons change as you go from left to right across a period ?
Ans: In a Periodic Table, as we go left to right across a period, the number of shells remains same but the atomic number gradually increases . Due to which nuclear charge increases .Due to increase in nuclear charge which tends to pull the electrons closer to the nucleus . As a result the effective nuclear charge experienced by valence electron is increasing because the outermost electron is closer to the nucleus . Therefore, these can not be lost easily . That is why the tendency to lose electrons decreases as we go from left to right .
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