(Chapter – 5) (Periodic Classification of Elements)
(Class – X)
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 1:
(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.)
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 2:
(a) 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 3:
(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.
(a) 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).
(b) 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).
(c) 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).
(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 4:
(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.
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)
(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
|
|
−
|
−
|
|
−
|
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?
Answer 6:
(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 7:
|
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.
Question
8:
How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer 8:
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 9:
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 10:
|
|
Mendeleev’s periodic
table
|
|
Modern periodic table
|
|
1.
|
Elements are arranged in the increasing order of their atomic masses.
|
1.
|
Elements are arranged in the increasing order of their atomic numbers.
|
|
2.
|
There are a total of 7 groups (columns) and 6 periods (rows).
|
2.
|
There are a total of 18 groups (columns) and 7 periods (rows).
|
|
3.
|
Elements having similar properties were placed directly under
one another.
|
3.
|
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.
|
|
4.
|
The position of hydrogen could not be explained.
|
4.
|
Hydrogen is
placed above alkali
metals.
|
|
5.
|
No distinguishing positions for metals and
non-metals.
|
5.
|
Metals are present
at the left hand side of the periodic table whereas non-metals are present at the right hand
side.
|
Question 1:
Did Dobereiner’s triads
also exist in the columns
of Newlands’ Octaves?
Compare and
find out.
Answer
1:
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
|
H
|
Li
|
Be
|
B
|
C
|
N
|
O
|
|
F
|
Na
|
Mg
|
Al
|
Si
|
P
|
S
|
|
Cl
|
K
|
Ca
|
Cr
|
Ti
|
Mn
|
Fe
|
|
Co and Ni
|
Cu
|
Zn
|
Y
|
In
|
As
|
Se
|
|
Br
|
Rb
|
Sr
|
Ce and La
|
Zr
|
−
|
−
|
Question 2:
What were the limitations of Dobereiner’s classification?
Answer 2:
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 3:
Limitations of Newlands’ law of octaves:
a) 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.
b) Those elements that were discovered after Newlands’
octaves did not follow the law of octaves.
c) The position of cobalt and nickel in the group of the elements (F, Cl) of different properties could not be explained.
d) 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 1:
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 2:
Scandium and germanium
Question 3:
What were the criteria used by Mendeleev
in creating his Periodic Table?
Answer 3:
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 4:
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
1:
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 2:
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 3:
(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 4:
(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 5:
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?
Ga, Ge, As, Se, Be
Answer 6:
Since Be lies to the extreme left hand side of the periodic
table, Be is the most metallic among the given
elements.
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