PERIODIC TABLE
1.
What is the fundamental property on
which Mendeleev periodic table is based.
ANS: the Mendeleev periodic table is based on increasing
order of atomic masses and similarity in the chemical properties of elements.
2.
State Mendeleev’s periodic law.
ANS: it states that the physical and chemical properties
of the element are periodic function of their atomic mass.
3.
Name a group of elements which was
not known when Mendeleev formulated his periodic table.
ANS: noble gases were not known.
4.
Name three elements for which
Mendeleev left gap in his periodic table.
ANS: Scandium, gallium, and
Germanium
5.
List two anomalous pairs in
Mendeleev’s periodic table.
ANS: i) Cobalt and Nickel
ii) Tellurium and Iodine
6.
State modern periodic table.
ANS: it states that the physical and chemical properties of the element
are periodic function of their atomic number.
7.
Mention the number of groups and
periods in Mendeleev and Modern periodic
table is based.
ANS: In Mendeleev’s periodic
table there were 6 periods and VIII groups.
In Modern periodic table, there are 7 periods and 18 groups.
8.
Mention the number of valence
electrons present in elements belonging to
(a) Group 17 (b)
group 1 (c) group 16 (d) group 13
ANS : (a) 7, (b)
1 (c) 6 (d) 3
9.
How does the atomic radii change
(a) On moving across the period in the periodic table?
(b) On moving down the group.Give reason.
ANS (a) On moving from left to right in a period of the periodic table, the
atomic radius of elements decreases.
REASON:
-
(a)
this is because the number of shell remain same but the nuclear charge
increases.
-
Due
to large positive charge on the nucleus, the electrons are pulled in more close
to the nucleus and the size of the atom decreases.
(b)
On
moving down the group the atomic size increases. Although, the nuclear charge
increases but due to the addition of a new energy shell the valence electrons
become far away from nucleus. Hence, they are not held effectively by the
nucleus thus increasing its size.
10.
How does the non metallic character change
(a) On moving across the period in the periodic table?
(b) On moving down the group in the periodic table?
ANS: (a) On moving from
left to right in a period, the non metallic nature (electronegative character ) increases
REASON:
-The non metals accept
electrons and form negative ions, so non metals are electronegative in nature.
[-The greatest metallic
character is found in the elements on the extreme left side of the period and
the greatest non metallic character is found in elements on the extreme right
side of a period.]
- due to increase in
nuclear charge, (increase in no.of protons)the valence electrons are pulled in
strongly by the nucleus and it becomes easier for the atoms to gain electrons.
(b)On going down the
groups, non metallic character decreases.
REASON:
As we go down in a group of periodic table, one more electron shell is
added to the atom and its size increases.
The attraction of nucleus for the incoming electron decreases, due to
which atom cannot form negative ions and electronegative character decreases.
11.(a) What name
is given to elements belonging to group 1 of the periodic table and why?
(c) Name two elements for this group which can be easily cut with a knife?
ANS:( a) the elements
belonging to group 1 are collectively known as ALKALI METALS. They are so
called as they vigorously react with water to form highly soluble bases called
alkalis. E.g. NaOH, KOH
(c)
sodium
and Potassium
12. Chlorine has 2 isotopes Cl-35 and Cl-37. Would
you place them in different slots because their atomic masses are different or
would you place them in the same position because their chemical properties are
same?
ANS:
according to Mendeleev’s periodic table Cl-35 and Cl-37 should be placed
at different position as the basis of classification is increasing atomic
masses but since their chemical properties are same , they should be placed in
the same position in the periodic table.
13.Account the following:
(a) Element in a group of the periodic table have similar chemical
properties.
(b)
Elements of group 18 are called zero
valent.
ANS: (a) the chemical properties of element are
determined by its valence electron. So, in a group all the elements have same
number of valence electron. Hence they exhibit similar chemical properties.
(c)
Because
group 18 elements have their outermost shell complete.
14. Element M is in the first group of the periodic table. Write the formula
of its oxide.
