In the previous section, we completed the discussion on oxidation and reduction. In this section, we will discuss about the Periodic table.
A large number of elements are present in nature. We need to arrange them in order. Proper arrangement is essential in every field. Consider the example of a shop. Goods like soaps, detergents etc., will be kept at one particular place. Goods like fruits, vegetables etc., will be kept at a different place, away from soaps and detergents. In the section for soaps itself, there will be sub-sections. Bathing soaps will be kept at one place, while washing soaps will be kept at another place.
In the next section, we will discuss more features of the Periodic table.
A large number of elements are present in nature. We need to arrange them in order. Proper arrangement is essential in every field. Consider the example of a shop. Goods like soaps, detergents etc., will be kept at one particular place. Goods like fruits, vegetables etc., will be kept at a different place, away from soaps and detergents. In the section for soaps itself, there will be sub-sections. Bathing soaps will be kept at one place, while washing soaps will be kept at another place.
Let us now try to arrange the
elements. For that, first, we will make some cards. One card for each
element. A sample card (the green square) is shown in fig.4.1 below. It is the card for Iron (Fe).
The above card, and the others, that we will use for our present discussion, are taken from Wikimedia commons. Each card will show many important details about the element. For our present discussion, we need only the following details:
 |
| Fig.4.1 |
• The atomic number Z of the
element
• Symbol of the element
• Name of the element
• Electronic configuration
of the element
First we
need cards upto atomic number Z = 18, that is Argon. Once we prepare
those cards, we will put them side by side in sequential order. This
is shown in fig.4.2 below (right click, and select 'open in new tab' for an enlarged view):
Fig.4.2 shows the cards from 1 to
18 arranged side by side. After argon, we can continue with any number of
elements that we like, and arrange like this. But such an
arrangement will take up a large horizontal space from left to right.
Also, such an arrangement will not serve any special purpose. We want
metals to be together at one place, non-metals to be together at
another place, gases to be together at yet another place, etc., So
let us make some modifications to the above arrangement.
 |
| Fig.4.2 |
Let us take out the elements
after Z= 10. That is., we take out the elements from Sodium with Z = 11,
upto Argon with Z = 18. We take them and put them under the first 10
elements. This is shown in fig.4.3 below(right click, and select 'open in new tab' for an enlarged view):
• Care must be taken to see
that, Sodium No.11 comes under Lithium No.3.
 |
| Fig.4.3 |
Now look at the
electronic configurations carefully.
♦ Lithium and sodium have 1
electron in their outer most shells, and now, both are grouped
together in one column
♦ Beryllium and magnesium
have 2 electrons in their outer most shells, and now, both are
grouped together in one column
♦ Boron and aluminium have 3
electrons in their outer most shells, and now, both are grouped
together in one column
We find the above pattern for
all the elements from 3 to 18. Let us write it down:
• All the elements in a
column have the same number of electrons in the outer most shells
• Also, when we move from left to right, with the passing of every one column, the number of electrons in the outer most shell increase by 1
We find that hydrogen and
helium are left out. Let us give them appropriate positions:
• Take out hydrogen, and put it
above lithium No.3
• Take out helium, and put it
above neon No.10
The modified table is shown in
fig.4.4 below:
• Now, the first column has 3
elements. All of them have 1 electron in their outer most shells
 |
| Fig.4.4 |
• The last column also has 3
elements. But the number of electrons in the outer most shell are not
the same. Helium has 2, and the others have 8. We can think about it
in this way:
The elements in this column
have a stable configuration. That is., maximum number of electrons in
the outer most shell. These elements in the last column do not
usually take part in reactions because, they are stable.
• So now our table has 3
horizontal rows. Each of these horizontal rows is called a 'period'.
• Also, the table has 7 vertical columns. Each of these vertical columns is called a 'group'.
Do we see any relation ship between the following two:
• Also, the table has 7 vertical columns. Each of these vertical columns is called a 'group'.
Do we see any relation ship between the following two:
♦ The position of any period
♦ The electronic configuration
of the elements in that period?
• Indeed there is a relation.
Look at the 1st period. Take a closer look at the electronic
configuration of the elements in that period. The configuration has
only 1 digit. That means, all the elements in the 1st period has
only 1 shell.
