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School of Elements
1
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Q.I
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(A) Fill in the blanks :
The horizontal rows in the periodic table are ....................... .
From left to right, the atomic size of an atom ....................... .
There are ....................... periods in the periodic table..
An element is placed in group II A, so the number of valence electrons is
....................... .
An element is placed in 2nd period, so it has ....................... shells.
Noble gases have valency as ....................... .
Metals have a tendency to ....................... electrons..
Valency across a period ....................... .
Valency down the group ....................... .
Elements of group I A and II A are ....................... .
An atom is said to be a non-metal if it ....................... electrons.
....................... is the only element in duplet state.
The electronic configuration of Neon is ....................... .
Sodium is a ....................... .
Silicon and antimony are ....................... .
Group 1A is called ....................... .
Group II A is called as ....................... .
Group VII A is called as ....................... .
The chemical properties down the group ....................... .
First period contains ....................... elements.
Moseley arranged elements according to ....................... .
The non-metals are present in ....................... .
The number of shells down the group ....................... .
The transition character of elements is from ....................... to
....................... .
The formula of chloride of metal is MCl 2 , the metal M belongs to
....................... group.
....................... group contains all gases at room temperature.
The arrangement of elements in a group of three is called ....................... .
The law used by Newlands to arrange elements is called ....................... .
Elements showing properties of both metals and non-metals are called as
....................... .
The element eka-Aluminium is called as ....................... .
Answers :
1. periods
3. 7
5. 2
7. donate
9. ramains the same
11. gains or shares
13. 2, 8
15. metalloids
17. alkaline earth metals
19. remain the same
21. Atomic number
23. increases
25. II A
27. triad
29. metalloids
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decreases
2
0
varies gradually
metals
Helium
metal
alkali metals
halogens
2
upper right hand corner
metallic to Non-metallic
18 or zero
Newlands Law of octaves
gallium.
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Q.I
1.
(i)
(ii)
(iii)
Ans.
(B) Match the following :
Column I
Mendeleev
Dobereiner
Newlands
(i - b), (ii - c), (iii - a).
2.
(i)
(ii)
(iii)
(iv)
Ans.
Column I
Alkali metals
Alkaline earth metals
Halogens
Noble gases
(i - b), (ii - a), (iii - d), (iv - c).
3.
(i)
(ii)
(iii)
(iv)
Ans.
(a)
(b)
(c)
Column II
Octaves
Periodic law
Triads
(a)
(b)
(c)
(d)
Column II
II A group
I A group
zero group
VII A group
Column I
Helium
Chlorine
Hydrogen
Magnesium
(i - d), (ii - a), (iii - c), (iv - b).
(a)
(b)
(c)
(d)
Column II
Halogen
Alkaline earth metal
H
Noble gas
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*(i)
(ii)
(iii)
(iv)
Ans.
Column I
Sodium
Sulphur
Manganese
Cerium
(i - c), (ii - a), (iii - d), (iv - b).
(a)
(b)
(c)
(d)
Column II
Non-metal
Lanthanide
Metal
Transition metal
5.
(i)
(ii)
(iii)
Ans.
Column I
Mendeleev
Dobereiner
Newlands
(i - a), (ii - b), (iii - c).
(a)
(b)
(c)
Column II
Russian scientist
German scientist
Englishman
Q.I
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(C) State whether the following statements are true or false. If false
write the corrected statement :
Atomic number is the number of protons or electrons in the nucleus.
True.
Fourth and fifth periods are longest periods.
False. Fourth and fifth periods are long periods, 6th period is the longest
period
Newlands arranged elements based on their atomic number.
False. Newlands arranged elements based on their atomic mass.
Mendeleev was the first scientist to create a periodic table for the elements.
True.
Mendeleev arranged 63 elements in the periodic table.
True.
Eka boron is known as Germanium.
False. Eka boron is known as Scandium
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17.
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18.
Ans.
Tellurium, Polonium are metalloids.
True.
Isotopes have same atomic masses.
False. Isotopes are atoms of the same element having same atomic number
but different atomic masses.
Isotopes have similar chemical properties.
True.
La stands for Lutetium.
False. La stands for Lanthanum.
s-block includes I A, II A and hydrogen.
True.
d-block elements are called normal elements.
False. d-block elements are called transition elements.
f-block elements are metalloids.
False. f-block elements are metals.
Metalloids are also called as semi-metals.
True.
Bordering the zig-zag line are elements called as metalloids.
True.
In an atom of an element, extra-nuclear electrons take part in the chemical.
reaction.
True.
The number of shells increases down the group.
True.
Helium is the only element in octet state.
False. Helium is the only element in duplet state.
Q.I
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(D) Find the odd man out :
Carbon, Nitrogen, Oxygen, Sulphur.
Sulphur. It is in third period while the rest are in second period.
Hydrogen, Helium, Neon, Argon.
Hydrogen. The rest are noble gases.
Lithium, Sodium, Iodine, Potassium.
Iodine as a halogen and rest belong to group 1A.
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*5.
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MEMORISE :
Fluorine and Chlorine are gases, bromine is a liquid, iodine and astatine
– Extra Information.
are solids.
Boron, Silicon, Arsenic, Sodium.
Sodium belongs to group IA and rest are metalloids.
Li, Sr, K, Na.
Sr is an element of a different triad whereas Li, Na, K are elements of
same triad.
B and Al, O and S, Be and Mg, N and S.
N and S. It does not show similarities in properties according to Newlands
law of octaves whereas the others show similarities.
Electrons, protons, neutrons, lanthanides.
Lanthanides. It is a series and rest all make up an atom.
