Q & A of Periodic table

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.