Chapter 5 Periodic Classification of Elements
Question 1:
Did Dobereiner’s triads also exist
in the columns of Newlands’ Octaves? Compare and find out.
Answer:
Only one triad of Dobereiner’s
triads exists in the columns of Newlands’ octaves. The triad formed by the
elements Li, Na, and K of Dobereiner’s triads also occurred in the columns of
Newlands’ octaves.
Dobereiner’s triads
Li Ca Cl
Na Sr Br
K Ba I
Newlands’ octaves
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H
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Li
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Be
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B
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C
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N
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O
|
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F
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Na
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Mg
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Al
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Si
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P
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S
|
|
Cl
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K
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Ca
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Cr
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Ti
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Mn
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Fe
|
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Co and Ni
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Cu
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Zn
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Y
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In
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As
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Se
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Br
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Rb
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Sr
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Ce and La
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Zr
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−
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−
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Question 2:
What were the limitations of Dobereiner’s classification?
Answer:
Limitation of Dobereiner’s classification:
All known elements could not be
classified into groups of triads on the basis of their properties.
Question 3:
What were the limitations of Newlands’ Law of Octaves?
Answer:
Limitations
of Newlands’ law of octaves:
(i)
It was not applicable throughout the arrangements. It
was applicable up to calcium only. The properties of the elements listed after
calcium showed no resemblance to the properties of the elements above them.
(ii)
Those elements that were discovered after Newlands’
octaves did not follow the law of octaves.
(iii)
The position of cobalt and nickel in the group of the
elements (F, 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.
Question 1:
Use Mendeleev’s Periodic Table to
predict the formulae for the oxides of the following elements:
K, C, Al, Si, Ba.
Answer:
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.
Question 2:
Besides
gallium, which other elements have since been discovered that were left by
Mendeleev in his Periodic Table? (any two) Answer:
Scandium and
germanium Question
3:
What were the criteria used by Mendeleev in creating his
Periodic Table?
Answer:
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.
Question 4:
Why do you think the noble gases are placed in a 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.
Question 1:
How could the Modern Periodic Table
remove various anomalies of Mendeleev’s Periodic Table?
Answer:
Mendeleev was unable to give fixed
position to hydrogen and isotopes in the periodic table. In Mendeleev’s
periodic table, the increasing manner of atomic mass of the elements is not
always regular from one to its next. It was believed that a more fundamental
property than atomic mass could explain periodic properties in a better manner.
It was Henry Moseley who
demonstrated that atomic number of an element could explain periodic properties
in a better way than atomic mass of an element and arranged the elements in
increasing order of their atomic numbers. Then it was found that the various
anomalies of Mendeleev’s periodic table were removed by the modern periodic
table.
Question 2:
Name two elements you would expect to show chemical
reactions similar to magnesium.
What is the basis for your choice?
Answer:
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.
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.
Answer:
(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.
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 an unreactive gas and neon is a gas of
extremely low reactivity. What, if anything, do their atoms have in common?
Answer:
(a)
Yes. The atoms of all the three elements lithium,
sodium, and potassium have one electron in their outermost shells.
(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.
Question 5:
In the Modern Periodic Table, which are the metals among the
first ten elements?
Answer:
Among the first ten elements, lithium (Li) and beryllium
(Be) are metals.
Question 6:
By considering their position in
the Periodic Table, which one of the following elements would you expect to
have maximum metallic characteristic?
Answer:
Since Be lies to the extreme left
hand side of the periodic table, Be is the most metallic among the given
elements.
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.
Answer:
(c) The atoms lose
their electrons more easily.
(On moving
from left to right across the periods of the periodic table, the non-metallic
character increases. Hence, the tendency to lose electrons decreases.) 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 Answer:
(b) X
would most likely be in the same group of the Periodic Table as magnesium (Mg).
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?
Answer:
(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).
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?
Answer:
(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.
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) Answer:
(a) The atomic
number of this element is 17.
(b) It would be
chemically similar to F(9) with configuration as 2, 7.
Question 6:
The position of three elements A, B and C in the Periodic
Table are shown below −
|
Group 16
|
Group 17
|
|
−
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−
|
|
−
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A
|
|
−
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−
|
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B
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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?
Answer:
(a) A
is a non-metal.
(b) C
is less reactive than A, as reactivity decreases down the group in halogens.
(c) C will be smaller in size than B as moving across a period, the nuclear
charge increases and therefore, electrons come closer to the nucleus.
(d) A will form an anion as it accepts an electron to
complete its 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?
Answer:
|
Element
|
K L
M
|
|
Nitrogen
Phosphorus
|
2 5
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.
Question 8:
How does the electronic
configuration of an atom relate to its position in the Modern Periodic Table?
Answer:
In the modern periodic table, atoms
with similar electronic configurations are placed in the same column. In a
group, the number of valence electrons remains the same.
Elements across a period show an increase in the number of
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?
Answer:
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).
Question 10:
Compare and contrast the
arrangement of elements in Mendeleev’s periodic Table and the Modern Periodic
Table.
Answer:
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Mendeleev’s periodic
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table
|
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Modern periodic table
|
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1.
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Elements are arranged in the increasing order of their
atomic masses.
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1.
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Elements
are arranged in the increasing order of their atomic numbers.
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2.
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There are a total of 7
groups
(columns) and 6 periods (rows).
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2.
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There
are a total of 18 groups (columns) and 7 periods (rows).
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3.
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Elements having similar properties were placed directly
under one another.
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3.
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Elements
having the same valence shell are present in the same period while elements
having the same number of valence electrons are present in the same group.
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4.
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The position of hydrogen could not be explained.
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4.
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Hydrogen
is placed above alkali metals.
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5.
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No distinguishing positions for metals and non-metals.
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5.
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Metals
are present at the left hand side of the periodic table whereas non-metals
are present at the right hand side.
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