MEMORISE :
Electrons, Protons and neutrons are called as sub-atomic or elementary
– Extra Information.
particles.
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Q.I
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(E) Write the correlated terms :
Newlands : Octaves :: ....................... : Triads.
Dobereiner.
Ca, Sr, Ba : Triad II :: ....................... Triad I.
Li, Na, K.
Fluorine : ....................... :: Chlorine : Third period.
Second period.
Neon : Inert element :: Bromine : ....................... .
Halogen/normal element.
Fifth period : 18 elements :: Sixth period : ....................... .
32 elements.
Alkali metals : Monovalent :: Alkaline earth metal : ....................... .
Divalent.
Tellurium : ....................... :: Radium : Metal.
Metalloid.
(F) Complete the following table :
Element
Symbol
Group
Sodium
Lithium
Fluorine
Barium
Calcium
...........
...........
...........
...........
...........
IA
IA
VII A
II A
II A
Ans. Element
Symbol
Group
Na
Li
F
Ba
Ca
IA
IA
VII A
II A
II A
Sodium
Lithium
Fluorine
Barium
Calcium
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Q.I
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Molecular formula of
Valency
compound with chlorine
NaCl
...........
...........
1
ClF
...........
BaCl 2
2
CaCl 2
2
Molecular formula of
Valency
compound with chlorine
NaCl
1
LiCl
1
ClF
1
BaCl 2
2
CaCl 2
2
MEMORISE :
To get the molecular formula of a compound:
Eg: Sodium chloride
Symbol
Na
Cl
Valency
1
1
Cl 1
Criss cross Na 1
i.e. NaCl
(G) Give two names of the following
Member of 1st period
:
nd
:
Member of 2 period
Alkali metals
:
Alkaline earth metals
:
:
Member of 3rd period
Elements of zero group
:
Elements of halogen family
:
Metalloid
:
SCHOOL SECTION
– Extra Information.
:
Hydrogen, Helium
Carbon, Nitrogen.
Lithium, Sodium
Magnesium, Calcium
Phosphorus, Sulphur
Neon, Argon
Fluorine, Chlorine
Silicon, Antimony
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Q.II
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(A) State the following :
Dobereiner’s triads law.
He arranged chemically similar elements in the increasing order of their
atomic weights in a group of three which is known as “Dobereiner’s triads”.
In a Triad, atomic mass of the middle element is approximately equal to
the mean of atomic masses of other two elements in that triad.
Newlands law of octaves.
When the elements are arranged in increasing order of their atomic masses,
the properties of every eighth element are similar to the first, as in the
octave of music.
Mendeleev’s periodic law.
The physical and chemical properties of elements are periodic function of
their atomic masses.
Modern periodic law.
The physical and chemical properties of elements are periodic function of
their atomic numbers.
(B) Define the following :
Groups.
Vertical column of elements in a periodic table are called as a groups.
Periods.
Horizontal row of elements in a periodic table are called as a periods.
Inert gas elements.
The elements in which all the shells including the outermost shell is
completely filled are called as inert gas elements.
Normal elements.
The elements in which only the last shell is incompletely filled are called
as normal elements.
Transition elements.
The elements in which last two shells are incompletely filled are called
transition elements.
Inner transition elements.
The elements in which last three shells are incompletely filled are called
inner transition elements.
Metalloids.
Elements showing properties of both metals and non-metals are called as
metalloids
Lanthanides.
14 elements (post lanthanum) with atomic numbers 58 to 71 (Ce to Lu) are
called lanthanides.
Actinides.
14 elements (post actinium) with atomic numbers 90 to 103 (Th to Lr) are
called actinides.
Q.III (A) Give scientific reasons :
1. Group VII A elements are strong non-metals and group I A elements are
strong metals.
Ans. 1. Non-metals are electron acceptors and metals are electron donors.
The elements of group VII A have seven electrons in their outermost
shell. So, they have a strong tendency to accept one electron and form
a negatively charged ion (anion) with octet state.
2. The elements of group I A have one electron in their outermost shell.
So, they have a strong tendency to donate one electron and form
positively charged ion (cation) with octet state or duplet state.
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MEMORISE :
Electropositivity is the property of an atom to donate electrons when
combined in a compound. Electronegativity is the property of an atom to
accept electrons when combined in a compound. – Extra Information.
Metallic character decreases across a period.
1. Metals have a tendency to lose electrons and are said to be electro positive.
2. As we move across a period, the atomic number and the nuclear charge
increases. So, the atomic size decreases.
3. As a result, electrons are more tightly bound to the nucleus and more
amount of energy is needed to remove an electron from the outermost
shell. So, metallic character decreases across the period.
MEMORISE :
Nuclear charge is the positive charge on the nucleus of an atom.
– Extra Information.
Non-metallic character increases across a period.
1. Non-metals have a tendency to gain electrons.
2. As we move across a period, the atomic number and the nuclear charge
increases. So, the atomic size decreases.
3. As a result, the electron that is gained is pulled more strongly. So,
non-metallic character increases across the period.
Atomic size decreases across a period.
1. Atomic size is defined as the distance between the centre of the nucleus
of an atom to its outermost shell.
2. Across a period, the nuclear charge increases. Nuclear charge is the
positive charge on the nucleus of an atom. Also, across the period
electrons are added to the same shell. So, electrons experience greater
pull from the nucleus.
3. Therefore, atomic size decreases across a period.
Atomic size increases down the group.
1. Atomic size is defined as the distance between the centre of the nucleus
of an atom to its outermost shell.
2. As we go down the group, the number of shells increases and so the
nuclear distance from valence electrons increases.
