NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements

NCERT Intext Questions

Page Number 81

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

Ans. Yes, Dobereiner’s triads also existed in the columns of Newland’s Octaves. For example, Li, Na, K.

If we consider lithium (Li) as the first element, then sodium (Na) is eighth element. If we consider sodium as the first element, then potassium is the eighth element. From this we conclude that Dobereiner’s triads also exist in the columns of Newland’s octaves.

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

  • Ans. Limitations of Dobereiner’s classification are:
  • (i) He could find only three triads i.e., total of 9 elements only, however the total number of elements were more.
  • (ii) The classification could not be applied to all groups or elements.
  • (iii) It was not applicable to the elements of very low or high atomic mass.

3. What were the limitations of Newlands’ law of octaves ?

  • Ans. The limitations of Newland’s law of octaves are:
  • (i) Newland’s law of octaves was applicable to the classification of elements upto calcium only. After calcium every eighth element did not possess the properties similar to that of the first element.
  • (ii) Newlands 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’ law of Octaves.
  • (iii) In order 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. For example, the two elements cobalt (Co) and nickel (Ni) were put together in just one slot and that too in the column of elements like fluorine, chlorine and bromine which have very different properties from these elements.
  • (iv) Iron (Fe) element which resembles elements like cobalt and nickel in properties, was placed far away from these elements.
Page Number 85

1. Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements : K, C, Al, Si, Ba.

  • Ans. Oxides are formed when oxygen reacts with an element, its valancy = 2.
  • K – G IA – Valancy 1 – Formula of oxide = K2O
  • C – G IVA – Valancy 4 – Formula of oxide = C2O4/CO2
  • Al – G IIIA – Valancy 3 – Formula of oxide = Al2O3
  • Si – G IVA – Valancy 4 – Formula of oxide = Si2O4/SiO2
  • Ba – G IIA – Valancy 2 – Formula of oxide = Ba2O2/BaO

2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his periodic table? (any two)

Ans. Scandium and Germanium.

3. What were the criteria used by Mendeleev in creating his Periodic Table?

  • Ans. He used the following two criteria:
  • (i) Elements are arranged in increasing order of their atomic mass in horizontal rows (periods) and vertical columns (group).
  • (ii) Grouping of elements according to their similar chemical properties.

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

  • Ans. Noble gases are placed in a separate group because of following characteristics of noble gases:
  • (i) They are chemically inert and do not react with other elements, Xe is the exception.
  • (ii) They occur in lower concentration in the atmosphere.
  • (iii) They have a stable electronic configuration of ns2 np6.
Page Number 90

1. How could the Modern Periodic table remove various anomalies of Mendeleev’s periodic Table?

  • Ans. The anomalies of Mendeleev’s periodic table are removed by Modern Period Table as follows:
  • (i) The modern periodic table is based on atomic number, while Mendeleev’s periodic table was based on atomic mass.
  • (ii) The isotopes of an element have same number of protons (or atomic number). So they are alloted the same position in modern periodic table.
  • (iii) Cobalt and nickel are placed at 9th and 10th position respectively.
  • (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. Mg (12) = 2, 8, 2 – two electrons in outermost shell

Oxide of Mg = MgO

Beryllium (Be) and Calcium (Ca).

Both Be (atomic number 4) and Ca (atomic number 20) have similar electronic configuration, i.e., two electrons in outermost shells.

Be 2, 2

Ca 2, 8, 8, 2

Both Be and Ca react with oxygen to give basic oxides, BeO and CaO.

3. 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 outer most shells.

Ans. (a) Lithium : Atomic number – 3, 1s2, 2s1 (2, 1); Sodium : Atomic number – 11, 1s2, 2s22p6, 3s1 (2, 8, 1);

Potassium : Atomic number – 19, 1s2, 2s22p6, 3s2 3p6, 4s1 (2, 8, 8, 1).

(b) Beryllium : Atomic number – 4 (2, 2); 1s2, 2s2 Calcium : Atomic number – 20, 1s2, 2s22p6, 3s23p6, 4s2 (2, 8, 8, 2)

(c) Helium : Atomic number – 2, 1s2(2); Neon : Atomic number – 10, 1s2, 2s2 2p6 (2, 8);

Argon : Atomic number – 18, 1s2, 2s2, 2p6, 3s23p6 (2, 8, 8).

4. (a) Lithium, sodium, potassium are all metals that react with water to liberate 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 ?

Ans. (a) The elements Li, Na and K are having similar properties as:

(i) They belong to the same group i.e., Group 1 in periodic table.

(ii) All having one valance electron in outermost shell.