ANS: M2 O
15. An atom has electronic configuration 2,8,2
(a) What is the atomic number of the element.
(b) What is its valency?
(c) To which element would it chemically similar and why: Be (4), O(8)
ANS:(a) 12 (b)
2
(c)Be, because Be has two electron in
its valence shell as in element mentioned.
16. Why is position of Hydrogen not justified in the periodic table.
ANS:
Hydrogen with atomic number 1 shows properties similar to both alkali metals in
group 1 and also to halogens in group 17. Thus, its position in group 1 with
alkali metals is controversial.
17. An element M forms an oxide M2O3. It belongs to
third period in the modern periodic table. Write the atomic number and valency
of the metal.
ANS: the element is
aluminium with atomic number 13 and valency 3.
18. Atomic number of few
elements given below: (5 mks)
10, 20, 7, 14
(a) Identify these elements
(b) Identify group number of these elements
(c) Period of the elements
(d) Electronic configuration of the elements
(e)
Determine the valency of these
elements.
ANS:
(a) 10 – Neon 20 – Calcium 7- nitrogen
14 – Silicon
(b)Neon – 18 group Calcium -2 group
nitrogen – 3 group Silicon-14 group
19.How does the valency
vary
(a) On moving down the group?
(b) On moving across the period?
ANS: (a) On
moving down the group, the valency of elements remains same in a particular
group.
(c)
On
moving across the period, the valency first increases from 1 to 4 and then
decreases from 4 to zero.
20.Chlorine and bromine
are kept in the same group in the periodic table. Why?
ANS: Cl and Br have been
kept in the same group as they have same number of electrons and thus, show
similar chemical properties.
21.There are 7 electrons
in the outermost ‘L’ shell of an element . Predict the period and group in the
periodic table this element belongs to.
ANS: name of the element
is Fluorine
Group : 17 period: 2
NOTES ( NO WRITTEN WORK)
There are 18 groups of elements divided into
9 main groups. They are
I, II,
III, IV, V, VI, VII, VIII and 0 groups. The groups I to VII has two sub
groups
each called A – sub group and B – sub group. Group VIII has 3
rows of
elements and 0 group has one row of elements.
The A sub group elements are called normal
elements.
The B sub group elements are called
transition elements.
Lanthanides and Actinides are called inner
transition elements.
Group 1 (I A ) elements are called alkali
metals
Group 2 (II A) elements are called alkaline
earth metals.
Group 17 (VII A) elements are called
halogens.
Group 18 (0 group) are called noble gases.
In a group all the elements have the same
number of valence
electrons.
Group I elements have 1 valence electron, Group II elements
have 2
valence electron, Group III elements have 3 valence electrons
etc.
In a period all the elements contain the
same number of shells.
Mendeleev classified elements in the
increasing order of their atomic masses and similarities in their properties.
The formulae of the oxides and hydrides
formed by the elements was also the basis for the classification of the
elements.
Mendeleev’s periodic table has 6 horizontal
rows called periods and 8 vertical rows called groups. The groups 1 to 7 had
two sub groups called A sub group and B sub group. Group 8 had 3 rows of
elements. Elements having similar properties were placed in the same groups.
There are some spaces left vacant in the table to accommodate the elements to
be discovered in future.
Merits of Mendeleev’s periodic table
:-
i) Elements were classified on a more
fundamental basis of their atomic masses and properties.
ii) Spaces were left vacant to accommodate the
elements to be discovered in future.
iii) It
could predict the properties of the elements which helped in the discovery of
new elements.
iv) The
inert gas elements discovered later could be placed in a separate group
without disturbing the table.
Defects of Mendeleev’s periodic table
:-
i) Some elements are not arranged in the
increasing order of their atomic masses. Co is placed before Ni, Te is placed before I etc.
ii) Position of hydrogen is not clear because
it shows properties similar to metals as well as non metals.
iii) The
position of isotopes of elements is not clear.
