Class 10 Science Solution
Chapter 5 Periodic Classification Of Elements
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.
2. What were the limitations of Dobereiner’s classification ?
Ans:
> It failed to arrange all the then known elements
> Dobereiner could identify only three triads from the elements known that time.
3. What were the limitations of Newlands’ law of octaves ?
Ans:
(i) Newlands law of octaves was applicable to the classification of elements upto calcium only.
(ii) Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future.
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:
K2O, CO2, Al2O3, SiO2, 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:
Mendeleev used atomic mass to arranged the elements in periodic table.
4. Why do you think the noble gases are placed in a separate group ?
Ans:
Similar inert behaviour and similar electronic configuration, they are justified to be placed in a separate group.
Page Number: 90
1. How could the modern periodic table remove various anomalies of Mendeleev’s periodic Table ?
Ans:
(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:
Calcium and Beryllium are similar to Magnesium because all the three elements belong to the same group and have 2 valence electrons in their outer shell.
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
Ans:
Lithium (Li), Sodium (Na) and potassium (k) have a single electron in their outermost shells.
Magnesium (Mg) and Calcium (Ca) have two electrons in their outermost shells
Neon (Ne), Argon (Ar and Xenon (Xe) filled outermost shells
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) They’ve one valence electron in their outermost shells and as a result of this, they are very unstable. So, they readily react with water to liberate hydrogen. They are also called alkali metals.
b) Their outermost shells are full leading to high stability. They react only in extreme circumstances and hence are called noble gases.
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 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 characteristic? Ga Ge As Se Be
Ans:
Among the elements listed in the question. Be and Ga are expected to be most metallic. Out of Be and Ga, Ga is bigger in size and hence has a greater tendency to lose electrons than Be. Therefore, Ga is more metallic than Be.
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.
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
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. v
(e) twice as many electrons in its second shell as in its first shell ?
Ans:
(a) Neon (2, 8)
(b) Magnesium
(c) Silicon (2, 8, 4)
(d) Boron (2, 3)
(e) Carbon (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) Valency equal to 3.
(b) Valency equal to 1.
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 this element is 17.
(b) It would be chemically similar to F(9) with configuration as 2, 7.
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?
Ans:
a) Element A is a non-metal
b) Element C is less reactive than Element A
c) C is smaller in size than B
d) A will form anion
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:
Nitrogen (7): 2, 5
Phosphorus (15): 2, 8, 5
Since, electronegativity decreases with moving from top to bottom in a group, thus Nitrogen will be more electronegative.
8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Ans:
The number of valence electrons decides an atom’s position in the periodic table while the electronic configuration decides the number of valence electrons.
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 element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).
10. Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.
Ans:
Mendeleev’s Periodic Table:
> Elements are arranged in the increasing order of their atomic masses.
> There are 8 groups
> Each groups are subdivided into sub group ‘a’ and ‘b’
> There is no place for noble gases.
> There was no place for isotopes.
Modern Periodic Table:
> Elements are arranged in the increasing order of their atomic numbers.
> There are 18 groups
> Groups are not subdivided into sub-groups.
> A separate group is meant for noble gases.
> Isotope are kept at the same place because their atomic number is same.
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