NCERT Solutions for class 10th Science Chapter 5 Periodic Classification of Elements

NCERT INTEXT QUESTIONS

Question 1. Did Dobereiner’s triads also exist in the columns of Newland’s Octaves? Compare and find out.

Sol. Yes, Dobereiner triads also exist in the columns of Newland’s octaves. As only one triad exist in the column of Newlands’ octaves. It can get clearer by observing the given table.
Dobereiner triads are:

The triad formed in Dobereiner’s triads is Li (lithium), Na (sodium), K (potassium) which also occur in the columns of Newland’s octaves. So, Dobereiner triads also exist in the columns of Newlands’ octaves.

Question 2. What were the limitations of Dobereiner’s classification?

Sol. The limitations of Dobereiner’s classification can be described into the following points:
(i) Dobereiner had found only four triads at that time, out of which only three were accepted, and the remaining elements had not been classified as Dobereiner’s triads.
(ii) Dobereiner stated that all elements occurred in groups of three, when arranged in increasing order of atomic masses having similar properties, then the atomic mass of the middle element was roughly the average of the atomic mass of the other two elements. Three elements nitrogen, phosphorus and arsenic have same chemical properties, but these three elements do not follow Dobereiner ‘s rule.

Question 3. What were the limitations ofNewlands’ Law of octaves?

Sol. The limitations of Newland ‘s law of octaves given are as follows:
(i) Newland’s slaw of octaves states that when elements are arranged in the increasing order of their atomic masses, every eighth element has similar properties to the first element. But after calcium, every eighth element did not possess similar properties to that of first. Hence, this law was applicable upto calcium only.
(ii) Newland accepted that only 56 elements will subsist in nature and no more elements will be discovered in future. But ultimately, new elements were discovered whose properties did not fit into the Law of Octaves.
(iii) Newland adjusted two elements in the same slot having different properties in order to fit elements into his table. For example: iron, whose properties resembles with cobalt and nickel, was placed far away from these elements.

Question 4. Use Mendeleev’s periodic table to predict the formulae for the oxides of following elements :
K, C, Al, Si, Ba

Sol. In Mendeleev’s periodic table, oxygen is placed in VI -A group, having valency 2. Similarly we can easily find out the valency of the elements given from their respective groups, which can help us to predict the formulae for the oxides.

K (Potassium)- It belongs to I-A group and has valency 1. Thus, the formula ofoxide formed is O.

C (Carbon)– It belongs to IV-A group and has valency 4. Thus, the formula of oxide formed is C20 4 or CO2.
Al (Aluminium)– It belongs to III-A group and has valency 3. Thus, the
formula of oxide formed is Al203 .
Si (Silicon)– It belongs to IV-A group and has valency 4. Thus, the formula of oxide formed is Si02.
Ba (Barium)- It belongs to II-A group and has valency 2. Thus, the formula
of oxide formed is BaO.

Question 5. Besides gallium, which other elements have since been discovered to fill the gaps left by Mendeleev in his periodic table? (any two)

Sol. In Mendeleev’s periodic table, elements were arranged on the basis of the fundamental property, atomic mass, and chemical properties. He founded three places in the periodic table where elements appeared to be missing. He predicted that these elements will have similar properties to Eka-boron, Eka-aluminium and Eka-silicon later discovered and named as scandium, gallium and germanium respectively. So, besides gallium, germanium and scandium were discovered that were left by Mendeleev in his Periodic table.

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

Sol. The criteria that were used by Mendeleev in creating his periodic table were as follows:
(i) Elements were arranged in the order of their increasing atomic masses.
(ii) Considering the chemical properties of the elements, he focused on the compounds that were formed by hydrogen and oxygen. The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification.
(iii) He also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties. On the basis of which he stated that ‘the properties of elements are the periodic function of their atomic masses’.

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

Sol. Noble gases are placed in a separate group because of the following reasons:
(i) Noble gases are inert in nature hence occur in a very low concentration in the atmosphere.
(ii) They have completely filled outer electronic shell configuration.Atoms with full valence electron shells are extremely stable and therefore do not tend to form chemical bonds and have little tendency to gain or lose electrons.
(iii) The properties exhibited by noble gases are completely different from all the other elements and show similar properties with each other. Therefore, they are placed in a separate group.
Note:Xenon is considered as the most reactive element among noble gases. Xenon com­ pounds are reactive because the electrons are not strongly held, owing to the big size of xenon.

Question 8. How could the modern periodic table remove various anomalies of Mendeleev’s periodic table?

Sol. Mendeleev examined the relationship between the atomic masses of the elements and their physical and chemical properties. But still there were various anomalies in this periodic table like:
(i) Position of hydrogen in a periodic table was not fixed.
(ii) Position of isotopes was also not fixed.
(iii) Lanthanide and actinide were placed outside the periodic table in two separate columns.
It was Henry Moseley who demonstrated that atomic number of an element could explain periodic properties in a better way than atomic mass of an element and arranged the elements in increasing order of their atomic numbers, which removed the various anomalies of Mendeleev’s periodic table.

