5
Q 1. Give formula for the
following: (i) Bromide of element X of second group. (ii) Oxide of
element Y of third group. (iii) Chloride of element Z of fourth group.
Solution
(i) Bromide of element X
of second group = XBr2 (ii) Oxide of element Y of third
group = Y2O3 (iii) Chloride of element Z of
fourth group = ZCl4
Q 2. (a) Did Dobereiner's
triads also exist in the columns of Newlands law of octaves? Explain your
answer. (b) What were the limitations of Dobereiner's classification of
elements?
Solution
(a) Yes, Dobereiners
triads also exist in the columns of Newlands Octaves. Consider the
elements lithium (Li), sodium (Na) and potassium (K) which are present in the
second column of Newlands classification of elements. Now, if we start with lithium
as the 1st element, then the 8th element
from it is sodium and according to Newlands law of octaves, the properties of
the 8th element, sodium should be similar to that of the 1st element,
lithium. Again, if we take sodium as the 1st element, then
the 8th element from it is potassium and according to Newlands
law of octaves, the properties of the 8th element, potassium
should be similar to that of the 1st element, sodium. This
means that according to Newlands law of octaves, the elements
lithium, sodium and potassium should have
similar chemical properties. We also know that lithium, sodium and
potassium form a Dobereiner's triad having similar chemical properties.
From this, we conclude that Dobereiners triads also exist in the columns of Newlands
Octaves. (b) The main limitation of Dobereiner's classification of
elements was that it failed to arrange all the then known elements in the form
of triads of elements having similar chemical properties. Dobereiner could
identify only three triads from the elements known at that time. So, his
classification of elements was not much successful. Another limitation was that
Dobereiner failed to explain the relation between atomic masses of elements and
their chemical properties.
Q 3. Hydrogen can be placed
in group 1 and group 7 of periodic table. Why?
Solution
Hydrogen forms both
positive ions like alkali metals (group 1) and negative ions like halogens
(group 7). Thus, it can be placed in both group 1 and group 7. In
Mendeleev"s Periodic table, the position of hydrogen was not clear.
Q 4. Give reasons for the
need of classification of elements.
Solution
The continuous discovery
of new elements and their compounds led to confusions. It became difficult to
study, remember and recall the properties of all the elements. Thus, the need
to classify and place them in certain groups was felt.
Q 5. Given below are some
elements of the modern periodic table : 4Be, 9Fe, 14Si, 19K, 20Ca
(i) Select the element that has one electron in the outermost shell and write
its electronic configuration. (ii) Select two elements that belong to the same
group. Give reasons for your answer. (iii) Select two elements that belong to
the same period. Which one of the two has bigger atomic size?
Solution
(i) 19K
has one electron in the outermost shell and its electronic configuration is 2,
8, 8, 1. (ii) 4Be and 20Ca belongs to same
group i.e. Group 2. Electronic configuration: 4Be - 2, 2 20Ca
- 2, 8, 8, 2 4Be and 20Ca have same number of
valence electrons in outermost shell i.e. 2 so they belong to same group.
(iii) 9F and 4Be belongs to the same period
i.e. period 2. Electronic configuration: 9F - 2, 7 4Be
- 2, 2 4Be has bigger atomic size than 9F
because the atomic radius decreases as we move from left to right due to an
increase in nuclear charge which tends to pull the electrons closer to the
nucleus and hence size of atom reduces.
Q 6. How does atomic size
vary in a group from top to bottom and in a period from left to right? Explain.
Solution
(i) Atomic size increases
as we move from top to bottom because each time one new shell is added and
electrons in the outermost shell move away from the nucleus. (ii) Atomic size
decreases across the period, as electrons are added in the same shell,
effective nuclear charge increases and thus, atomic size decreases.
Q 7. An element X has mass
number 35 and number of neutrons 18 (a) Write the atomic number of X. (b) Give
electronic configuration of X. (c) To which group and period does it belong?
Solution
(a) Atomic number = 35 -
18 = 17 (b) 2, 8, 7 (c) 17thgroup and 3rdperiod
Q 8. Name two elements
whose properties were correctly predicted by Mendeleev. Mention their present
day name.
