The fifth chapter in the NCERT Solutions for Class 10 Science revolves around the Periodic Classification of Elements. The chapter begins with an introduction to the early attempts at classifying elements i.e. Dobereiner’s Triads and Newlands’ Law of Octaves. You will get to learn about Mendeleev’s Periodic Table and its merits as well as demerits. The Modern Periodic Table by Henry Moseley is also studied in detail. The chapter further analyses the position of elements and the trends in the modern periodic table.
Topics in Chapter 5 include:
The Periodic Table is one of the most important concepts in Chemistry. With Home Revise, you can build a strong grasp of the subject and give yourself an advantage.
Q.1 Did Dobereiner's triads also exist in the columns of Newland's Octaves? Compare and find out.
Ans: Only one triad of Dobereiner's triads exists in the columns of Newland's octaves. The triad formed by the elements Li, Na, and K of Dobereiner's triads also occurred in the columns of Newlands' octaves.
Q.2 What were the limitations of Dobereiner's classification?
Ans: Limitation of Dobereiner's classification:
All known elements could not be classified into groups of triads on the basis of their properties.
Q.3 What were the limitations of Newland's Law of Octaves?
Ans: Limitations of Newland's law of octaves:
(i) It was not applicable throughout the arrangements. It was applicable up to calcium only.
(ii) Those elements that were discovered after Newland's octaves did not find a place in Newlands table.
(iii) The position of cobalt and nickel in the group of the elements containing F and Cl of different properties could not be explained.
(iv) Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.
Q.4 Use Mendeleev's Periodic Table to predict the formulae for the oxides of the following elements:
K, C, Al, Si, Ba.
Ans: K is in group 1. Therefore, the oxide will be K2O.
C is in group 4. Therefore, the oxide will be CO2.
Al is in group 3. Therefore, the oxide will be Al2O3.
Si is in group 4. Therefore, the oxide will be SiO2.
Ba is in group 2. Therefore, the oxide will be BaO.
Q.5 Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)
Ans: Scandium and germanium
Concept insight: Recall that Mendeleev was able to predict the properties of 3 unknown elements which he had named as eka-boron, eka-aluminium and eka-silicon. Eka-boron was later named as scandium, eka-aluminium as gallium and eka-silicon as germanium.
Q.6 What were the criteria used by Mendeleev in creating his Periodic Table?
Ans: Mendeleev's periodic table was based on the observation that the properties of 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 their properties get repeated after regular intervals.
Q.7 Why do you think the noble gases are placed in a separate group?
Ans: Noble gases are inert elements. Their properties are different from all the other elements. Therefore, the noble gases are placed in a separate group.
Q.8 How could the Modern Periodic Table remove various anomalies of Mendeleev's Periodic Table?
Ans: Modern periodic table is based on atomic numbers whereas Mendeleev's periodic table was based on atomic masses. So, properties of elements could be explained with more precision when elements were arranged on the basis of increasing atomic number
For example:
The position of isotopes is taken care of when the elements are arranged in the ascending order of their atomic numbers as they have the same atomic number they do not need to be given separate slots.
Modern Periodic table can justify the position of hydrogen in group 1 as it has one valence electron like other elements of the group.The anomaly regarding few elements such as tellurium (Te), iodine (I) was solved in Modern table. If arranged in increasing order of their atomic numbers, (Te is 52 and I is 53.) Te will naturally come before I.
Q.9 Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Ans: Calcium (Ca) and Strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements. And since chemical properties are due to valence electrons, they show same chemical reactions.
Q.10 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 shells.
Ans: (a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells.
(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.
(c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.
Concept Insight: Recall that elements of group I have 1 valence electron, group II have 2 valence electrons while group XVIII have completely filled valence shell.
Q.11 (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 non-reactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Ans: (a) Yes. The atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shell.
(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.
Q.12 In the Modern Periodic Table, which are the metals among the first ten elements?
Ans: Among the first ten elements, lithium (Li) and beryllium (Be) are metals.
Q.13 By considering their position in the Modern Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic? Ga, Ge, As, Se, Be
Ans: Galium (Ga)
Q.14 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.
Q.15 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
X would most likely be in the same group of the Periodic Table as magnesium (Mg).
Q.16 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 has two shells, both of which are completely filled with electrons (2 electrons in K shell and 8 electrons in L shell).
(b) Magnesium has the electronic configuration 2, 8, 2.
(c) Silicon has a total of three shells, with four electrons in its valence shell (2 electrons in K shell, 8 electrons in L shell and 4 electrons in M shell).
(d) Boron has a total of two shells, with three electrons in its valence shell (2 electrons in K shell and 3 electrons in L shell).
(e) Carbon has twice as many electrons in its second shell as in its first shell (2 electrons in K shell and 4 electrons in L shell).
Q.17 (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 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 (7). Hence, they all have valency equal to 1.
Q.18 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: The atomic number of this element is 17.
(b) It would be chemically similar to F (9) with configuration as 2, 7.
Q.19 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.
(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) A is a non-metal.
(b) C is more reactive than A
(c) C will be smaller in size than B
(d) A will form an anion
Q.20 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: Element K L M
Nitrogen 2 5
Phosphorus 2 8 5
Nitrogen is more electronegative than phosphorus. On moving down a group, the number of shell increases. Therefore, the valence electrons move away from the nucleus and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electronegativity decreases.
Q.21 How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
In the Modern Periodic Table, atoms with similar electronic configurations are placed in the same group. In a group, the number of valence electrons remains the same. Elements across a period show an increase in the number of valence electrons in the same shell.
Q.22 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: Calcium has an atomic number of 20, and thus has an electronic configuration of 2, 8, 8, 2. Thus, calcium has 2 valence electrons. The electronic configuration of the element having atomic number 12 is 2, 8.2. Thus, this element with 2 valence electrons resemble calcium the most.
Q.23 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 |
| Elements are arranged in the increasing order of their atomic masses. | Elements are arranged in the increasing order of their atomic numbers. |
| There are 8 groups | There are 18 groups |
| Each groups are subdivided into sub group ‘a’ and ‘b’ | Groups are not subdivided into sub-groups. |
| Groups for Noble gas was not present as noble gases were not discovered by that time | A separate group is meant for noble gases. |
| There was no place for isotopes. | This problem has been rectified as slots are determined according to atomic number. |