NCERT Solutions for Class 10 Science Chapter-5 “Periodic Classification of Elements”
Answer The Following Question.
1. Did Dobereiner’s triads also exist in
the columns of Newland’s Octaves? Compare and find out?
Answer
: Yes,
following triads can be identified:
Li, Na, K
Be, Mg, Ca
The atomic mass of middle atom in the above triads is
approximately equal to the average mass of elements on the left and right hand
side.
2. What were the limitations of
Dobereiner’s classification?
Answer
:
Dobereiner could identify only three triads from the
elements known at that time. Hence, this classification of elements was not
found to be useful.
3. What were the limitations of Newland’s
law of octaves?
Answer
:
The limitations of Newlands’ Law of Octaves :
(a) it was found that Law of Octaves was applicable
only upto calcium.
After calcium eighth element did not possess
properties similar to that of first.
(b) Newlands’ assumed that only 56 elements existed in
nature and no more elements would be discovered in future.
(c) To fit the element into his table, Newlands
adjusted two elements in the same slot but also put some unlike elements under
the same note.
4. Use Mendeleev’s Periodic Table to
predict the formulae for the oxides of the following elements :
K, C, Al, Si, Ba
Answer
:
K2O, CO2, SiO2, BaO
5. Besides gallium, which other elements
have since been discovered that were left by Mendeleev in his Periodic Table?
(any two)
Answer
:
Scandium and Germanium.
6. What were the criteria used by
Mendeleev in creating his Periodic table?
Answer
:
Mendeleev created his Periodic Table on the basis of
their fundamental property the atomic mass and also on the similarities of
chemical properties. Among chemical properties he concentrated on the compounds
formed by elements with oxygen and hydrogen.
7. Why do you think the noble gases are
placed in separate group?
Answer
:
Noble gases are inert elements. Their properties are
different from the all other elements. Therefore, the noble gases are placed in
a separate group.
8. How could the Modern Periodic table
remove various anomalies of Mendeleev’s Periodic Table?
Answer
:
Modern Periodic Table settled the placement of
isotopes as isotopes have same atomic number. Position of controversial
position of hydrogen is also settled in modern periodic table.
9. Name two elements you would expect to
show chemical reactions similar to magnesium. What is the basis for your
choice?
Answer
:
Calcium and Strontium would show chemical reactions
similar to magnesium. They have the same number of electrons in the outermost
shell.
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.
Answer
:
(a) Lithium, sodium and potassium have a single
electron in their outermost shells.
(b) Magnesium and calcium have two electrons in their
outermost shells.
(c) Helium, neon and argon have filled outermost
shells.
11. (a) Lithium, sodium and 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 un-reactive gas and neon
is a gas of extremely low reactivity. What, if anything, do their atoms have in
common?
Answer
:
(a) Lithium, sodium and potassium have same number of
electrons in the outermost shell.
(b) Helium and argon have completed outermost shell, 2
electrons in case of helium and 8 electrons in case of argon.
12. In the Modern Periodic Table, which
are the metals among the first ten elements?
Answer
:
Lithium and Beryllium are the metals among the first
ten elements in the Modern Periodic Table.
13. 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
Answer
:
Metallic character of an element increases down a
group and decreases from left to right in period. On this basis Be is expected
to have maximum metallic character.
14. Which of the following statements is
not 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 valance electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides becomes more acidic.
Answer
:
(c) The atoms lose their electrons more easily.
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) AI (d) Si
Answer
:
(b) Mg
16. Which element has
(a) two cells, 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 valance shell?
(d) twice as many electrons in its second shell as in
its first shell?
Answer
:
(a) Neon
(b) Magnesium
(c) Silicon
(d) Carbon
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 common?
Answer
:
(a) All the elements in the in the same column as
boron have three electrons in the valence shell. That is all the elements are
trivalent.
(b) All the elements in the same column as fluorine
has one electron in the valance shell, which is all the elements, are
monovalent.
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)
Answer
:
Chlorine has the electronic configuration 2, 8, 7.
(a) Atomic number of element is 17.
(b) F (9)
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 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?
Answer
:
(a) A is a non-metal.
(b) C is less reactive than A.
(c) C will be smaller than B.
(d) A will form anion.
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?
Answer
:
Nitrogen atomic number 7 has got 7 electrons with
electronic configuration 2, 5.
Phosphorus with atomic number 15 has got 15 electrons
with electronic configuration 2, 8, 5.
Non-metallic character decreases as we move down the
group. Therefore, nitrogen will be more electronegative than phosphorus.
21. How does the electronic configuration
of an atom relate to its position in the Modern Periodic Table?
Answer
:
Group number on an element can be predicted from the
number of electrons in the outermost shell. Period number of an element can be
predicted from the number of shells with filled electrons. Knowing the
electronic configuration, we can find the number of electrons in the outermost
shell and the number of shells with filled electrons. This can help to relate
its position in the Periodic Table.
22. 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?
Answer
:
Elements in a group have similar properties. Elements
with atomic numbers 12 and 38 lie in the same group as calcium. Therefore, they
will have properties resembling calcium.
23. Compare and contrast the arrangement
of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.
Answer :
|
Mendleeve’s Periodic Table |
Modern Periodic Table |
|
(i) Elements are arranged in the order of increasing
atomic mass. |
(i) Elements are arranged in the order of increasing
atomic number.
|
|
(ii) There
are nine vertical columns called groups.
|
(ii) There are eighteen vertical columns called
groups, |
|
(iii) There is no place for noble gases. |
(iii) Noble gases are placed on the right hand side
of the table |
|
(iv) There is no place for isotopes.
|
(iv) Isotope are kept at the same place because
their atomic number is same.
|
|
(v) Transition elements are placed together in group
Vlll. |
(v) Transition elements are placed in the middle of
the long period (Group 3 to 12). |
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