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LONG ANSWER TYPE QUESTIONS
1. What physical and chemical
properties of elements were
used by Mendeleev in creating
his periodic table? List two
observations which posed a
challenge to
Mendeleev’s Periodic Law.
2. (a) Why did Mendeleev have gaps
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in his periodic table?
(b) State any three limitations of
Mendeleev’s classification.
(c) How does electronic configuration
of atoms change in a period with
increase in atomic number?
3. An element is placed in 2nd group
and 3rd period of the Periodic
Table, burns in presence of
oxygen to form a basic oxide.
(i) Identify the element.
(it) Write the electronic configuration.
(Hi) Write the balanced equation when
it burns in the presence of air.
(iv) Write a balanced equation when
this oxide is dissolved in water.
(v) Draw the electron dot structure
for the formation of this oxide.
Group18 elements are called ..
PERIODIC
CLASSIFICATION OF
ELEMENTS
Important points:
Classification means grouping of elements on
the basis of similarities and properties.
It is difficult to study each and every element
individually and to know their properties
and uses. Therefore, they have been
classified into groups on the basis of their
similarities.
Dobereiner’ Triads: When the elements are
arranged in groups of three in increasing
order of atomic masses the middle
elements of a group has the atomic mass
and properties roughly the average of other
two elements. Limitations:
He could identify only three triads. He was
unable to prepare the triads of all the
elements discovered at that time.
Newlin Law of Octaves: When elements are
arranged in increasing order of atomic
mass the properties of the eight element are
a kind of repetition of the first, just like the
notes of music. Limitations:
1. The law was applicable to elements upto
calcium.
2. It contained only 56 elements.
3. In order to fit elements into the table, he
adjusted two elements like cobalt and
nickel in the same slot and also put some
unlike elements in same slot (note of
music).
Mendeleev’s Periodic Table
Mendeleev’s periodic law: The properties of
elements are periodic functions of their
atomic masses. Characteristics of
Mendeleev’s law:
1. He arranged all the 63 elements in increasing
order of their atomic masses.
2. The table contained vertical columns called
groups and horizontal rows called periods.
3. The elements with similar physical and
chemical properties came under same
group.
Advantages and Disadvantages
1. He could classify all the 63 elements
discovered at that time on the basis of
similarities in properties.
2. He left gaps for yet to be discovered
elements.
3. He produced the properties of undiscovered
elements and thus helped in the discovery
of these elements later on.
4. He named them by prefixing a Sanskrit
numeral Eka (1), Divi (2), Tri (3) to have
of proceeding elements of the same group,
for example, Eka Boron, Divi Magnesium,
etc.
5. He helped in correction of atomic weights of
certain elements on the basis of their
position in the periodic table, for example,
Berelium.
Limitations:
1. No fix position was given to hydro -gen in
the Mendeleev’s periodic table.
2. Position of isotopes of all elements was not
certain according to his periodic table.
3. Atomic masses did not increase in a regular
manner is going from one element to the
next. For example, Cobalt and Nickel.
4. He did not provide any place for noble gases
which were discovered later.
Modern Periodic Table: Properties of
elements are periodic functions of their
atomic number. It was given by Henry
Moseley.
Characteristics:
1. Modern periodic table consists of 18 groups
and 7 periods.
2. Group 1 elements are called Alkali metals.
3. Group 2 elements are called Alkaline earth
metals.
4. Group 17 elements are called Halogen.
5. Group 18 elements are called Noble gases.
Properties of a Period
III Period
Na Mg A1 Si
11 12 13
P
S
CI A
14 15 16 17 18
2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2, 8,6 2, 8, 7 2, 8,8
+1
+2
+3
4 -3 -2
-1
0
1. Number of valence electrons: As we move
from left to right in a period the no. of
valence electrons go on increasing.
I Fill in the blanks
(i) Elements are classified on the basis of in
their
(ii) Dobereiner grouped the elements into
and Newlands gave the
(iii) Mendeleev’s periodic law was based
upon while Modern periodic law is based
upon
(iv) Elements in the Modern periodic table
are arranged in called and called
(v) Valency of an element is determined by
the number of present in the of its
atom.
II. True or False
(i) In order to fit elements into his table,
Newlands adjusted three elements in the
10th
Class
special
2. Valency: The valency of elements first
increases & then decreases.
3. Size of an atom: The size of an atom
decreases as we move from left to right in
a period. In a period, the nuclear charge
increases progressively by one unit.
Increased nuclear charge pulls the electrons
closer to the nucleus and reduces the size
of the atom.
4. Metallic and non-metallic property:
Metallic characters decreases across a
period. Because the effective nuclear
charge acting on the valence shell electrons
increases across a period. The tendency to
loose electrons decreases.
5.Nature of oxide: The first three elements
have basic oxides and rest of elements
acidic oxides.
Properties of a Group
1. Number of valence electrons: The number
of valence electrons remains same in a
group as we move from top to bottom.
2. Valency: All the elements have same
valency in a group.
3. Atomic size: The size of an atom go on
increasing down the group this increases
the distance between the outermost
electrons and the nucleus so that atomic
size increases in spite of increase in nuclear
charge.
4. Metallic characters: The metallic character
same slot, but also put some unlike
elements in the same plot.
(ii) Electronic configuration of hydrogen
resembles that of alkaline earth metals.
(iii The term atomic size refers to the
distance between the centre of the
nucleus and the penultimate sh,ell of an
isolated atom.
(iv) In the Modern periodic table, a zig-zag
line segments metals from non-metals.
The borderline elements - boron, silicon,
germanium, arsenic, antimony, tellurium
and polonium are intermediate in
properties and are called metalloids or
semi-metals.
(v) Metallic character decreases across a
increases down a group. The effective
nuclear charge experienced by valence
electrons is decreasing because the
outermost electrons are farther away from
the nucleus. Thus these can be lost easily
and non-metallic character decrease on
moving down to group.
5. Nature of oxide: The oxides of elements on
left side of the periodic table are basic and
oxides of elements on the right side of the
periodic table are acidic.
Practice Questions
VERY SHORT ANSWER TYPE QUESTIONS
1. How many triads were identified by
Dobereiner? Name any one of them.
2. How many elements are classified by
Newlands ?
3. Name any two elements which were
discovered later after Mendeleev’s periodic
table.
4. The elements of first group form with
oxygen which type of oxides?
5. How many groups and periods are there in
the modern periodic table?
6. The element A belong to 13th group. What
is the formula of its chloride?
SHORT ANSWER TYPE QUESTIONS
7. Give any two charges in the properties of Igroup elements of modern periodic table,
if we move downwards.
8. What is meant by the term valency? How it
changes on moving down a group?
9. What happens to the metallic character on
moving down a group and moving across
the period respectively?
P. Sanjeev
PGT
Hyderabad
period and increases down a group
because the effective nuclear charge
acting on the valence shell electrons
increases across a period and decreases
down the group.
Answers
I.
(i) Similarities, properties.
(ii) Triads, law of octaves.
(iii) Atomic mass, atomic number.
(iv) 18 vertically columns, group,
7 horizontal rows, periods.
(v) Valence electrons, outermost shell.
II. (i) False (ii) False (iii) False (iv) True
(v) True