3. The nuclear charge also increases but increase in number of shells
dominates over increase in nuclear charge. So, atomic size increases
down the group.
Valency varies gradually across a period.
1. In the modern periodic table, the elements are arranged in increasing
order of atomic number (Z).
2. Atomic number is related to the electronic configuration.
3. As the atomic number increases, the number of valence electrons
increases. The first element has one electron in its outermost shell
while the last element in a period has either completed duplet or octet.
4. So, valency varies gradually across a period.
Elements in a particular group have similar chemical properties. OR
Elements in the same group show same valency
1. Position of an element in the periodic table is related to its electronic
configuration. The elements of a particular group have the same number
of electrons in the outermost shell (i.e. valence electrons).
2. Chemical properties of an element depends on the number of electrons
in the valence shell. Eg, Chemical properties of sodium and potassium
in group I A are similar.
3. Hence, elements in a particular group have similar chemical properties.
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The family of noble gases is called as zero group.
1. Zero group or group 18 is a family of inert gases also called as noble
gases or rare gases.
2. In their atoms, all the electronic shells including the outermost shell
are completely filled. They have stable electronic configuration with
complete duplet (Helium) or complete octet (Neon, Argon etc).
3. So, atoms do not gain, lose or share electrons and hence have zero
valency.
4. Hence, the family of noble gases is called as zero group.
MEMORISE :
Noble gases have a stable electronic configuration, they are in duplet or
octet state. So, they don't lose, gain, share electrons. So, inert gases
exist in their free atomic state.
– Extra Information.
Chlorine is placed in VII A group in the modern periodic table.
1. Number of valence electrons is equal to the group number.
2. Chlorine has atomic number 17 and hence its electronic configuration
is 2, 8, 7.
3. There are 7 electrons in the valence shell.
4. Hence, Chlorine is placed in VII A group in the modern periodic table.
Q.III (B) Answer the following questions in short :
1. From the list of elements given below, make three triads and name the
family of each triad.
List : Cl, Li, Ca, Br, Na, Sr, I, K, Ba.
Ans. Alkali metal triad : Li Na K.
Alkaline earth metal : Ca, Sr, Ba.
Halogens triad : Cl, Br, I.
2. The elements A and B obey the law of octaves. How many elements are
there between A and B?
Ans. There are 6 elements between A and B.
3. What was the basis of classification of elements in Mendeleev's periodic
table?
Ans. Basis of Mendeleev's periodic table :
1. Similarities in the chemical properties of the elements.
2. Increasing order of atomic mass of the elements.
4. What are transition elements ? Which amongst the following are
transition elements ? K, Mn, Ca, Cr, Cu, Cs, Fe
Ans. 1. The elements in which last two shells are incompletely filled are called
transition elements.
2. The elements Mn, Cr, Cu, Fe are transition elements.
5. Name alkali metals, to which group do they belong?
Ans. 1. The alkali metals are Lithium, Sodium, Potassium, Rubidium, Cesium,
Francium.
2. These elements belong to group I A.
6. Name alkaline earth metals, to which group do they belong?
Ans. 1. The alkaline earth metals are Beryllium, Magnesium, Calcium,
Strontium, Barium, Radium.
2. These elements belong to group II A.
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Name halogens, to which group do they belong?
1. The halogens are Fluorine, Chlorine, Bromine, Iodine, Astatine.
2. These elements belong to group VII A.
Oxygen (Z = 8) and sulphur (Z = 16) belong to the same group of the
periodic table. On the basis of electronic configuration, name the group
and period to which they belong.
1. Group number indicates number of valence electrons.
2. Period number indicates no of shells.
3. The electronic configuration of oxygen is 2, 6. So, it belongs to Group VI
A and 2nd period.
4. The electronic configuration of sulphur is 2, 8, 6.
So, it belongs to Group VI A and 3rd period.
Name the noble gases, to which group do they belong?
1. The noble gases are Helium, Neon, Argon, Krypton, Xenon, Radon
2. These elements belong to group 18.
What are the names of the periods and how many elements do they
have ?
1. First period is the shortest period with 2 elements.
2. Second, Third period are short periods with 8 elements.
3. Fourth, Fifth period are long periods with 18 elements.
4. Sixth period is the longest period with 32 elements.
5. Seventh period is the incomplete period with 30 elements.
An element is in the second period and group VI A of the periodic table.
1. Is it metal or non-metal.
Non-metal.
2. Number of valence electrons.
6.
3. Electronic configuration.
2, 6.
4. Name of the element.
Oxygen.
5. Formula of compound with sodium.
Na 2O.
Label each of the following as either metal, metalloid or a non-metal :
K, Ni, As, Br, S, Al.
Metals : K, Ni, Al
Non-metals : Br, S
Metalloid : As.
Which are the three elements having single electron in the outermost
shell?
Lithium, Sodium, Potassium.
Which are the three elements with filled outermost shell?
Helium, Neon, Argon.
Which are the three elements having seven electrons in the valence
shell?
Chlorine, Bromine, Iodine.
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Q.IV
1.
Ans.
Distinguish between :
Periods and Groups.
Periods
Groups
1. Modern periodic table has 7 1. Modern periodic table has 18
horizontal rows called as periods.
vertical columns of elements
called groups.
2. The number of shells present in 2. The number of electrons present
the atom of an element
in the outermost shell of an atom
determines the period number.
of the element determines the
group number.
3. Elements show gradual variation 3. Elements belonging to a
in chemical properties along a
particular group show strong
period. Neighbouring elements
resemblance in their chemical
show some similarities but nonproperties like valency, formulae
neighbouring elements show
of compounds and chemical
appreciable dissimilarity in their
reactions.
properties.
