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Exam Focus:
Historical classification, Döbereiner's Triads,
Newlands' Law of Octaves, Mendeleev's Periodic Table,
Modern Periodic Law, groups, periods, valency,
atomic size, metallic/non-metallic character and
periodic trends.
A. Very Short Answer Questions
Question 1
Why was classification of elements necessary?
Classification made it easier to study a large number of
elements by arranging elements with similar properties in
an organised manner.
Question 2
What is a Döbereiner's triad?
A group of three elements with similar properties, in which
the atomic mass of the middle element is approximately the
average of the atomic masses of the other two elements.
Question 3
State Newlands' Law of Octaves.
When elements were arranged in increasing order of atomic
mass, every eighth element had properties similar to the
first element.
Question 4
On which basis is the modern periodic table arranged?
The modern periodic table is arranged according to increasing
atomic number.
Question 5
What are groups and periods?
Vertical columns are called groups and horizontal rows are
called periods.
Question 6
What are valence electrons?
Valence electrons are the electrons present in the outermost
occupied shell of an atom.
Question 7
What is atomic number?
Atomic number is the number of protons present in the nucleus
of an atom.
B. Historical Classification
Question 8
How were Döbereiner's Triads identified?
Elements having similar chemical properties were grouped in
sets of three. When arranged in increasing atomic mass, the
middle element had an atomic mass approximately equal to the
average of the other two.
Question 9
Why did Döbereiner's classification fail to classify all
known elements?
Only a limited number of elements could be arranged into
suitable triads, so the system could not classify all known
elements.
Question 10
Why did Newlands' Law of Octaves fail for heavier elements?
The pattern worked reasonably well only for some lighter
elements. With increasing atomic mass, the repetition of
properties after every eighth element no longer remained
valid.
Question 11
Mention two limitations of Newlands' Law of Octaves.
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It worked mainly for lighter elements.
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New elements could not always be fitted correctly
into the arrangement.
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Some dissimilar elements were placed together.
C. Mendeleev's Periodic Table
Question 12
On what basis did Mendeleev arrange elements?
Mendeleev mainly considered increasing atomic masses along
with similarities in chemical properties.
Question 13
Why were gaps left in Mendeleev's periodic table?
Gaps were left for elements that had not yet been discovered.
Mendeleev could then predict some of their properties.
Question 14
State two achievements of Mendeleev's periodic table.
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Similar elements were grouped systematically.
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It allowed predictions about undiscovered elements.
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It provided an organised framework for studying
chemical properties.
Question 15
Mention important limitations of Mendeleev's periodic table.
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The position of hydrogen was not completely
satisfactory.
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Isotopes could not be assigned separate positions
using atomic mass alone.
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Some pairs did not fit perfectly into strict
increasing atomic-mass order.
D. Modern Periodic Table
Question 16
State the Modern Periodic Law.
The properties of elements are periodic functions of their
atomic numbers.
Question 17
Why is atomic number a better basis for classification than
atomic mass?
Atomic number is directly related to the number of protons and
determines the electronic arrangement of the atom. The modern
periodic arrangement based on atomic number gives a consistent
periodic relationship between elements.
Question 18
How many groups and periods are present in the modern
periodic table?
The modern periodic table has 18 groups and 7 periods.
E. Electronic Configuration, Groups and Periods
Question 19
How is the period of an element related to its electronic
configuration?
The number of occupied electron shells generally indicates
the period in which the element is placed.
Question 20
Why are valence electrons important?
Valence electrons largely determine how an element combines
with other elements and therefore influence its chemical
properties and valency.
Question 21
An element has two occupied electron shells. What can be
inferred about its period?
It belongs to the second period.
F. Valency
Question 22
What is valency?
Valency is the combining capacity of an atom.
Question 23
How does valency generally vary across a period for
main-group elements?
It generally increases from one to four and then decreases
from four to zero across a period.
G. Atomic Size and Periodic Trends
Question 24
Why does atomic size generally decrease from left to right
across a period?
Nuclear charge increases across the period while electrons
are added to the same principal shell. The stronger
attraction between the nucleus and electrons generally
reduces atomic size.
Question 25
Why does atomic size generally increase down a group?
Additional electron shells are added as we move down the
group, increasing the size of the electron cloud.
H. Metallic and Non-Metallic Character
Question 26
How does metallic character change across a period?
Metallic character generally decreases from left to right
across a period.
Question 27
How does metallic character change down a group?
Metallic character generally increases down a group.
Question 28
How does non-metallic character vary across a period?
Non-metallic character generally increases from left to right.
Question 29
How does non-metallic character change down a group?
Non-metallic character generally decreases down a group.
I. Reasoning-Based Questions
Question 30
Why do elements in the same group often show similar chemical
properties?
Elements in the same main group have related outer electronic
configurations. Their valence-electron arrangement contributes
strongly to similar chemical behaviour.
Question 31
Why do chemical properties repeat periodically?
As atomic number increases, electronic configurations show
recurring patterns in the outer shell. These recurring
electronic patterns lead to periodic repetition of properties.
Question 32
Why is an element's position in the periodic table useful
for predicting its chemical behaviour?
Its position reflects information about atomic number,
electron-shell arrangement and valence electrons, all of
which influence chemical behaviour.
J. Competency-Based Questions
Question 33
Two elements X and Y belong to the same group. Element X is
above element Y. Which element would generally have the
larger atomic size and why?
Element Y would generally have the larger atomic size because
an additional electron shell is present as we move down the
group.
Question 34
Elements A and B are in the same period. A lies to the left
of B. Which one would generally show greater metallic
character?
Element A would generally show greater metallic character
because metallic character decreases from left to right
across a period.
Question 35
An element has a large atomic size compared with another
element in the same group. What can you infer about their
relative positions?
The element with the larger atomic size is generally lower in
the same group because additional electron shells are added
down the group.
Question 36
A classification system leaves gaps and predicts properties
of undiscovered elements. Which historical system does this
describe, and why was it useful?
It describes Mendeleev's periodic table. The gaps allowed room
for undiscovered elements and enabled predictions about some
of their properties.
K. Long Answer Questions
Question 37
Compare Döbereiner's Triads, Newlands' Law of Octaves and
Mendeleev's Periodic Table.
Döbereiner:
grouped certain elements in sets of three.
Newlands:
arranged elements by increasing atomic mass and proposed
repetition every eighth element.
Mendeleev:
organised elements mainly using atomic mass and chemical
properties, left gaps for undiscovered elements and made
predictions.
Question 38
Explain the major periodic trends in the modern periodic
table.
Atomic size generally decreases across a period and increases
down a group.
Metallic character generally decreases across a period and
increases down a group.
Non-metallic character generally increases across a period and
decreases down a group.
These trends are connected to atomic size, effective nuclear
attraction and electronic configuration.
Question 39
Explain how electronic configuration is related to the
position of an element in the modern periodic table.
The number of occupied electron shells generally indicates
the period. The arrangement of valence electrons helps
explain group position and chemical behaviour, especially for
main-group elements.
L. Quick Practice
Try answering these without looking at the solutions:
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State the Modern Periodic Law.
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Why did Newlands' arrangement fail for heavier elements?
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What is the significance of valence electrons?
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Why does atomic size increase down a group?
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Why does metallic character decrease across a period?
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Explain why elements in the same group often show
similar chemical properties.