Q1: What are the defects in Modern Periodic
Table?
Answer: Following are the defects:
Position of hydrogen still dicey. It is not fixed till now.
Position of lanthanides and actinides has not been given
inside the main body of periodic table.
It does not reflect the exact distribution of electrons of
some of transition and inner transition
elements.
Q2: What are the salient features of Modern
Periodic Table?
Answer:
In a period of the periodic table, the number of valence
electrons increases as atomic number increases. As a
result, elements change from metal to metalloid to
nonmetal to noble gas.
Atomic size is a periodic property. As atomic number
increases in a period, atomic radius
decreases. As atomic number increases in a group,
atomic radius increases.
Positive ions have smaller atomic radii than the neutral
atoms from which they derive. Negative ions have larger
atomic radii than their neutral atoms.
Positive ions in the same group increase in size down
the group.
In a group, each element has the same number of
valence electrons. As a result, the elements
in a group show similar chemical behavior.
Metallic character decreases from left to right in a
period because of the increase in the
effective nuclear charge.
Non-metallic character increase from left to right in a
period because of ncrease in
effective nuclear charge.Non-metallic character
decreases down the group because of increase in the
size of the
atom.
Q3: Two elements X and Y have atomic numbers 12
and 16 respectively. Write the electronic
configuration for these elements. To which period
of the modern periodic table do these two
elements belong? What type of bond will be formed
between them and why?
Answer: Electronic configuration of X (Z=12): 2, 8, 2
Electronic configuration of Y (Z=16): 2, 8, 6
Both these elements belong to third period. An ionic
bond is formed between X and Y due to transfer of two
electrons from X to Y.
Q4: The present classification of elements is based
on which fundamental property of elements?
Answer: Atomic Number is the basis.
Q5: Li, Na and K are the elements of a Dobereiners
Triad. If atomic mass of Li is 7 and that of K is 39.
What would be the atomic mass of Na?
Answer: According of Dobereiner’s law of triads, the
atomic mass of the middle element, in this case Na,
should be the arithmetic mean of Li and K.
Thus,
arithmetic mean of Li and K = (7 + 39)/2 = 23
(Answer)
Q6: Define Dobereiner’s law of triads.
Answer: It states, "when elements are placed in order
of the ascending order of atomic masses, groups of
three elements having similar properties are obtained.
The atomic mass of the middle element of the triad is
equal to the mean of the atomic masses of the other
two elements of the triad."
Q7: Why did Dobereiner's system of classification
fail?
Answer:
Major drawback of Deberneir’s classification was that it
was valid only for a few groups of elements known
during that time. He was able to identify three triads
only.
Also with more accurate measurements of atomic
masses showed that the mid element of the triad did
not really have the mean value of the sum of the other
two elements of the triad.
For elements of very low mass or very high mass, the
law did not hold good. For example Flourine(F),
Chlorine (Cl), Bromine(Br). Atomic mass of Cl is not an
arithmetic mean of atomic masses of F and Br
Q8: Explain about position of metalloids in modern
periodic table.
Answer : In the Modern Periodic Table, a zig-zag line
separates 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.
Q9: Why does Silicon classified as metalloid?
Answer: Silicon is classified as a semi-metal or metalloid
because it exhibits some properties of both metals and
non-metals.
Q10: State Newland's law of octaves.
Answer: Elements are arranged in increasing order of
their atomic masses such that the properties of the
eighth element is repetition of the properties of first
element ( similar to eighth note in an octave of music)
Q11: X and Y are the two elements having similar
properties which obey Newlands law of octaves.
How many elements are there in between X and Y?
Answer: The law states there are eight elements in an
octave (row). No. of elements between X and Y is six.
Q12: What are the drawbacks of Newland's law of
octaves?
Answer: Following are the major drawbacks:
1. Worked well with lighter elements (upto Calcium). After
that elements in eighth column did not possess
properties similar to element in first column.
2. Newland assumed only 56 elements existed so far. Later
new elements were discovered which did not fit into
octaves table.
3. Newland adjusted few elements in a same slot though
their properties were quite different. E.g. Cobalt and
nickel are in the same slot and these are placed in the
same column as fluorine, chlorine and bromine which
have very different properties than these elements. Iron,
which resembles cobalt and nickel in properties, has
been placed far away from these elements.