2.
Ans.
Alkali metals and Alkaline earth metals.
Q.V
1.
Ans.
Answer the following in detail :
Explain Dobereiner’s law of triads giving examples.
1. He arranged chemically similar elements in the increasing order of
their atomic masses in a group of three which is known as “Dobereiner’s
Triads”. In a triad atomic mass of middle element is approximately
equal to the mean of atomic masses of other two elements in that
Triad.
Eg.: Lithium (Li), Sodium (Na), Potassium (K) form a triad as they show
similar chemical properties. The atomic weights are 6.9, 23, 39
respectively.
2. According to Dobereiner’s law of triads,
6.9  39
45.9
=
= 22.95 ~ 23 = Atomic weight of sodium.
2
2
Thus, the atomic weight of sodium (23) is the average of the atomic
weights of lithium (7) and potassium (39). The other examples are:
Alkali metals
Alkaline earth metals
1. In the modern periodic table, IA 1. In the modern periodic table, IIA
group
elements
including
group
elements
including
lithium, sodium, potassium,
beryllium, magnesium, calcium,
rubidium,
caesium
and
strontium, barium, radioactive
radioactive francium are alkali
radium are alkaline earth
metals.
metals.
2. Atoms of these elements have 2. Atoms of these elements have
one electron in the valence shell.
two electrons in their valence
shell.
3. They are monovalent.
3. They are divalent.
4. Their oxides and hydroxides 4. Their oxides and hydroxides
dissolve readily in water.
dissolve slightly in water.
Elements
Calcium
Strontium
Barium
10
Atomic
(Ca)
(Sr)
(Ba)
Mass
40.1
87.6
137.3
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Elements
Chlorine
Bromine
Iodine
Atomic
(Cl)
(Br)
(I)
Mass
35.5
79.9
126.9
Elements
Sulphur
Selenium
Tellurium
Atomic
(S)
(Se)
(Te)
Mass
32
79
128
Explain the limitations of Dobereiner's law of triads.
1. All the elements known at that time could not be classified as triads
(i.e. law was applicable to a few elements only.
2. It does not take into account the isotopes of elements.
3. It cannot be applied to other chemically similar elements.
Eg. :
Triads
Elements
Atomic weights
Mean of atomic weights
3.
Ans.
4.
Ans.
III
IV
Cu
Ag
Au Zn
Cd
Hg
63.5 108
197 65 112.5 200
130.25
132.5
The above set of triads do not obey Dobereiner's law.
Explain Newlands law of octaves giving examples.
Newlands Law : When the elements are arranged in increasing order of
their atomic masses, the properties of the eighth element are similar to
that of the first.
Examples :
1. The eight element from lithium is sodium. Similarly, eight element
from sodium is potassium. Lithium, sodium, potassium have similar
chemical properties.
2. The eighth element from fluorine is chlorine. Fluorine and chlorine
have similar chemical properties.
Explain the limitations of Newlands law of octaves.
1. Newlands law was applicable to elements with low atomic masses only.
He could arrange elements only upto calcium out of total 56 elements
known.
2. The law fails to explain similarities in properties of elements with higher
atomic weights. After calcium, every eighth element did not possess
properties similar to the first.
3. Newlands thought only 56 elements existed, but later several elements
were discovered.
4. In order to fit the existing elements, Newlands adjusted two elements
in the same position which differed in their properties.
5. This periodic table did not include inert gases as they were not
discovered.
Eg. :
Sa (Do)
H
F
Cl
SCHOOL SECTION
Re (Re)
Li
Na
K
Ga (Mi)
Be
Mg
Ca
Ma (Fa)
B
Al
Ti
Pa (So)
C
Si
Cr
Da (La)
N
P
Mn
Ni(Ti)
O
S
Fe
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In the above table, Ti is the eighth element with respect to Al, but does not
show similar properties. Similarly, Cr is different from Si. The elements
Ti, Cr, Mn, Fe and Co, Ni do not show similarities as per Newlands law.
Explain the merits of Mendeleevs periodic table.
1. Mendeleev was the first who successfully classified all known elements.
2. Mendeleev kept some blank places in his periodic table. These vacant
spaces were for elements that were yet to be discovered. He also
predicted properties of these elements even before they were discovered.
Later they were found to be correct.
3. In the periodic table, some gaps were left by Mendeleev for unknown
elements that could be found in the future. Three such unknown
elements were named as Eka-Boron, Eka-Aluminium and Eka-Silicon.
Even the properties of these unknown elements were predicted and
these were found to accurate.
Predicted element
Eka-boron
Eka-Aluminium
Eka-Silicon
Actual element discovered after
Scandium
Gallium
Germanium
Comparison of properties of eka-aluminium and gallium.
Eka-Aluminium
Atomic weight
About 68
Density of solid
6.0g/cm3
Melting point
Low
Valency
3
Method of discovery Probably from its spectrum
Oxide
Ea2O 3
Density = 5.5 g/cm3
Soluble in acids and bases
4.
Gallium
69.72
5.9g/cm3
29.78
3
Spectroscopically
Ga 2 O 3
Density = 5.8 g/cm3
Soluble in acids
and bases
When noble gases were discovered later, they were placed in
Mendeleev’s periodic table without disturbing the positions of other
elements.

MEMORISE :
Spectrum : A vacuumised cylindrical discharge tube is taken and filled
with hydrogen gas (whose spectrum is to be obtained). Electric current is
passed through it.