Solution
Two elements whose
properties were correctly predicted by Mendeleev are: Eka-aluminium - Gallium
Eka-silicon - Germanium
Q 9. Lithium, sodium and
potassium are all elements that react with water to liberate hydrogen gas. Is
there any similarity in the atoms of these elements? Explain.
Solution
Yes, there is a
similarity in the atoms of lithium, sodium and potassium
elements.
All these elements have a similar electronic configuration having one
electron each in their valence shells. The electronic configurations of
lithium, sodium and potassium are given below: Lithium: 2, 1
Sodium: 2, 8, 1 Potassium: 2, 8, 8,
1
Q 10. Group → Period
↓ 1 2 13 14 15 16 17 18 I a
j II b e g h k III c
f i l IV d
Consider the table given above and answer the following questions: (i)
Name the most reactive metal. (ii) How many shells does 'd' have? (iii) Name
the element (s) having valency 2. (iv) How many valence electrons does 'j'
have? (v) Which is more non - metallic, 'h' or 'i'? (vi) The atom of which
element is bigger in size, 'e' or 'h'?
Solution
(i) Element 'b' is the
most reactive. (ii) 'd' has 4 shells. (iii) Elements 'e' and 'g' have valency
2. (iv) 'j' has zero valence electrons. (v) Element 'h' is more non-metallic.
(vi) Element 'e' is bigger in size.
Q 11. Account for the
following: (a) How is metallic character of an element expressed? (b) Noble
gases are placed in a separate group.
Solution
(a) The metallic
character of an element is expressed in terms of electron releasing tendency of
its atom. As a result, a positive ion is formed. Metals are hence
electropositive in nature. (b) Noble gases like helium, neon and argon were
discovered very late because they are very inert and present in extremely low
concentrations in our atmosphere. Hence, they were placed in a new separate
group without disturbing the existing order.
Q 12. Explain why the
number of elements in the third period is 8.
Solution
According to the 2n2 rule,
the maximum number of electrons in the third period = 2 x (3)2 =
18. But, the last shell cannot accommodate more than 8 electrons so, the number
of electrons in third period is 8. Hence, the number of elements is also 8.
Q 13. An element 'M' has
atomic number 12. (a) Write its electronic configuration. (b) State the group
to which 'M' belongs. (c) Is 'M' a metal or a non-metal? (d) Write the formula
of its oxide.
Solution
(a) The electronic
configuration of M is 2, 8, 2. (b) M belongs to 2nd group. (c)
M is a metal. (d) MO
Q 14. (a) How does the
tendency of an element to lose electrons change in a group? (b) How does this
tendency change in a period?
Solution
(a) On moving down a
group, the number of shells increases at each succeeding element. As a result,
the valence electrons lies farther away from the nucleus. Therefore, the force
of attraction between nucleus and the electrons decreases and tendency to
lose electrons increases down the group. (b) As we move left to right in a
period, number of shells remain same but number of valence electrons increases.
As a result, effective nuclear charge increases and the tendency to lose
electrons decreases.
Q 15. Account for the
following: (a) Elements of group 18 are called zero valent. (b) Elements in a
group of periodic table have similar chemical properties.
Solution
(a) Elements of group 18
have their outermost shell completely filled. So, they neither gain electron
nor lose any electron and hence have zero valency. Thus, they are called zero
valent. (b) Chemical properties of elements are determined by the number of
valence electrons. Since in a group, all the elements have same number of
valence electrons, thus they exhibit similar chemical properties.
Q 16. a. How does valency
vary in a group on going from top to bottom? b. How does the number of valence
electrons vary in a period on going from left to right and from top to bottom
in a group?
Solution
(a) Valency remains same
on moving from top to bottom in a particular group. This is because the
outermost electronic configuration of all the elements in a group remains same.
(b) On moving from left to right in a period, the number of valence electrons
in elements increases from 1 to 8. The elements in a period have consecutive
atomic numbers. On moving down a group in the periodic table, the number of
valence electrons in the elements remains the same.
