We have seen how the
elements are arranged in the periodic table. The arrangement is based
on rules 7 and 8 that we saw in the previous section. When the
elements are arranged in this way, we obtain some favourable 'side
effects'. What are those side effects? Let us examine:
In the next section, we will discuss more Periodic properties.
Look at the group I. The
elements in this group are: Li, Na, K, Rb. Cs and Fr
• All of them have 1 electron in
their outer most shells.
• These elements have similar
properties. All of them form alkalies when reacting with water. For
example, When sodium (Na) reacts with water, sodium hydroxide (NaOH)
is formed.
• Because of the similarities in
the properties, these elements can be regarded as 'members of a
single family'. This family is given a special name: Alkali metals.
• Similarly, all elements in
group II belong to the family called Alkaline Earth metals. The
properties of all the elements in this family are also similar. Just like Alkali metals, this family also form alkalies. Then why the term 'earth'?
• Because, in early days, these elements were seen only as compounds, in 'minerals obtained from earth'. They do not occur in free form in nature. Later, scientists were able to isolate them from the minerals. But the name stuck.
The table below gives the full
list of the families in the periodic table:
Group I: Alkali metals
Group II: Alkaline earth
metals
Group III: Boron family
Group IV: Carbon family
Group V: Nitrogen family
Group VI: Oxygen family
Group VII: Halogens
Group VIII: Noble gases
The elements helium,
neon, argon, krypton, xenon and radon, which belong to group VIII are
called Noble gases. They are monoatomic molecules. Because they do
not need to combine with another atom to attain stability. (We have
seen the details here also see here). They already have octet configuration.
Normally, they do not combine with other elements also. Hence they
are called inert gases. As they are found only in very small
quantities, they are also called rare gases.
• Helium is used in weather
balloons.
• Neon is used in discharge
lamps to obtain orange colour
• Argon is used in electrical
bulbs to prevent the evaporation of the filament
• Radon is a radioactive gas
So we find that elements with similar properties fall together in the periodic table. This is one of the many 'favourable side effects' that we obtain, while making a systematic arrangement of elements. We will see more as we continue our discussion.
Representative elements
The first 10 elements show a
periodicity in electron filling. This is shown in the table below:
Look at the column for
L-shell. The electron filling is in sequential order from 1 to 8. In the periodic table, these first 10 elements reside at the top most portions in their
respective groups. They are the first elements in their families, and
are called Representative elements.
• The position of hydrogen is
still under debate. Some of it's properties makes it eligible to be a
member of the Alkali metal family (group I)
• Some of it's other properties
makes it eligible to be a member of the halogen family (group VII).
Let us examine:
1. Hydrogen loses one electron in some chemical reactions. Alkali metals also lose one electron in chemical reactions
1. Hydrogen loses one electron in some chemical reactions. Alkali metals also lose one electron in chemical reactions
2(a) Hydrogen is a non metal.
Halogens are also non metals
(b) Hydrogen is diatomic. Halogens
are also diatomic
(c) Hydrogen is not a solid.
Halogens are also not solid. (Alkali metals are solid)
(d) Hydrogen is not a metal.
Halogens are also not metals. (Alkali 'metals' are metals)
(e) Hydrogen has high ionisation
energy. Halogens also have high ionisation energy. (Alkali metals have low ionisation energy). We will learn about ionisation energy
later in the next section.
(g) Hydrogen gains one electron in
some chemical reactions. Halogens also gain one electron in chemical
reactions
Transition elements
Consider the groups I and II.
The elements in these two groups are metals. Consider the groups III
to VIII. They are non metals. [However, some elements (inside green
squares coming under groups III, IV, V and VI in fig.4.8) show properties of
both metals and non metals. They are classified as Metalloids] The
elements in the space between group II and group III are called Transition elements. Because, they form a transition from metals to
non metals. The following are some of the properties of transition
elements:
• They are metals
• They form coloured compounds
• They show similarities in
chemical properties in groups as well as in periods. That is:
♦ If we take any single vertical
group from within the transition elements, all the elements in that
group will have similar chemical properties
♦ If we take any single
horizontal period from within the transition elements, all the
elements in that period will have similar chemical properties
• In compounds, they exhibit
different oxidation states. For example: Fe2+ and Fe3+
Periodic trends in the Periodic table
The word 'period' means an
interval of time. An example for the usage of the word 'periodic' is:
Periodic checkup of a car. It
means, that, the car has to undergo regular checkup at the ends of definite periods. So the event 'checkup of the car' occurs at the end
of regular periods. In other word, it occur periodically.
In the same way, some of the
properties of elements occur periodically. Let us see an example:
■ Point your finger on
Lithium No.3
• It has 1 electron in it's
outer most shell.
• Now add 8 to it's number.
We get 3 + 8 = 11. Element no. 11 is sodium.
• It also has 1 electron in
it's outer most shell
■ Point your finger on
Fluorine No.9
• It has 7 electrons in it's
outer most shell.
• Now add 8 to it's number.
We get 9 + 8 = 17. Element no. 17 is Chlorine.
• It also has 7 electrons
in it's outer most shell
■ So 8 is the period. This
period '8' works for elements upto magnesium No.12.
Let us see the period for the elements after magnesium:
Let us see the period for the elements after magnesium:
■ Point your finger on
Potassium No. 19.
• It has 1 electron in it's
outer most shell.
• Now add 18 to it's number.
We get 19 + 18 = 37. Element no. 37 is Rubidium.
• It also has 1 electron in
it's outer most shell
■ Point your finger on
germanium No. 32.
• It has 4 electrons in it's
outer most shell.
• Now add 18 to it's number.
We get 32 + 18 = 50. Element no. 50 is Tin.
• It also has 4 electron in
it's outer most shell
■ So 18 is the period for
the elements after 12.
What is the difference between
the two periods '8' and '18'?
The difference is 18 – 8 = 10.
Note that 10 is the 'number of groups' of the transition elements
It is not just the 'number of electrons in the outer most shell'. Chemical properties of the element obtained by adding the appropriate period 8 or 18 will also be similar.
• We have not checked
the period for any transition elements.
It is not just the 'number of electrons in the outer most shell'. Chemical properties of the element obtained by adding the appropriate period 8 or 18 will also be similar.
• For the transition elements,
we may not get the same number of electrons in the outer most shell
after the period 18.
• But still, the element that we get by adding 18, will have similar chemical properties.
• But still, the element that we get by adding 18, will have similar chemical properties.
• Within the transition
elements, after barium No.56, the period will become '32' instead of '18'. This is because of the presence of 14 Lanthanides and Actinides. [18 + 14 = 32]
Now let us see some important properties that vary periodically:
Size of atoms in a group
When we move down any group,
the size of atom increases. This is because, the number of shells
increases.
Example: In group II, Ca has 4
shells. Sr, which is just below Ca, has 5 shells. Naturally, the atom
with 5 shells will be larger than that with 4 shells. Some Bohr models showing the details of shells can be seen here.
Size of atoms in a period
We want to know how the size
of atoms vary when we move from left to right in any period. Let us
write an analysis:
1. In any period, the 'number
of shells' is same for all the elements in that period
2. In any period, when we move
from left to right, the number of electrons get increased by '1'
3. The number of electrons is
same as the number of protons in the nucleus. So the number of
protons also increase as we move from left to right.
4. Greater number of protons and
electrons means that there will be greater force of attraction
between the positively charged protons and the negatively charged
electrons.
5. The greater force of
attraction pulls the shells more and more towards the nucleus.
6. As a result, the overall size
of the atom decreases as we move from left to right in a period.
In the next section, we will discuss more Periodic properties.
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