Question 9. Name two elements you would expect to show same kind of chemical reactivity as magnesium. What is the basis for your choice?

Sol. Since the chemical properties are due to valence electrons present in there outermost shell. Magnesium belongs to group 2 in a periodic table having valency 2. So, the two elements which show same kind of chemical reactivity as magnesium are beryllium (Be) and calcium (Ca), which belongs to the same group as that of magnesium and have valency 2.
Note: Apart from beryllium (Be) and calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra) are also the members of group-2 having valency 2. They are also termed as alkaline earth metals.

Question 10. Name:
(a) three elements that have only a single electron in their outermost shells.
(b) two elements that have two electrons in their outermost shells.
(c) three elements with filled outermost shells.

Sol. (a) The three elements that have only single electron in their outermost shell are lithium (Li), sodium (Na) and potassium (K).
The alkali metals found in group 1 of the periodic table are having only one electron in their outer shell. Therefore, they are ready to lose that one electron in ionic bonding with other elements. Apart from these elements, rubidium (Rb), caesium (Cs) and francium (Fr) are also included in the same group.
(b) The two elements that have only two electrons in their outermost shell are beryllium (Be) and magnesium (Mg).
Alkali earth metals include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). They have capability to lose two electrons present in their outer shell. The general electronic configuration of Group 2 elements is ns2.
(c) The three elements with filled outermost shells are helium (He), neon (Ne) and argon (Ar).
Group-18 elements also termed as noble gases which includes helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn), all have completely filled outermost shells. They are inert gases and found in very low concentration in nature.

Question 11. (a) Lithium, sodium, potassium are all metals that react with water toliberate hydrogen gas. Is there any similarity in the atoms of these elements?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Sol.
(a) Lithium (Li), sodium (Na) and potassium (K) are those elements which belongs to same group i.e. I-A group, also known as alkali metals. All the elements present in alkali group have only one electron in their outermost shell. Hence, show similar physical and chemical properties. This is the reason why lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas.
(b) Both helium (He) and neon (Ne) are noble gas and are inert in nature. They have completely filled valence shell i.e. helium has duplet in its K-shell and neon has octet in its L- shell. Thus the atoms have very low or zero reactivity. This is the reason why helium is an unreactive gas and neon is a gas of extremely low reactivity.

Question 12.In the modern periodic table, of the first ten elements, which are metals?

Sol. Modern periodic table arranges the elements by increasing atomic number and is based upon the electronic configuration of the elements. The first ten elements in the modem periodic table are hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon. Out of which only two are metals i.e. lithium (Li) which is an alkali metal and beryllium (Be) which is an alkaline earth metal

Question 13. By considering their position in the periodic table, which one of the following elements would you expect to have the most metallic characteristics?
Ga Ge As Se Be

Sol. In a modern periodic table, metallic character decreases from moving towards left to right and increases on moving down the group. The table given below is showing the position of the given elements, in a periodic table:

As beryllium (Be) has lowest group number among the given elements, so it shows the maximum metallic characteristics.
Note: Metals are those which have a tendency to lose electrons from their outermost shell to form cations. Greater the tendency to lose electrons, stronger is the metal.

Excersice:

Question 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.

Sol. (c)On moving left to right across the period of the periodic table, metallic character decreases. Metals tends to lose electrons while forming bonds, that is, they are electropositive in nature. As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease.

Question 2. Element X forms a chloride with the formula XC12, 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

Sol. (b) As Na (sodium) is an alkali metal which has valency 1, when it combines with Cl2 (chlorine), formation ofNaCl takes place. Similarly Al (aluminium) belongs to group 13 having valency 3, which combines with Cl2 to form Al2Cl3 and Si (silicon) belongs to group 14 having valency 4 and after combining with Cl2 , it forms SiC14. Magnesium is an alkaline earth metal so its valency is 2 and when Cl2 reacts with Mg (magnesium), it forms MgC12, which is similar to the given formula i.e. XC12, so the correct answer will be magnesium (Mg).

Question 3. Which element has :
(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its first shell?