Q 17. (a) The atomic number
has been chosen as the basis for classifying elements. Why? (b) By considering
their positions in the Periodic Table, which one of the following elements
would you expect to have maximum metallic character? Na, Mg, Al.
Solution
(a) Atomic number decides
the electronic configuration of the elements which is responsible for the
chemical properties of the elements. (b) Na will have maximum metallic
character since metallic character decreases on going from left to right in a
period.
Q 18. Elements A, B, C, D,
E have following electronic configurations: A: 2, 3 B: 2, 8, 3 C: 2, 8, 5 D: 2,
8, 7 E: 2, 8, 8, 2 (i) Which of these belong to same group? (ii) Which of these
belong to same period?
Solution
(i) Element A and B
belong to same group since they have the same number of valence electrons. (ii)
Element B, C and D belong to same period since they have same number of shells
in their atoms.
Q 19. Name the element
which has twice as many electrons in its second shell as in its first shell. Write
its electronic configuration also.
Solution
Carbon has twice as many
electrons in its second shell as in its first shell. K-shell- 2 electrons
L-shell- 4 electrons
Q 20. Consider the
following elements: 20Ca, 8O, 18Ar, 16S, 4Be, 2He
Which of the above elements would you expect to be: (i) Very stable (ii) In
group 2 of the periodic table (iii) In group 16 of the periodic table. Give
reasons.
Solution
(i) 18Ar
and 2He (Noble gases) are very stable because of completely
filled shells. (ii) 20Ca and 4Be belong to
group 2 since they have two shells only. (iii) 8O and 16S
belong to group 16 since they both have 6 valence electrons in their outermost
shell.
Q 21. (a) Why physical
properties and chemical properties of an element are periodic function of their
atomic numbers? (b) How does Modern Periodic Law justifies one position for
isotopes?
Solution
(a) Physical and chemical
properties of elements are determined by their atomic numbers which is equal to
the number of electrons. By using the number of electrons, we can find
out the number of valence electrons and hence the physical and chemical
properties. (b) According to Modern Periodic Law, elements are arranged
in the Modern Periodic Table in the increasing order of their atomic numbers.
Isotopes have the same atomic number and different atomic mass. So, though they
have different atomic masses still they are given the same position in the
Modern Periodic Table.
Q 22. Three elements A, B
and C have atomic numbers 7, 8 and 9 respectively. (a) What would be their
positions in the modern periodic table (Mention group and period both)? (b)
Arrange A, B and C in the decreasing order of their size. (c) Which one of the
three elements is most reactive and why?
Solution
(a) Position of A = 15th group,
2nd period Position of B = 16th group, 2nd period
Position of C = 17th group, 2nd period (b) A
> B > C since atomic size decreases as we move from left to right along a
period. (c) C is most reactive as it has smallest size, so it can easily gain
electrons.
Q 23. Arrange the following
elements in the decreasing order of metallic character: (i) Si, Mg, Na, P (ii)
B, Al, Mg, K
Solution
On going down a group,
metallic character of elements increases. On moving from left to right
in a period, metallic character of elements decreases. (i) Na > Mg
> Si > P (ii) K > Mg > Al > B
Q 24. A part of modern
periodic table is given below. Answer the following questions based on this
table. H He Li Be B C N O F Ne Na Mg
Al Si P S Cl Ar (a) Why do H, Li and Na show similar properties? (b)
Atomic size of Mg is bigger than Be. Why? (c) Why are He, Ne and Ar called
noble gases? (d) Write a common name of the family to which F and Cl belong.
(e) Write the trend of non - metallic character in the horizontal row from Na
to Cl. (f) How does the atomic size vary as we move from Li to F in the second
period of the periodic table?
Solution
(a) H, Li and Na show
similar properties because they have one electron in their valence shell and
belong to same group. (b) Atomic size of Mg is bigger than Be because Mg
consists of three shells whereas Be consist of 2 shells. This increases the
distance between outermost electrons and the nucleus. (c) He, Ne and Ar are
called noble gases because their outermost shell is complete and their
combining capacity is zero i.e. they are least or less reactive. (d) Halogen
family (e) Non-metallic character increases from Na to Cl. (f) Atomic size
decreases as we move from Li to F in the second period of the periodic table.