♦ In fact, 'all the elements in
the world, which have only 1 shell', are included in the 1st period. Because, after hydrogen and helium, the next element is
lithium, which has 2 shells.
• Look at the 2nd period.
Take a closer look at the electronic configuration of the elements in
that period. The configuration has 2 digits. That means, all the
elements in the 2nd period has two shells.
♦ In fact, 'all the elements in
the world, which have 2 shells', are included in the 2nd period.
Because, after neon, the next element is sodium, which has 3 shells.
• Look at the 3rd period. Take
a closer look at the electronic configuration of the elements in that
period. The configuration has 3 digits. That means, all the
elements in the third period has 3 shells.
♦ In fact, 'all the elements in
the world, which have 3 shells', are included in the 3rd period. Because, after argon, the next element is potassium, which
has 4 shells.
Let us write a summary of the
discussion that we had so far.
1. The horizontal rows are called Periods
2. The vertical columns are
called Groups
3. All the elements in a period will have the same number of shells
4. All the elements in a group
will have the same number of electrons in the outer most shell.
5. As we move from top to bottom in the table, with the passing of each period, 1 shell gets added
5. As we move from top to bottom in the table, with the passing of each period, 1 shell gets added
6. As we move from left to right in the table, with the passing of each new group, 1 electron gets added in the outermost shell
7. The 'name of the period' will indicate:
♦ the number of shells present, in each element in that period
• For example, each elements in period 3 will have 3 shells. K, L and M
8. The 'name of the group' will indicate:
♦ the number of electrons present in the outer most shell of each element in that group
• For example, each elements in Group V will have 5 electrons in it's outer most shell
7. The 'name of the period' will indicate:
♦ the number of shells present, in each element in that period
• For example, each elements in period 3 will have 3 shells. K, L and M
8. The 'name of the group' will indicate:
♦ the number of electrons present in the outer most shell of each element in that group
• For example, each elements in Group V will have 5 electrons in it's outer most shell
So we have arranged the first 18 elements. Let us now arrange the rest. Rules 7 and 8 written above will help us.
■ The next element is No.19
Potassium.
• It has 4 shells. K, L, M and N. So, according to rule 7
above, it falls in period 4
• It has 1 electron in the outer
most shell. So, according to rule 8, it will fall in group I
• Based on the above 2,
potassium is the first element in the 4th
period. So it will fall just below sodium as shown in the fig.4.5 below:
 |
| Fig.4.5 |
■The next element is No.20
calcium.
• It has 4 shells. K, L, M and N. So, according to rule 7, it
falls in period 4
• It has 2 electrons in the
outer most shell. So, according to rule 8, it will fall in
group II
• Based on the above 2, calcium is the second element in the 4th
period. So it will fall just below
magnesium. This is also shown in the fig.4.5 above.
■ The next element is No.21
scandium.
• It has 4 shells. K, L, M and N. So, according to rule 7, it
falls in period 4
• It has 2 electrons in the
outer most shell. So, according to rule 8, it will fall in
group II
• Based on the above 2, scandium
is the second element in the 4th
period. So it will fall just below
magnesium in the fig.4.4.
• So here we encounter a
problem. We have already assigned the 'position below magnesium' to
calcium. Now, scandium is also claiming the same position. Scandium
has the claim because, it too has '4 shells', and '2 electrons in the
outer most shell'.
■ Let us try the next element.
The next element is No.22 titanium.
• It has 4 shells. K, L, M and N.
So, according to rule 7, it falls in period 4
• It has 2 electrons in the
outer most shell. So, according to rule 8, it will fall in
group II
• Based on the above 2, titanium
is the second element in the 4th
period. So it will fall just below
magnesium in the fig.4.4.
• Here also we encounter the
same problem. We have already assigned the 'position below magnesium' to calcium. Now, scandium and titanium are also claiming the same
position. Titanium has the claim because, it too has '4 shells', and
'2 electrons in the outer most shell'.
■ Let us try one more element. The next element is No.23 vanadium.
• It has 4 shells. K, L, M
and N. So, according to rule 7, it falls in period 4
• It has 2 electrons in the
outer most shell. So, according to rule 8, it will fall in
group II
• Based on the above 2, vanadium is the second element in the 4th
period. So it will fall just below
magnesium in the fig.4.4.