Q13: What periodic trends do we observe in terms
of atomic radii or atomic sizes in Modern periodic
table?
Answer: Following two trends are observed.
1. Within each column (group), atomic radius tends to
increase from top to bottom. This trend results
primarily from the increase in the number of the outer
electrons. As we go down a column, the outer electrons
have a greater probability of being farther from the
nucleus, causing the atom to increase in size.
2. Within each row (period), atomic radius tends to
decrease from left to right. The major factor influencing
this trend is the increase in the nuclear charge as we
move across a row. The increasing effective nuclear
charge steadily draws the valence electrons closer to
the nucleus, causing the atomic radius to decrease.
Q14: Name the scientist who proposed modern
periodic law? On which fundamental property of
elements it is based?
Answer: Henry Moseley . He proposed atomic number
should be chosen as fundamental property (instead of
atomic mass) to classify elements.
Q15: On what basis did Mendeleev arrange the
elements?
Answer: On the basis of atomic masses.
Q16 : An element R of atomic number 12 combines
separately with NO 3- and (SO 4 )2- , (PO 4 )3- radicals.
a) Write the electronic configuration of element R
b) Write the formulae of the three compounds so
formed.
c) To which group of the periodic table does the
element ‘R’ belong?
d) Does it form covalent or ionic compound? Why?
Answer:
a) Electronic configuration of R is 2,8,2.
b) Compounds formed are: R(NO 3 )2 , R(SO 4 ) and R
(PO4 ) 2
c) R has two valence electrons and hence it belongs to
second group.
d) It forms ionic compound by loosing two electrons to
attain stable electronic configuration.
Q17 Why do you think the noble gases are
placed in a separate group?
Answer: Due to following reasons:
1. Similar inert behaviour
2. Similar electronic configuration
3. Available in low concentration.
Q18 Besides gallium, which other
elements have since been discovered that were left
by Mendeléev in his Periodic Table? (any two)
Answer: Scandium (Sc) and Germanium(Ge)
Table?
Answer: Following are the defects:
Position of hydrogen still dicey. It is not fixed till now.
Position of lanthanides and actinides has not been given
inside the main body of periodic table.
It does not reflect the exact distribution of electrons of
some of transition and inner transition
elements.
Q2: What are the salient features of Modern
Periodic Table?
Answer:
In a period of the periodic table, the number of valence
electrons increases as atomic number increases. As a
result, elements change from metal to metalloid to
nonmetal to noble gas.
Atomic size is a periodic property. As atomic number
increases in a period, atomic radius
decreases. As atomic number increases in a group,
atomic radius increases.
Positive ions have smaller atomic radii than the neutral
atoms from which they derive. Negative ions have larger
atomic radii than their neutral atoms.
Positive ions in the same group increase in size down
the group.
In a group, each element has the same number of
valence electrons. As a result, the elements
in a group show similar chemical behavior.
Metallic character decreases from left to right in a
period because of the increase in the
effective nuclear charge.
Non-metallic character increase from left to right in a
period because of ncrease in
effective nuclear charge.Non-metallic character
decreases down the group because of increase in the
size of the
atom.
Q3: Two elements X and Y have atomic numbers 12
and 16 respectively. Write the electronic
configuration for these elements. To which period
of the modern periodic table do these two
elements belong? What type of bond will be formed
between them and why?
Answer: Electronic configuration of X (Z=12): 2, 8, 2
Electronic configuration of Y (Z=16): 2, 8, 6
Both these elements belong to third period. An ionic
bond is formed between X and Y due to transfer of two
electrons from X to Y.
Q4: The present classification of elements is based
on which fundamental property of elements?
Answer: Atomic Number is the basis.
Q5: Li, Na and K are the elements of a Dobereiners
Triad. If atomic mass of Li is 7 and that of K is 39.
What would be the atomic mass of Na?
Answer: According of Dobereiner’s law of triads, the
atomic mass of the middle element, in this case Na,
should be the arithmetic mean of Li and K.