Hydrogen (Z=1), so
Total number of protons = total number of electrons = 1
The electron present in the first shell absorbs electrical energy and goes
in an excited state, i.e jumps to the next shell. As a result, the atom
becomes unstable. To gain stability, the electron emits the same amount
of energy that was absorbed and comes back to its ground state. This
energy emitted is in the form of light. This light when passed through a
prism, gets dispersed to give coloured bands. This is spectrum. This similar
to dispersion of white light when passed through the prism. The spectrum
of each element is unique as fingerprints vary from person to person. This
method of obtaining the spectrum is spectroscopy. – Extra Information.
12
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*6.
Ans.
7.
Ans.
*8.
Ans.
9.
Ans.
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Explain the demerits of Mendeleevs periodic table.
1. Hydrogen resembles alkali metals as well as halogens. Therefore, no
fixed position could be given to hydrogen in the periodic table.
2. Isotopes of same elements have different atomic masses; therefore
each one of them should be given different position. On the other hand
as isotopes are chemically similar, they had to be given same position.
3. At certain places, an element of higher atomic mass has been placed
before an element of lower atomic mass. For eg, cobalt (Co = 58.93) is
placed before nickel (Ni = 58.71)
4. Some elements placed in the same sub-group had different properties.
Eg. Manganese (Mn) is placed with halogens which totally differs in the
properties.
Explain the modern periodic table.
1. In 1913, Henry Moseley, an English physicist found that it was atomic
number (Z) and not atomic mass which was the fundamental property
of an element. Thus atomic number is useful for arrangement of
elements in the periodic table.
2. The modern periodic table is also called as long form of periodic table.
3. In the modern periodic table, elements are arranged in the increasing
order of atomic number (Z).
4. The modern periodic table contains 7 horizontal rows called as periods
and 18 vertical columns called as groups.
5. Apart from the seven rows, there are two additional rows called
lanthanide series and actinide series placed at the bottom of the periodic
table.
6. Based on electronic configuration, the modern periodic table is divided
into four blocks namely, s-block, p-block, d-block and f-block.
7. The blocks gives rise to 4 types of elements namely normal elements,
zero group elements, transition elements and inner-transition elements.
8. Elements belonging to a particular group show strong resemblance in
their chemical properties like valency, formulae of compound and
chemical reactions.
9. Elements show gradual variation in chemical properties along a period.
10. Neighbouring elements within a period show some similarities but nonneighbours show appreciable dissimilarity in their chemical properties.
Explain the merits of modern periodic table. OR
How could the modern periodic table remove various anomalies of
Mendeleev's table?
1. The modern periodic table was classified based on atomic number. All
isotopes of the same elements have different masses but same atomic
number. Therefore, they occupy the same position in the modern periodic
table.
2. When elements are arranged according to their atomic numbers, the
anomaly regarding certain pairs of elements in Mendeleev's periodic
table disappears eg.atomic number of cobalt and nickel are 27 and 28
respectively. Therefore, cobalt will come first and then nickel although
its atomic mass is greater.
3. Elements are classified according to their electronic configuration into
different blocks.
Explain the four blocks of periodic table
Based on electronic configuration, the modern periodic table is divided
into four blocks namely, s-block, p-block, d-block, f-block.
SCHOOL SECTION
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s-block

10.
Ans.

11.
Ans.
14
p-block
d-block
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f-block
No. of shells
incomplete/
valence
electrons
Outermost
shell
incomplete
(1 or 2
valence
electrons).
Position
I A, II A and III A to VII A and
hydrogen.
zero group.
Group IIIB
to II B along
with group
VIII.
Lanthanides
and actinides
placed
separately at
the bottom of
the periodic
table.
Includes
All metals
except
hydrogen.
Metals, non-metals,
metalloids, zero
group elements.
Metals.
Metals.
Types of
elements
Normal
elements.
Normal and inert
elements.
Transition
elements.
Innertransition
elements.
Outermost shell
Last two
Last three
incomplete except
shells
shells
zero group elements incomplete. incomplete.
that have completely
filled shells
(3 to 8 valence
electrons).
MEMORISE :
Position of hydrogen : Hydrogen is at the head of table 1A and it has one
valence electron much like the other members of group. However hydrogen
is not considered as part of group 1A because chemically it is a non-metal
whereas all other members of group are strong alkali metals. In fact
position of hydrogen in periodic table is unresolved. – Extra Information.
Explain the zig-zag line in the periodic table.
1. On the basis of electronic configuration of elements, the periodic table
is divided into four blocks namely, s-block, p-block, d-block and f-block.
2. The elements of s-block (except hydrogen), d-block and f-block are all
metals.
3. In the p-block, all the three types of elements, i.e metals, non-metals
and metalloids are present.
4. A zig-zag line separates the metals on the left side form the non-metals
on the right side of the periodic table.
5. The bordering elements along the zig-zag line are the metalloids. These
are Antimony (Sb), Germanium (Ge), Boron (B), Silicon (Si),
Arsenic (As), Tellurium (Te), Polonium (Po). They show intermediate
properties and are called as metalloids or semi-metals.
MEMORISE :
The zigzag line on the periodic table, sometimes referred to as the
Hays-McDaniel line, distinguishes between the metals and non metals.
It is also called as stair case.
– Extra Information.
Explain the term periodic properties and explain the following trends of
the periodic table with respect to :
1. Valency 2. Metallic character
*3. Atomic size.
1. Valency :
(a) In the modern periodic table, the elements are arranged in
increasing order of atomic number (Z). Atomic number is related to
the electronic configuration.
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(b) As the atomic number increases, the number of valence electrons
increases. The first element has one electron in the outermost
shell while the last element in a period has either completed duplet
or completed octet. So, valency varies gradually across a period.