Li (3) – 1s2, 2s1 – (2, 1)
Na (11) – 1s2, 2s2 2p6, 3s1 – (2, 8, 1)
K (19) – 1s2, 2s2 2p6, 3s2 3p6, 4s1 (2, 8, 8, 1)

Hence these metals (Li, Na, K) react with water to librate hydrogen gas.

(b) The outmost shell of both the atoms of He and Ne is completely filled.

He (2) – 1s2 (2)
Ne (10) – 1s2, 2s2 2p6 (2, 8)

Hence both He and Ne are very less reactive elements.

5. In the modern periodic table, which are the metals among the first ten elements?

Ans. The first ten elements in modern periodic table are hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon. Out of these, lithium and beryllium are metals, whereas others are non-metals.

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. As we move from left to right in a period, the metallic character decreases as the size of an atom also decreases and thus, the valence electrons are held tightly by nucleus. So it becomes hard to loose electrons resulting in decrease in metallic character.

The given elements Ga, Ge, As, Se belong to the same period 4 and Be occupies the most left position so it is most metallic among them.

NCERT Exercise Questions

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.

Explanation: When we move from left to right across the periods, the size of an atom decreases and the number of valence electrons increases, the nucleus is tightly bound to increase the metallic character. So they can not loose their electrons easily.

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

Ans. (b) Mg

Explanation: This is becuase both Na and Mg belong to same period, having same properties.

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 ?
  • Ans. (a) Neon (10) – 1s2, 2s2 2p6 (2, 8)
  • (b) Magnesium (10) – 1s2, 2s2 2p6, 3s2 (2, 8, 2)
  • (c) Silicon (14) – 1s2, 2s2 2p6, 3s2 3p2 (2, 8, 4)
  • (d) Boron (5) – 1s2, 2s2 2p1 (2, 3)
  • (e) Carbon (6) – 1s2, 2s2 2p2 (2, 4)

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?

Ans. (a) Elements in the same column or group as boron have valency of three and have three valence electrons.

(b) Elements in the same column or group as fluorine form acidic oxides and have seven electrons in their outermost shells and have valency of one.

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)
  • Ans. (a) The atomic number of the given element is 2 + 8 + 7(= 17).
  • (b) N(7) = 2, 5, F = 2, 7, P = 2, 8, 5, Ar = 2, 8, 8 It would be chemically similar to fluorine [F(9)] because its electronic configuration is 2, 7.

6. The positions of three elements A, B and C in the periodic table are shown below:

(a) State whether A is a metal or non-metal.

Group 16 Group 17
A
B C
  • (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 element A ?
  • Ans. (a) Since the valency of group 17 elements is 1 and all these elements accept electrons, thus A is a non-metal.
  • (b) C is less reactive than A as reactivity decreases down the group in halogens.
  • (c) C is smaller in size than B because B and C both are related to the same period and the size decreases as one moves from left to right in a period.
  • (d) A will form anion as it accepts one electron to complete its octet.

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 (7) = 1s2, 2s2 2p3 = 2, 5
Electronic configuration of phosphorus (15) = 1s2, 2s2 2p6, 3s5 = 2, 8, 5

Nitrogen will be more electronegative because outermost shell is nearer to nucleus and therefore nucleus will attract electrons more strongly. In a group of the periodic table, electron attracting tendency decreases as we move from top to bottom.

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. For example, if an element has 1 or 2 electrons in its outermost shell, then it would belong to group 1 or group 2. And if it has 3 or more electrons in its outermost shell, then it would belong to group 10, 4-the number of electrons in the outermost shell.

All the alkali metals have one electron in their outermost shell, so they are placed in group 1. Thus, all the group 2 elements have 2 electrons in their outermost shell. In group 15 elements, there are 5 electrons in their outermost shell. Similarly, the number of shells in an element indicates its period number. For example, the atomic number of magnesium is 12 and its electronic configuration is 2, 8, 2. Thus it is an element of 3rd period.

9. In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic number 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?

  • Ans. Ca (20) = 2, 8, 8, 2
  • The electronic configuration of elements with : 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
  • Elements with atomic number 12, 21 and 38 will have same physical and chemical properties as the valence electron in outermost shell is same as in Ca.

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

Ans.

Mendeleev’s Periodic Table Modern Periodic Table
(i) Elements are arranged in the order of increasing atomic mass. Elements are arranged in the order of increasing atomic number.
(ii) It contain 8 groups and 6 periods. It contain 18 groups and 7 periods.
(iii) There is no place for noble gases. Noble gases are placed on the right hand side of the table.
(iv) There is no place for isotopes. Isotopes are kept at the same place because their atomic number is same.
(v) Transition elements are placed together in group VIII. Transition elements are placed in the middle of the long period (Group 3 to 12).
(vi) Dissimilar metals are placed together. Dissimilar metals are placed in different groups.