Thus,
arithmetic mean of Li and K = (7 + 39)/2 = 23
(Answer)
Q6: Define Dobereiner’s law of triads.
Answer: It states, "when elements are placed in order
of the ascending order of atomic masses, groups of
three elements having similar properties are obtained.
The atomic mass of the middle element of the triad is
equal to the mean of the atomic masses of the other
two elements of the triad."
Q7: Why did Dobereiner's system of classification
fail?
Answer:
Major drawback of Deberneir’s classification was that it
was valid only for a few groups of elements known
during that time. He was able to identify three triads
only.
Also with more accurate measurements of atomic
masses showed that the mid element of the triad did
not really have the mean value of the sum of the other
two elements of the triad.
For elements of very low mass or very high mass, the
law did not hold good. For example Flourine(F),
Chlorine (Cl), Bromine(Br). Atomic mass of Cl is not an
arithmetic mean of atomic masses of F and Br
Q8: Explain about position of metalloids in modern
periodic table.
Answer : In the Modern Periodic Table, a zig-zag line
separates 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.
Q9: Why does Silicon classified as metalloid?
Answer: Silicon is classified as a semi-metal or metalloid
because it exhibits some properties of both metals and
non-metals.
Q10: State Newland's law of octaves.
Answer: Elements are arranged in increasing order of
their atomic masses such that the properties of the
eighth element is repetition of the properties of first
element ( similar to eighth note in an octave of music)
Q11: X and Y are the two elements having similar
properties which obey Newlands law of octaves.
How many elements are there in between X and Y?
Answer: The law states there are eight elements in an
octave (row). No. of elements between X and Y is six.
Q12: What are the drawbacks of Newland's law of
octaves?
Answer: Following are the major drawbacks:
1. Worked well with lighter elements (upto Calcium). After
that elements in eighth column did not possess
properties similar to element in first column.
2. Newland assumed only 56 elements existed so far. Later
new elements were discovered which did not fit into
octaves table.
3. Newland adjusted few elements in a same slot though
their properties were quite different. E.g. Cobalt and
nickel are in the same slot and these are placed in the
same column as fluorine, chlorine and bromine which
have very different properties than these elements. Iron,
which resembles cobalt and nickel in properties, has
been placed far away from these elements.
Q13: What periodic trends do we observe in terms
of atomic radii or atomic sizes in Modern periodic
table?
Answer: Following two trends are observed.
1. Within each column (group), atomic radius tends to
increase from top to bottom. This trend results
primarily from the increase in the number of the outer
electrons. As we go down a column, the outer electrons
have a greater probability of being farther from the
nucleus, causing the atom to increase in size.
2. Within each row (period), atomic radius tends to
decrease from left to right. The major factor influencing
this trend is the increase in the nuclear charge as we
move across a row. The increasing effective nuclear
charge steadily draws the valence electrons closer to
the nucleus, causing the atomic radius to decrease.
Q14: Name the scientist who proposed modern
periodic law? On which fundamental property of
elements it is based?
Answer: Henry Moseley . He proposed atomic number
should be chosen as fundamental property (instead of
atomic mass) to classify elements.
Q15: On what basis did Mendeleev arrange the
elements?
Answer: On the basis of atomic masses.
Q16 : An element R of atomic number 12 combines
separately with NO 3- and (SO 4 )2- , (PO 4 )3- radicals.
a) Write the electronic configuration of element R
b) Write the formulae of the three compounds so
formed.
c) To which group of the periodic table does the
element ‘R’ belong?
d) Does it form covalent or ionic compound? Why?
Answer:
a) Electronic configuration of R is 2,8,2.
b) Compounds formed are: R(NO 3 )2 , R(SO 4 ) and R
(PO4 ) 2
c) R has two valence electrons and hence it belongs to
second group.
d) It forms ionic compound by loosing two electrons to
attain stable electronic configuration.
Q17 Why do you think the noble gases are
placed in a separate group?
Answer: Due to following reasons:
1. Similar inert behaviour
2. Similar electronic configuration
3. Available in low concentration.
Q18 Besides gallium, which other
elements have since been discovered that were left
by Mendeléev in his Periodic Table? (any two)
Answer: Scandium (Sc) and Germanium(Ge)