(c) The group number indicates number of valence electrons. So,
elements in a group have the same number of valence electrons.
Therefore, down the group, valency remains the same.

MEMORISE :
Valency is the combining power of an atom or it is the number of electrons
donated or accepted or shared to attain the nearest inert gas configuration.
– Extra Information.
2. Metallic character : The tendency of an element to lose electrons and
form positively charged ions (cations) is called as metallic character.
Non-metallic character : The tendency of an element to gain electrons and
form negatively charged ions (anions) is called as non-metallic character.
As we go down a group, the atomic size increases. So, the nuclear
attraction on the outermost electron decreases and electrons can be
lost more easily. So, down the group, metallic character increases and
non-metallic character decreases.
Across a period, the nuclear charge increases. Consequently, the
attraction on the valence electrons increases and so more energy is
required to lose the electron. So, across a period, metallic character
decreases and non-metallic character increases.

MEMORISE :
Factors affecting metallic and non-metallic character:
1. Atomic size : The greater the atomic size, the electron can be lost
more easily.
2. The energy required to remove the electron is also less. So, lower the
energy, more is the metallic nature of element.
The energy required to remove a loosely bound electron from the
outermost shell of an isolated gaseous atom is called as ionization
potential.
– Extra Information.
3. Atomic size is defined as the distance from the centre of the nucleus to
the outermost shell of an atom. Across a period, the number of shells
remain the same. So, as nuclear charge increases due to increase in
atomic number, the attraction to the outermost electron increases and
pulls the electrons inwards. So, atomic size decreases across a period.
Li
Be
B
C
N
O
F
Down the group, the number of shells increases. So, although the
nuclear charge increases, the attraction to the outermost electron
decreases. So, down the group, the atomic size increases.
F
Cl
Br
I
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12.
Ans.
13.
Ans.
14.
Ans.
15.
Ans.
*16.
Ans.
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What are the causes of the periodicity of elements?
Periodicity in properties of elements are seen because
1. In the modern periodic table, the elements are arranged according to
their increasing atomic numbers.
2. The physical and chemical properties of elements largely depends upon
the outermost electronic configuration, that is the valence electrons.
3. Hence the outermost electronic configuration for each group remain
the same.
4. Since periodicity in properties of elements are seen due to same
outermost electronic configuration.
State Dobereiners law of triads. P, Q, R are elements of a Dobereiner's
triad. If atomic mass of P is 35.5 and R is 127, what would be the
atomic mass of Q?
Dobereiner's law of triads : He arranged chemically similar elements in
the increasing order of their atomic masses in a group of three which is
known as "Dobereiner's Triads". In a Triad, atomic mass of middle element
is approximately equal to the mean of atomic masses of other two elements
in that Triad.
Atomic weight of P  Atomic weight of R
Atomic weight of Q =
2
35.5  127
162.5
=
=
= 81.25.
2
2
Why is atomic number said to be a fundamental property?
1. Atomic number (Z) is defined as the number of protons or electrons in
an atom.
2. It is more important than atomic mass because atomic number tells us
about the chemical properties of an element.
3. It tell us about the number of valance electrons and valance electrons
decide the chemical properties. It tell us about the nature of an element
and its electronic configuration.
Explain the features of Mendeleev's periodic table.
Main features of Mendeleev’s periodic table are as follows :
1. The horizontal rows in the periodic table are called periods. There are
seven periods. These are numbered from 1 to 7.
2. Properties of elements in a particular period show regular gradation
from left to right.
3. Vertical columns in the periodic table are called groups. There are
eight groups numbered from I to VIII. Groups I to VII are further divided
into A and B subgroups.
Among the first twenty elements, which are metals, non-metals and
metalloids?
Metals : Lithium, Beryllium, Sodium, Magnesium, Aluminium, Potassium,
Calcium.
Non-metals : Hydrogen, Helium (zero group), Carbon, Nitrogen, Oxygen,
Fluorine, Neon(zero group), Phosphorus, Sulphur, Chlorine, Argon (Zero
group).
Metalloids : Boron, Silicon.
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ACTIVITY BASED QUESTIONS
ACTIVITY : 1.3
Q.
Ans.
Q.
Ans.
Q.
Ans.
Consider isotopes of oxygen 16O and 18O. Would you able to place them
in Mendeleev’s periodic table ?
No, we cannot place 16O and 18O in Mendeleev’s periodic table.
Find resemblance between hydrogen and alkali metals by writing
compounds of both with chlorine, sulphur and oxygen.
Alkali metals are : Li, Na, K, Rb, Cs, Fr.
H2O
Na 2 O
H2S
Na 2 S
HCl
NaCl
Hydrogen and alkali metals form similar formulae with, oxygen, sulphur
and chlorine.
Find out a pair of elements from the periodic table where higher mass
element is placed before lower mass element.
Co
Ni
;
Te
I
(58.9)
(58.7)
(12.8)
(12.7)
ACTIVITY : 1.4
Q.
Ans.
Q.
Ans.
Q.
Ans.
How are isotopes of different elements placed in the Modern Periodic
Table ?
Isotopes have same atomic number, but different atomic mass. In modern
periodic table the elements are arranged in increasing order of atomic
numbers. So, all isotopes of an element can be given the same position.
What should be the position of Hydrogen in the Modern Periodic Table ?
Why?
In modern periodic table, elements are arranged in the increasing order of
their atomic number, so a Hydrogen must be the first element in modern
periodic table, because it has the least atomic number.
Write the name, symbol and electronic configuration of first ten
elements.
Elements
Symbol
Atomic number
E.C.