Q 25. (a) What were the achievements
of Mendeleev's periodic table? (b) What were the basic properties of
elements considered by Mendeleev for their classification in the periodic
table?
Solution
(a) The achievements of
Mendeleev's periodic table were: Mendeleev kept some blank spaces in the
periodic table for the elements that were yet to be discovered. Predicted
element Actual element discovered later Eka-boron Scandium Eka-aluminium
Gallium Eka-silicon Germanium He also predicted properties of some elements
even before their discovery and were later found to be correct. Property
Eka-aluminium Gallium Atomic mass 68 69.7 Formula of oxide
E2O3
Ga2O3 Formula of chloride
ECl3
GaCl3 Mendeleev’s periodic table could accommodate noble
gases when they were discovered. (b) Mendeleev took the formulae of the
oxides and hydrides formed by the elements as the basic properties of elements
for their classification in the periodic table.
Q 26. Write one property of
hydrogen which makes it resemble with (a) Alkali metals (b) Halogens.
Solution
Resemblance with alkali
metals: Hydrogen has the same outermost electronic configuration as that of
alkali metals. Resemblance with halogens: Hydrogen exists as diatomic molecule
as halogens.
Q 27. Define atomic radius
of an element. How does it vary along the period and group?
Solution
The distance between the
centre of the nucleus and the outermost orbit of the atom is called the atomic
radius of the atom of an element. Variation along the period: - The atomic
radius decreases as we move from left to right along the period. This is due to
an increase in nuclear charge which tends to pull the electrons closer to the
nucleus and reduces the size of the atom. Variation along the group: - The
atomic radius increases down the group. This is because new shells are being
added as we go down the group. This increases the distance between the
outermost electrons and the nucleus so the atomic size increases inspite
of the increase in nuclear charge.
Q 28. How can the valency
of an element be determined if its electronic configuration is known? What will
be the valency of an element of atomic number 9?
Solution
An element's valency is
determined by the number of electrons in its outer shell. Hence, the number of
valence electrons obtained from the electronic configuration of the element
gives the valency i.e. the number of electrons lost, gained or shared by the
element to attain the noble gas configuration. The valency of an element of
atomic number 9 would be 1 since the number of valence electrons in its
outermost shell is 7 so it needs only one electron to attain the noble gas
configuration.
Q 29. (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? If yes, write the similarity.
(b) Helium is a non-reactive gas and neon is a gas of extremely low reactivity.
What, if anything, do their atoms have in common?
Solution
(a) Yes, the atoms of all
the three elements lithium, sodium and potassium have one electron each 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.
Q 30. Which physical and
chemical properties of the elements were used by Mendeleev in creating his
periodic table? List two observations which posed a challenge to Mendeleev's
periodic table.
Solution
The creation of
Mendeleev's periodic table was based upon certain physical and chemical
properties: Physical properties: The atomic masses of the elements were taken
into account and the elements were arranged in order of increasing atomic
masses. This influenced some of their physical properties like melting points,
boiling points, density etc. Chemical properties: The distribution of the
elements into different groups was linked with formation of hydrides by
combining with hydrogen and formation of oxides by combining with oxygen. This
is linked with the valency of the elements. The two main observations which
posed challenge to Mendeleev's periodic table are as follows: (i) Position of
isotopes: Since the isotopes of an element differ in their atomic masses, they
must be assigned separate positions in the periodic table. (ii) Anomalous
positions of some elements: In the Mendeleev's periodic table, certain elements
with higher atomic masses precede or placed before the elements with lower
atomic masses. For example: the elements Argon (At. mass = 39.9) is placed
before the element Potassium (At. Mass = 39.1).
Q 31. An element X has
atomic number 19. (a) Write its electronic configuration. (b) To which group of
the modern periodic table does it belong? (c) State the nature of the compound
formed by element X with chlorine. (d) Write the valency of element X.