• Here also we encounter the
same problem. We have already assigned the 'position below magnesium' to calcium. Now, scandium, titanium and vanadium are also claiming the same
position. Vanadium has the claim because, it too has '4 shells', and
'2 electrons in the outer most shell'.
Why is there more than 1 claim
for a single position? Let us analyse:
Consider the electron
configuration of the elements that we have seen so far in Period 4:
• No.19: Potassium (K): 2,8,8,1
• No.20: Calcium (Ca): 2,8,8,2
• No.21: Scandium (Sc): 2,8,9,2
• No.22: Titanium (Ti): 2,8,10,2
• No.23: Vanadium (V): 2,8,11,2
We can see that, after No.20,
the electrons get added to the second outer most shell. So there is
no sequential increase in the number of electrons in the outer most
shells of scandium, titanium and vanadium.
We will learn more details
about such electron configuration in higher classes. At present, all
we need to know is this:
■ The electrons are getting added
to the second outer most shell. A phenomenon which creates more than
one claim for the same position.
This special situation
continues beyond No.23 vanadium. It continues up to No.30 zinc. After
that normalcy is restored. That is., after No.30, the electrons get added to the
outer most shell. So we have to provide a special place for the
elements from 21 to 30. This special place is created in between
Groups II and III. This is shown in fig.4.6 below:
 |
| Fig.4.6 |
How much space should be provided between Groups II and III?
• Enough space to accommodate elements from No.21 to No.30. This is shown in fig.4.7 below (right click, and select 'open in new tab' for an enlarged view):
 |
| Fig.4.7 |
A total of 10 elements (from
No.21 to No.30) are accommodated in the newly created space. We can
see that, the elements which come after No.30, that is., the elements
from 31 to 36, strictly follow both the rules 7 and 8.
Now, all the elements in the world, which have 4 shells, are accommodated in the Period 4.
Now, all the elements in the world, which have 4 shells, are accommodated in the Period 4.
■ Let us move to the next period
5. In this period also, there are some problem causing elements. They
have 5 shells, and thus comply with the rule 7. But they do not
comply with rule 8. So they are also placed in the portion between
Groups II and III. There are exactly 10 'problem causing elements',
just as in Period 4
■ Let us move to the next period
6. In this period also, there are some 'problem causing elements'.
They have 6 shells, and thus comply with the rule 7. But they do not
comply with rule 8. So they are also placed in the portion between
Groups II and III. In this period, the number of 'problem causing
elements' are more. There are a total of 24 such elements. Note that there are only 10 such elements in Periods 4 and 5. If we
include all 24 in between Groups II and III, then the
table will become very long from left to right. We will not be able to print or draw it
on a single sheet. So, 14 of them (starting from No.57 Lanthanum),
are separated from the main table. These 14 are given a special name 'Lanthanides', and are kept at the bottom of the main table. This is
indicated by the red arrow in the 'completed periodic table' shown in
the fig.4.8 below (right click, and select 'open in new tab' for an enlarged view). This table is obtained from the Wikimedia commons, and can be seen here. All the elements in the world,
which have 6 shells come under the red arrow in Period 6.
 |
| Fig.4.8 |
■ Let us move to the next period 7. In this period also, there are 24 'problem causing elements'. They have 7 shells, and thus comply with the rule 7. But they do not comply with rule 8. 14 of them (starting from No.89 Actinium) are separated from the main table. These 14 are given a special name 'Actinides', and are kept at the bottom of the main table, below the Lanthanides. This is indicated by the blue arrow in the fig.4.8. All the elements in the world, which have 7 shells come under the blue arrow in Period 7.
• Lanthanides are also known as rare earths
• Actinides are man made artificial elements (except Thorium and Uranium)
• Lanthanides are also known as rare earths
• Actinides are man made artificial elements (except Thorium and Uranium)
So we have discussed the
basics about the arrangement of elements in the tabular form. The
Modern Periodic table is based on the works of the Russian scientist
Dmitri Ivanovich Mendeleev. Works of the English scientist Henry
Moseley contributed to the modifications of the Table.
In the next section, we will discuss more features of the Periodic table.
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