Hydrogen
Helium
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Flurorine
Neon
H
He
Li
Be
B
C
N
O
F
Ne
1
2
3
4
5
6
7
8
9
10
(1)
(2)
(2,1)
(2,2)
(2,3)
(2,4)
(2,5)
(2,6)
(2,7)
(2, 8)
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ACTIVITY : 1.5
Q.
Ans.
Write the electronic configuration of elements from atomic number 11
to 18.
Elements
Sodium
Magnesium
Aluminium
Silicon
Phosphorous
Sulphur
Chlorine
Argon
Symbol
Atomic No.
Na
Mg
Al
Si
P
S
Cl
Ar
11
12
13
14
15
16
17
18
E.C.
(2,
(2,
(2,
(2,
(2,
(2,
(2,
(2,
8,
8,
8,
8,
8,
8,
8,
8,
1)
2)
3)
4)
5)
6)
7)
8)
Q.
Ans.
Q.
What similarities do you find in their electronic configuration ?
They all have three shells in their atoms.
How many valence electrons are present in sodium, aluminium and
chlorine ?
Ans.
Elements
No. of valence electrons
Sodium
Aluminium
Chlorine
Q.
Ans.
Write the electronic configuration of
1. Mg and Ca.
2. F and Cl.
Elements
Magnesium
Calcium
Flurine
Chlorine
Q.
Ans.
Q.
Ans.
18
1
3
7
Atomic number
Electronic configuration
12
20
9
17
(2, 8, 2)
(2, 8, 8, 2)
(2, 7)
(2, 8, 7)
Do these elements contain same number of valence electrons ?
Yes, they contain same number of valence electrons.
Write the electronic configuration of B, O, Na, Al, S and K.
Symbol
Atomic number
Electronic configuration
B
O
Na
Al
S
K
3
8
11
13
16
19
(2, 1)
(2, 6)
(2, 8, 1)
(2, 8, 3)
(2, 8, 6)
(2, 8, 8, 1)
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ACTIVITY : 1.6
Q.
Ans.
Classify elements from atomic number 11 to 18 as metals or non metals
depending on the electrons present in the outermost shell.
Symbol
Na
Mg
Al
Si
P
S
Cl
Ar
Atomic number
11
12
13
14
15
16
17
18
Electronic configuration
(2, 8, 1)
(2, 8, 2)
Metals
(2, 8, 3)
(2, 8, 4)
(2, 8, 5)
(2, 8, 6)
Non metals
(2, 8, 7)
(2, 8, 8)
ACTIVITY : 1.8
Q.
Ans.
Q.
Ans.
How do you calculate valency of an element from its electronic
configuration ?
Electronic configuration tells about the valence electrons. If the number of
valence electrons are 1, 2 or 3, then such elements will donate 1, 2 or 3
electrons their valency is also 1, 2 or 3 respectively.
If an element has 4 valence electrons, it shares all 4 electrons, so the
valency is 4.
If the number of valence electrons are 5, 6 or 7 electrons, then such elements
accept 3, 2 or 1 electrons respectively. Thus their valency is 3, 2 or 1.
When the valence electrons are 2 (for only one shell) or 8 then it is a
stable electronic configuration. Thus, the valency is zero.
What is the valency of elements with atomic number 8, 14, 17 and 20 ?
Atomic number
Electronic configuration
Valency
8
14
17
20
(2, 6)
(2, 8, 4)
(2, 8, 7)
(2, 8, 8, 2)
2
4
1
2
ACTIVITY : 1.9
Q.
Ans.
Atomic radii of elements of third period are given below.
Period 3 elements
S
Atomic radius (pm) 127
Na
Cl
P
Mg
Si
Al
190
99
128
160
132
143
1. Arrange them in the decreasing order of their atomic radii.
Ans.
Ans.
Ans.
Na
Mg
Al
Si
P
S
Cl
190
160
143
132
128
127
99
2. Are they arranged as in the periodic table ?
Yes.
3. Which are the atoms of highest and lowest atomic radii ?
Highest atomic radius = Na (190 pm)
Lowest atomic radius = Cl (99 pm)
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Q.
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4. What trend is observed in the atomic radii in a period from left to
right ?
Atomic radii decreases from left to right in a period.
Atomic radii of group 17 are given below :
Group 17 elements
I
F
Cl
Br
Atomic radius (pm)
133
72
99
114
1. Arrange them in the increasing order of thier atomic radii.
Ans.
Ans.
Ans.
Ans.
20
F
(72)
Cl
(99)
Br
(114)
I
(133)
2. Are they arranged as in the periodic table ?
Yes.
3. Which are the atoms of highest and lowest atomic radii ?
Highest atomic radii = Iodine (133).
Lowest atomic radii = Fluorine (72).
4. What trend is observed in the atomic radii down the group ?
Atomic radii increases down the group.
Atomic
number
Elements
configuration
Symbol
electrons
Electronic
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Hydrogen
Helium
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Flurine
Neon
Sodium
Magmesium
Aluminium
Silicon
Phosphorus
Sulphur
Chlorine
Argon
Potassium
Calcium
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
1
2
2,1
2, 2
2, 3
2, 4
2, 5
2, 6
2, 7
2, 8
2, 8, 1
2, 8, 2
2, 8, 3
2, 8, 4
2, 8, 5
2, 8, 6
2, 8, 7
2, 8, 8
2, 8, 8, 1
2, 8, 8, 2
Valence Valency
electrons
1
2
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
2
1
0
1
2
3
4
3
2
1
0
1
2
3
4
3
2
1
0
1
2
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HOTS QUESTIONS & ANSWERS
1.
Ans.
