Question: Define the modern periodic law. Discuss the construction of the long form of the periodic table. (AS – 1) (4 Marks)
Answer: Modern periodic law: ”The physical and chemical properties of elements are periodic function of electronic configuration of their atoms”. Construction of the long form of the periodic table:
The modern periodic table has 18 vertical columns known as groups and 7 horizontal rows known as periods.
The groups are represented by using Roman numeral I through VIII with letters A and B in traditional notation.
According to latest recommendation of the IUPAC, these groups are
represented by Arabic numerals 1 through 18 with no A and B designations.
There are seven periods in the modern periodic table and the first period starts with K – shell. It contains only one sub – shell, the 1s. It accommodates 2 electrons. They are 1s1(H) and 1s2 (He).
Therefore the first period contains only two elements.
Second period consists of two s – block elements and six p-block elements. So the total number of elements in second period is 8.
Third period starts with third main shell (M). It has 3 sub-shells, namely 3s, 3p and 3d, but while electrons are being filled into the shell ‘3d’ gets electrons only after 4s is filled. So 3rd period has 8 elements only.
4th main shell (N) has four sub-shells, 4s, 4p, 4d and 4f. But electrons enter the atoms in the order 4s, 3d and 3p. Due to this the 4th period contains 18 elements.
On the same line, we can explain why there are 18 elements in the 5th period. There are 32 elements in the sixth period.
Seventh period is incomplete. It contains 2 elements from s -block (7s), 14 elements from f – block (5f), 10 elements from d block (6d) and some elements from p-block (7p). The f- block elements known as lanthanoids and actinoids are shown separately at the bottom of the periodic table.
————————————-
Question: Explain how the elements are classified into s, p, d and f-block elements in the periodic table and give the advantage of this type of classification. (4 Marks)
Answer: Depending on to which sub – shell, the differentiating electron enters the atom of the given element, the elements are classified as s, p, d and f block elements.
s – block elements:
The differentiating electron enters into s – sub shell.
I A and II A groups of elements belong to s -block.
Except hydrogen, all s – block elements are metals.
The general electronic configuration of these elements is ns1 to ns2.
p – block elements:
The differentiating electron enters into p – sub shell.
III A to VIII A or zero groups of elements belong to p – block.
He is the only element whose differentiating electron does not enter into p-sub shell.
The general electronic configuration of these elements is ns2 np1 to ns2 np6.
d-block elements:
The differentiating electron enters into d -sub shell.
I B to VIII B groups of elements belong to d – block.
All d – block elements are metals. The general electronic configuration of these elements is ns2np6 (n – 1)d1 to ns2np6 (n – 1)d10.
f – block elements:
The elements in which f – orbitals are being filled in their atoms are called f-block elements.
The lanthanoids and actinoids below the periodic table are all f-block elements.
Note:
s and p – block elements together are called representative elements.
d – block elements are called transition elements.
f – block elements are called inner transition elements.
Advantages:
Division of elements into different block elements is useful to identify their physical and chemical properties easily. It is easy to choose substitute elements in the study of chemical reactions.