classification of elements and periodicity in properties

Question 1

what is the diff in approach b/w the Mendeleev’s periodic law and the modern periodic law

Answer 1

in the classification of elements Mendeleev’s periodic law is based on atomic weights whereas modern periodic law is based on electronic configuration.

Question 2

why do elements in the same group have similar physical and chem properties?

Answer 2

the properties of elements depend on valency shell electrons. the elements belonging to the same group possess the same outer shell configuration therefore, they have similar physical and chem properties.

Question 3

what are representative elements? give their valence shell configuration.

Answer 3

the elements of the S and P block except the zero group are called representative elements. their general electronic configuration is ns^1-2 np^1-5.

Question 4

justify the position of f-block elements in the periodic table

Answer 4

the 4f and 5f series are actually placed in the 6th and 7th periods of the lllB-group. the main reason for this is similarity in chem properties. these are placed at the bottom of the periodic table in two series lanthanides and actinides to maintain symmetry of the table.

Question 5

periodic law def

Answer 5

the physical and chemical properties of the elements are the periodic functions of their atomic weights.

Question 6

characteristics of Mendeleev’s periodic table

Answer 6

1. by considering increasing order of atomic weights and similarity in chem properties he arranged the elements in horizontal rows and vertical columns.
2. the vertical columns are called groups. there are nine groups from 0 to Vlll.
3. the groups I to Vll are further divided into A and B subgroups.
4. the noble gases or inert gases are placed in the 0 group.
5. Vlll group consists of three triads known as Mendeleev’s triads.
   Fe, Co, Ni
   Ru, Rh, Pd
   Os, Ir, Pt
6. the horizontal rows are called periods. there are 7 periods.
7. the first 3 periods are called short periods. the elements with low atomic weights are placed in these short periods.
8. the fourth, fifth and sixth periods are long periods.

Question 7

advantages of Mendeleev’s table

Answer 7

1. systematic study of elements
2. discovery of new elements: Mendeleev predicted the names and properties of some elements which were not known at that time and left gaps for them. this helped the chemists to discover these elements.
   Mendeleev’s name - new name
   Eka aluminium - Gallium
   Eka Silicon - Germanium
   Eka boron - Scandium
3. correction of atomic weights: Mendeleev corrected the atomic weights of some elements like Be, In, Au, Pt, U, etc.

Question 8

drawbacks of Mendeleev’s table

Answer 8

1. position of hydrogen: hydrogen is placed in the lA group even though it resembles with alkali metals and halogens in properties.
2. anomalous pairs of elements: the pair of elements which violates the increasing order of atomic weights are called anomalous pairs.
   Ar-K, Co-Ni, Te-I, Th-Pa
3. position of isotopes: this table does not describe the position of isotopes.

Question 9

from a study of the properties of neighbouring elements, the properties of an unknown element can be predicted justify with example

Answer 9

from the study of neighbouring elements and their compounds, Mendeleev was able to predict the properties of certain unknown elements. these predictions were found to be very accurate. in later days the predictions actually matched with the predictions.
ex:
Eka aluminium - gallium
Eka silicon - germanium
Eka boron - scandium

Question 10

s block elements

Answer 10

1. the elements in which differentiating electron enters into s-orbital are called s block elements.
2. the general electronic configuration is ns^1-2.
3. these are placed on the left side of the periodic table.
4. it contains 2 groups: alkali metals and alkaline earth metals (lA and llA).
5. they are all electropositive soft metals.

Question 11

p block elements

Answer 11

1. the elements in which differentiating electron enters into p-orbital are called p block elements.
2. the general electronic configuration is ns^2np^1-6.
3. these are placed on the right side of the periodic table.
4. it contains 6 groups from 13 to 18.
5. it contains metal, non-metals and metalloids.

Question 12

d block elements

Answer 12

1. the elements in which differentiating electron enters into d-orbital are called d block elements.
2. the general electronic configuration is
   (n-1)d^1-10ns^1-2
3. they are placed in b/w s and p block elements.
4. the groups are designated from 3 to 12.
5. they are all hard metals that exhibit variable oxidation states, colours, magnetic properties.

Question 13

f block elements

Answer 13

1. the elements in which differentiating electron enters into f-orbital are called f block elements.
2. the general electronic configuration is
   (n-2)f^1-14(n-1)d^0-1ns^2
3. they are placed at the bottom of the table.
4. it contains 2 series namely lanthanides and actinides with 14 elements each.
5. they commonly show +3 oxidation state.