2.
Ans.
3.
Ans.
4.
Ans.
5.
Ans.
22
X, Y, Z are the elements of Dobereiner’s triad. If the atomic mass of X
is 7 and that of Z is 39. What should be the atomic mass of y ?
In the triads, the atomic mass of the middle element is approximately the
mean of the atomic masses of the other two elements. Hence,
7  39
46
=
= 23
Y=
2
2
 Atomic mass of Y = 23.
Read the following descriptions and decide whether the element belongs
to metal or non-metal. Justify your answer.
(a) Element ‘X’ has atomic number 11.
(b) Element ‘Y’ has atomic number 16.
(a) Element X is a metal. Its electronic configuration is (2, 8, 1). The element
has a tendency to loose one electron making it metallic in nature.
(b) Element Y is a non-metal. Its configuration is (2, 8, 6). Therefore there
is a tendency to accept the electrons rather than to loose, making it
non-metallic in nature.
The atomic number of an element X is 17. Predict :
(a) Its valency.
(b) Whether it is a metal or non-metal.
(c) Its relatively atomic size with respect to other members in the
group.
(a) Valency is 1 (Electronic configuration 2, 8, 7).
(b) It is a non-metal. (Tendency to accept electron as the outermost orbit
has 7 electrons.)
(c) Atomic size is relatively smaller with respect to the other members in
a group. (Electrons in the outer shell are not released easily.)
Inert gases are neutral because ...................... .
(a) they do not combine with other elements under normal
circumstances.
(b) the number of electrons in their outer shell is 8.
(c) they do react with other elements, but the process remains
incomplete.
The number of electrons in their outer shell is 8.
Identify the main characteristic of the elements given below from their
description.
(a) Element P is in O group element.
(b) Element Q is in group 1 with atomic number 19.
(c) Element R is a d-block element and its two outermost shells are
incomplete.
(d) Element S is f-block element.
(e) Element T has two different atomic masses.
(a) It is an inert gas.
(b) It is a metal with one electron in the outer shell.
(c) It is called a transition element and is metallic in nature.
(d) They are either lanthanides or actinides with three incomplete
outermost shells and metallic in nature.
(e) They are isotopes of element T.
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GLOSSARY
Alkali metals
:
Alkali
Alkaline earth metal
:
:
Atomic number
:
Atomic mass number
:
Electronic configuration
Electrostatic force of
attraction
Element
:
:
Isotopes
:
Halogens
:
Noble gases
:
Metals
Non-metals
Metalloids
:
:
:
Valency
:
Periodicity
Cations
Anions
Nuclear charge
Ionisation potential
:
:
:
:
:
:
Elements of group IA are most reactive. They are
called alkali metals as they form alkali on
reaction with water.
Soluble bases which produce OH– ions in water.
Elements belonging to group II A. They form alkali
on reaction with water and their oxides are found
in earth. So, they are called alkaline earth
metals.
Number of protons (in nucleus) or extra nuclear
electrons.
the total number of protons and neutrons in the
nucleus of an atom.
Distribution of electrons shell wise.
A strong force of attraction between particles of
opposite charges.
A substance that cannot be divided further into
simpler substances.
They are different atoms of same element having
same atomic number but different mass number
Elements belonging to group VII A. The elements
of this group form salts and are called as salt
formers.
Elements of group 18 also called as rare gases/
noble gases.
Electron donors, hence electro-positive.
Electron acceptors, hence electro-negative.
Elements showing properties of both metals and
non-metals.
Combining power of an atom or number of
electrons donated/accepted/shared to attain the
nearest inert gas configuration.
A phenomenon occurring at regular intervals.
Positively charged ions.
Negatively charged ions.
The positive charge on the nucleus of an atom.
The energy required to remove a loosely bound
electron from the outermost shell of an isolated
gaseous atom is called as ionization potential.
AMAZING FACTS
 The noble gas Xenon lasers can cut through materials that are so tough
even diamond tipped blades will not cut.
 Astatine is the rarest element on Earth (approx 28g in the Earth's entire
crust.)

SCHOOL SECTION
23
S.S.C.
Marks : 20
CHAPTER 1 : SCHOOL OF ELEMENTS
Duration : 1 hr.
SCIENCE
Q.I
1.
2.
3.
4.
Q.I
1.
2.
3.
4.
[A] Fill in the blanks :
Noble gases have valency as ....................... .
Group 1A is called ....................... .
The formula of chloride of metal is MCl 2, the metal M belongs to
....................... group.
The element eka-Aluminium is called as ....................... .
2
[B] Match the columns :
Column A
Alkali metals
Alkaline earth metals
Halogens
Noble gases
2
(a)
(b)
(c)
(d)
Column B
II A group
I A group
zero group
VII A group
Q.I
1.
2.
[C] State whether True or False :
Eka boron is known as Germanium.
Isotopes have same atomic masses.
1
Q.II
1.
2.
3.
[A] Give scientific reasons : (Any Two)
Atomic size decreases across a period.
The family of noble gases is called as zero group.
Chlorine is placed in VII A group in the modern periodic table.
4
Q.II
1.
2.
[B] State the following :
Dobereiner’s triads law.
Modern periodic law.
4
Q.III
1.
2.
3.
[A] Answer the following in details : (Any One)
Explain the demerits of Mendeleevs periodic table.
Explain the four blocks of periodic table.
Explain the zig-zag line in the periodic table.
4
Q.III
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[B] Answer in one sentence each : (Any Three)
Name elements having single electron in their outermost shell.
Name halogens, to which group do they belong ?
Give two names of Member of 3rd period.
What are lanthanides ?
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