Solution
(a) Electronic
configuration of X = 2,8,8,1 (b) Group1 (c) 'X' will form ionic (electrovalent)
compound with chlorine. (d) Valency of X= +1
Q 32. An element X (atomic
number 17) reacts with an element Y (atomic number 20) to form a divalent
halide. (a) What will be the formula of the halide formed by X and Y? (b) What
is the position of elements X and Y in periodic table? (c) What will be the
nature of oxide of element Y. Identify the nature of bonding in the compound
formed.
Solution
X - non-metal Y - metal
(a) Since it is a divalent halide, so formula of halide is YX2 (b)
X: Position- Group 17, period - 3 Y: Position -
Group2, period - 4 (c) Basic oxide - YO, ionic bond will be formed between Y
(metal) and oxygen (non-metal).
Q 33. Properties of some
elements are given below. Identify in each case the element in the periodic
table to which it belongs. (a) A soft metal stored under kerosene. (b) An
element with variable valency stored under water. (c) An element which is tetra
valent and forms the basis of organic chemistry. (d) An element which is an
inert gas with atomic number 2. (e) A metal which burns with brilliant light
when ignited. (f) An element which is yellow solid at room temperature that
shows catenation and allotropy.
Solution
(a) Sodium (b) Phosphorus
(c) Carbon (d) Helium (e) Magnesium (f) Sulphur
Q 34. Ria and Rama are
students of Class-X. Ria is very much organized and maintained. All teachers
love her. She earns a great respect in the class where as Rama is un-organised
and always faces a lot of problems in handling life situations. (i) In your
opinion how does organization skill help in daily life? (ii) How can you relate
the above fact with the chapter classification of elements. (iii) How
classification of elements help us in studying them properly?
Solution
(i) Organisation makes
our life simple, easy and systematic. (ii) As classification and organization
help us in our daily life in the same way classification of elements has made
the study of elements easier and simple. (iii) We can know about the properties
of elements because of this classification. Associated Value: The learner will
be motivated to practice an organized life with discipline.
Q 35. (a)State Modern
periodic law. (b)How, (i) tendency to lose electron, (ii) Valency
of the elements varies along the period from left to right? Give reasons to
justify your answers.
Solution
(a)Modern periodic law
states that "properties of elements are periodic functions of their atomic
numbers". (b) (i) Tendency to lose electrons decreases because the
effective charge acting on the valence shell electron increases. (ii) Valency increase
from 1 to 4 and then decrease to zero. Group No. 1, 2, 13, 14, 15, 16, 17, 18
valency 1, 2, 3, 4, 3,
2, 1, 0
Q 36. In the modern
periodic table, the element Calcium (atomic number = 20) is surrounded by
elements with atomic number 12, 19, 21 and 38. Which of these elements has
physical and chemical properties resembling those of Calcium and why?
Solution
Ca: Electronic
configuration is 2, 8, 8, 2 The physical and chemical properties of
elements with atomic number 12 and 38 will resemble with calcium. This is
because they all belong to the second group and all of them have two electrons
in their valence shell.
Q 37. Why are isotopes of
an element having different atomic masses placed at the same position in the
periodic table?
Solution
All isotopes of an
element have same number of protons and hence they have the same atomic number.
Hence, they can be placed at the same position in the periodic table.
Q 38. Where are metals,
non-metals and metalloids located in the periodic table?
Solution
(i) metals - left (ii)
non -metals - right (iii) metalloids - middle
Q 39. The position of three
elements A, B and C in the periodic table are shown below: Group 16
Group 17 - - - B - A - 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?
Solution
(a) A - non-metal (b) C -
less reactive than A (c) C - smaller in size than B (d) A - will form anion
Q 40. A part of the
Periodic Table has been shown below. Group → 1 2 13 14 15 16 17
18 Period ↓
1
2 A C E G
3 B D F Answer the following
questions on the basis of position of elements in the above table. (i) Which
element is a noble gas? Give reason. (ii) Which element is most
electronegative? Give reasons. (iii) Write the electronic configuration of (i)
B and (ii) E.
Solution
(i) G - It is a noble gas
and has zero valency. (ii) E is the most electronegative element because of its
smallest atomic size and more electron affinity. (iii) B - 2, 8, 1
E - 2, 7
Comments
Post a Comment