Sol. (a) Neon is the element which has two shells, both of which are completely filled with electrons. In a modern periodic table, neon is a member of 18th group. Elements belonging to 18th group have completely filled valence shell of electrons. Neon has two shells Kand L, in which 2 electrons are present in K-shell and 8 electrons in its L-shell.
(b) Magnesium (Mg) has the electronic configuration 2,8,2. The electronic configuration 2,8,2 shows that there are 2 valence electrons in the outermost shell hence it belongs to group-2 and it has 3 shells (K, L, M) thus belongs to period three. Group number signifies the number of valance electrons and the period number signifies number of shells. Atomic number of the given element will be 12. So, the element is magnesium having electronic configuration 2,8,2.
(c) Silicon has a total number of three shells, with four electrons in its valence shell. As there are four electrons in the valence shell of this element, which signifies that the element belongs to group-14 and this element have three shells. So, the element will be silicon which has 2 electrons in K-shell, 8 electrons in L-shell and 4 electrons in M-shell.
(d) Boron is the element which has total number of two shells with three electrons in its valence shell. As there are three electrons in the valence shell of this element, which signifies that the element belongs to group- 13 and this element have two shells. So the element will be boron which has 2 electrons in K-shell and 3 electrons in L-shell.
(e) Carbon is the element which has twice as many electrons in its second shell as in its first shell. The element which has twice as many electrons in its second shell as in its first shell, states that in first shell there will be two electrons and in the second shell will be twice as many electrons in its second shell as in its first shell i.e. 4, so the atomic number of the given element will be (2+4) 6. So, the element will be carbon which has 2 electrons in K-shell and 4 electrons in L-shell.

Question 4. (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?

Sol. (a) The elements which belong to same column or a group will have same number of electrons in their outermost shell, these electrons are also known as valence electrons. So, all the elements in the same column as boron have the same number of valence electrons (3). Hence, they all have valency equal to 3.
(b) All the elements in the same column as fluorine have the same number of valence electrons i.e., 7. Hence, they all have valency equal to 1.

Question 5. An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
(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)

Sol. (a) The atomic number of the given element will be 2 + 8 + 7 = 17
In general, an electronic configuration of an atom is the method to represent the distribution of electrons among the orbital shells and subshells. And an atomic number of the element is the number of protons in the nucleus ofa single atom of that element (where number of protons is equal to the number of electrons). So, by doing the summation of the given electronic configuration, the atomic number can be easily calculated, which will be 17.
(b) The element which will be chemically similar to the given element is F(9)
i.e.fluorine.
As F(9) has 7 electrons in its valence shell and the given element also have 7 electrons in its valence shell. Hence both have same valency i.e.l. So, the element which would be chemically similar to the given element is fluorine.

Question 6. The position of three elements A, Band C in the periodic table are shown below:

(a) State whether A is a metal or non-metal.
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion will be formed by an element A?

Sol. (a) According to the position given in the periodic table of three elements
A, Band C, elements A is anon-metal.
From the given table it can be easily clear that element A belongs to group 17, which is also known as halogen family. Halogens are non -metallic elements which include six elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). SoA is a non metal.
(b) C will be less reactive than A.
In group-17 (halogen family), reactivity of the elements decreases down the group due to the fact that atomic radius increases in size and electronegativity will also gets by which there is a less electron pulling. The reactivity will be in the order of F >Cl> Br> I.
(c) C is smaller than element B.
It can be seen that both the elements B and C belongs to same period. And it is a fact that on moving left to right along a period, the atomic size decrease due to the increase in the number of protons and electrons across a period. So, C is smaller than B.
(d) Element A will form anion (A-).
Element A belongs to group 17. There are 7 electrons in its valance shell. Therefore, it tends to accept one electron to complete octet and forms anion.

Question 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?

Sol. Nitrogen (N) and phosphorous (P), both the elements belongs to 15th group in a periodic table. The electronic configuration of these elements are given as follows:
N(7)-2, 5
P (15)-2, 8, 5
The number of shells increases on moving down the group, which decreases the tendency to attract the electrons. Therefore, electronegativity decreases.
As the order of the elements present in the periodic table from up to down is nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), and moscovium (Mc). Nitrogen comes first on moving down the group so nitrogen will be more electronegative than phosphorous.

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

Sol. In the modern periodic table, elements are arranged in the increasing order of their atomic number. Those elements which have similar electronic configuration are placed in same group. In a group, all the elements have same number of electrons in their valence shell, hence possess same valency. So the position of an atom is related to the number of electrons present in the outermost shell.

Question 9. In the modern periodic table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 28. Which of these have physical and chemical properties resembling calcium?
Sol. Those elements which have same valency and same number of electrons present in their valence shell, will share similar chemical and physical properties. For the given elements, first we have to find out the electronic configuration from the given atomic number,which can be given as:

  • Atomic number 12 – 2, 8, 2
  • Atomic number 19 – 2, 8, 8, 1
  • Atomic number 20 – 2, 8, 8, 2
  • Atomic number 21 – 2, 8, 9, 2
  • Atomic number 38-2, 8, 18, 8, 2

Thus, it is clear that element having atomic number 12 and 38 will resemble from calcium having atomic number 21 because these elements have same number of electrons present in their valence shell.

Question 10. Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.

Sol. The given table represents the contrasting characters between the elements in Mendeleev’s periodic table and modern periodic table:

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