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NCERT Solution for Class 10 Science Chapter 5 : Periodic Classification of Elements

Periodic Classification of Elements
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Chapter 5 of NCERT class 10 science is the Periodic Classification of Elements. This chapter will take you through the arrangement of elements within a periodic table. There are a total of 10 questions in this chapter that help you understand the basic properties of all elements and remember their position in the periodic table.

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NCERT Solution for Class 10 Science Chapter 5 Periodic Classification of Elements PDF

Exercise 5.1

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

Solution:
Where the average atomic mass of the first and third element is equal to the atomic mass of the middle element this type of triad is called Dobereiner’s triad. Only one Dobereiner’s triad is found in the table of Newland’s Law of Octave.
Newland’s Octave

Question2.
What were the limitations of Dobereiner’s classification?

Solution:
Limitations of the Dobereiner’s Classifications is that depending on their properties only few elements can exhibit Dobereiner’s Triad while others cannot. For example F, Cl and Br doesn’t exhibit properties of triad while Cl, Br and I show the properties of Dobereiner’s triad.

Question 3.
What were the limitations of Newland’s Law of Octaves?

Solution:
Limitations of Newlands’ law of octaves:
(i) It was applicable only on element with low atomic mass (Calcium only).
(ii) The properties of elements having atomic mass greater than calcium does not have any resemblance to the properties of elements having atomic mass lower than calcium.
(iii) Co and Ni are transition elements but they were placed in the same group as halogens like F and Cl.
(iv)Properties of Iron resembles to that of Cobalt and Nickel but all Iron is placed in different column than Cobalt and Nickel.

Exercise: 5.2

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

Solution 1:
Potassium is an Alkali Metal therefore the oxide of Potassium will be K2O.
Carbon is member of Carbon Family that is group 14 therefore the oxide of carbon will be CO2.
Aluminium is member of Boron Family that is group 13 therefore the oxide of aluminium will be Al2O3
Silicon is member of Carbon Family that is group 14 therefore the oxide of Silicon will be SiO2.
Barium is member of 2nd group of periodic table therefore the oxide of Barium will be BaO.

Question 2:
Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (Any two)

Solution 2:
Besides Gallium, Scandium and Germanium are the other two discovered elements that were left by Mendeleev in his periodic table.

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

Solution 3:
Mendeleev used the element’s atomic mass as the basis of creating his periodic table. He found that the properties of all the elements are dependent on their atomic masses; in fact, the properties are a periodic function of their atomic masses. Elements in Mendeleev’s Periodic table are arranged in order of increasing atomic mass. The properties of these elements
reoccur after a definite interval.

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

Solution 4:
Noble gases are inert in nature. These gases do not react with any element of the periodic table like other elements due to this inert nature these gases are called inert gases. As these gases exhibit different properties from the rest of the elements of the table they are placed in a separate group.

Exercise: 5.3

Question1.
How could the Modern Periodic Table remove various anomalies of
Mendeleev’s Periodic Table?

Solution:
In 1913, a scientist named Henry Moseley found that the atomic number of an element more fundamentally define its properties than its atomic mass. Position of isotopes which were not considered in Mendeleev’s periodic table were decided in the modern periodic table. Position of Cobalt and Nickel which was debatable in Mendeleev’s periodic table resolved in the modern periodic table. Elements belonging to the same group of Modern periodic table show similarities in their properties and behaviour due to similarity in their outer electronic
configuration.

Question 2:
Name two elements you would expect to show chemical reactions similar to
magnesium. What is the basis for your choice?

Solution 2:
Strontium (Sr) and Barium (Ba) are expected to show similarities in chemical reactions to that of magnesium (Mg) because these elements have same outer electronic configuration that is they have same number of in electrons in their valence shell. Due to presence same number of electron in valence shell elements show same chemical reactions.

Question 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 outermost shells.

Solution 3:
(a) Lithium (Li), Sodium (Na) and Caesium (Cs) all these elements have a single electron in their outermost shells.
(b) Barium (Ba) and calcium (Ca) these two elements have two electrons in their outermost shells.
(c) Neon (Ne), argon (Ar), and xenon (Xe) are inert gases and have completely filled outermost shells.

Question 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 a nonreactive gas and neon is a gas of extremely low
reactivity. What, if anything, do their atoms have in common?

Solution 4:
(a) All the three elements lithium, sodium, and potassium have only one electron in their outer most orbit due to this they react aggressively with water to liberate hydrogen gas.
(b) Helium (He) and neon (Ne) are inert gas that is these gases do not react with any element easily. The outermost shells of these gases are completely filled. Helium has a two electrons in its outermost shell, while neon has an 8 electrons in its K, L shell making their outermost shell complete.

Question 5:
In the Modern Periodic Table, which are the metals among the first ten
elements?

Solution 5:
Lithium (Li) and Beryllium (Be) are the alkali metals that lie among the first ten elements of Modern Periodic Table.

Question 6:
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 and Be

Solution 6:
We know that, Metallic characteristics increase when we go from top to bottom in Modern Periodic Table. As Gallium and Beryllium has biggest size in the above elements, they are expected to exhibit most metallic character among the given elements.

Exercises Chapter 5

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.

Solution 1:
(c) The statement “The atoms lose their electrons more easily” is incorrect.
(In the periodic table as we move from left to right, the atomic radius decreases as the atomic number of an element increases this decrease in atomic radius makes it difficult for atoms to lose electrons).

Question 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

Solution 2:
(b) Mg
(According to question the valence of X is 2 and from given options, only Mg has two electrons in its valence shell.)

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?

Solution 3:
(a) Neon has only two shells, both of these shells are completely filled with electrons
(b) The electronic configuration of Magnesium is 2, 8, 2.
(c) Silicon has three shells, with four electrons in its outermost or valence shell
(d) Boron has two shells, with three electrons in its outermost or valence shell
(e) Carbon has 4 electrons in its second shell and 2 electrons in its first 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?

Solution 4:
(a) All the elements of the same column have the same electron number in their valence shell.
For ex: Boron family have the same number of electrons in their valence shell/outermost shell and their valency is equal to 3.
(b) All the halogens have the same number of electrons in their valence shell and they all have a valency of 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)

Solution 5:
(a) The atomic number of the element with a given electronic configuration is 17.
(b) The given element will be similar to Fluorine as the electronic configuration of fluorine is 2, 7 that is they have the same number of electrons in the outermost shell.

Question 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.
(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?

Solution 6:
(a) A is a non-metal and it has seven valence electrons.
(b) C is less reactive than A as reactivity decreases as we go from top to bottom in halogens.
(c) The atomic size decreases as we move from left to right, so the size of C is smaller than that of B.
(d) A is in group 17 that is halogens so it will have 7 valence electrons in its outermost shell. As it needs only one electron to complete its octet it would form an anion and accept an electron to complete the octet.

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?

Solution 7:
The electronic configuration of the Nitrogen (7) and Phosphorus (15) is:
Element Atomic Number Configuration
Nitrogen 7 2,5
Phosphorous 15 2,8,5
Electronegativity is the property of an atom of an element to attract the shared pair of electron between two bonded atoms towards itself. When we move from left to right in the periodic table the electro negativity of the element increases and while it decreases while we move from top to bottom in the table. In the case discussed above that is the case of group 15, the electro negativity decreases as we move from top to bottom in the table and hence Nitrogen is more electronegative than Phosphorous.

Question 8:
How does the electronic configuration of an atom relate to its position in
the Modern Periodic Table?

Solution 8:
In the modern periodic table, the Period number of an element is defined this period number of an element is equal to the number of shells in the atom of an element. Whereas the group number of an element is defined as is equal to the number of valence electrons for elements having up to two valence electrons and equal to the number of valence electrons plus 10 for
elements having more than two valence electrons.

Question 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?

Solution 9:
Atomic number Electronic Configuration
Calcium(20) 2,8,8,2
12 2,8,2
19 2,8,8,1
21 2,8,8,3
38 2,8,8,18,2
Element with atomic number 12 and 38 exhibits similar chemical properties to that of calcium because all three elements have the same number of electrons in their
outermost/valence shell.

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

Solution 10: Refer pdf.

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