Chapter 1: Language of Chemistry
Topic: Radicals, The formula of a Compound
Radicals: the word radical
in chemistry stand for “an atom or group of atoms behaving as a unit in a number of compounds.” Radicals are reactive species that
combine with other radical to form compounds.
There are
three types of radicals.
Acidic
radical: The part of
salt which comes from acid is known as acidic radical. It is always negatively
charged therefore it is also called an anion. It forms ionic compounds when join
with cation. If cation is hydronium ion or hydrogen ion then they form acid.
Names
and symbols of some anions
Basic radical: the part of salt which comes from the base is known as basic radical. It is positively charged and also known as a cation. When they combine with anion then form ionic compounds and if anion is a hydroxide ion (OH-), then they form base.
Names
and symbols of some cations
Free radical: The part of covalent compounds which form due to fission of bond in molecular reaction. They are neutral in nature therefore called free radicals. They are reactive due to presence of unpaired electron. Two free radicals join to each other and form covalent compound.
Some common free radicals
Formula of a Compound
A formula is a short representation of a
compound with the help of symbols of the constituent elements. For covalent
compound it is known as a molecular formula but in the case of ionic compound it
is known as formula unit.
(i)
Method to write a
Formula
Step 1: Identify the symbol of the cation (first
part of the name) and the anion
Step 2: Identify the valency or charge of each
symbol and place it in parenthesis just above the symbol
Step 3: Balance the total positive and negative
charge on the cation and anion. You ask yourself do the total positive charge
and total negative charge add up to zero. If the answer is no then we ask how
many of each ion must we have in order to balance charge. We must have the same
number of positive charges as we do of negative charges. Another way of saying
that is that they must add up to zero.
Step 4: Once you have determined the number of
units of the cation and anion those become the subscripts which are placed
right after the respective symbol.
Example 1: Formula of Copper (I) Oxide.
Identify the symbols of
the cation and anion- Copper
is Cu and Oxide is O
Identify the charge for
each and place above the symbol in parenthesis
For Copper I that would be 1+ and for Oxide
that would be 2-
Balance the positive
and negative charges
Since
each Copper is 1+ and each Oxide is 2- then it will take two Cu+ to
balance one oxide with a 2- so that
2(1+)
+ 1(2-) = 0. The numbers outside the parenthesis become the subscripts in the
formula
Write the formula
placing the subscripts right after the symbol they go with.
Cu2O
Notice
that we don't worry to place a subscript 1 after the Oxide symbol. That is
because a subscript one is understood to be so.
Example 2: Formula of Calcium Chloride.
Identify the symbols of
the cation and anion- Calcium is Ca and
Chloride is Cl
Identify the charge for
each and place above the symbol in parenthesis
For Calcium that would be 2+ and for Chloride
that would be 1-
Balance the positive
and negative charges
Since
Calcium is 2+ and each Chloride is 1- then it will take two Cl- to
balance one calcium ion with a 2+ so that
1(2+)
+ 2(1-) = 0. The numbers outside the parenthesis become the subscripts in the
formula
Write the formula
placing the subscripts right after the symbol they go with.
CaCl2
Notice
that we don't worry to place a subscript 1 after the Calcium symbol. That is
because a subscript one is understood to be so.
Example 3: Formula of Ethane
Step
1: When two free radicals combine then we get molecule of covalent compound.
Methyl free radical CH3 contains one unpaired electron.
Step
2: Another methyl free radical also contains one unpaired electron.
Step
3: Unpaired electrons of both free radicals share their unpaired electron and
form covalent bond. Then we get CH3 – CH3
Example 4: Formula of Methyl Chloride
Step
1: Methyl free radical CH3 contains one unpaired electron.
Step
2: The Chloride free radical Cl also
contains one unpaired electron.
Step
3: Unpaired electrons of both free radicals share their unpaired electron and
form covalent bond. Then we get CH3 – Cl.
(ii)
Information conveyed by a formula
It gives us some information about the
compound. They can be listed as follows:
(i) The formula tells us about the different
elements present in that compound.
(ii)
It tells us about the number of atoms of
each element present in one molecule of that compound.
(iii) It represents one molecule of that
compound.
(iv)
It represents a definite mass of that
compound which is equal to the molecular weight of that compound.
(v)
It also tells us about the mass of each
element involved in one molecule of that compound.
For
example, if we take KMnO4, the various facts which we collect from
this formula are as follows:
(i)
It tells us that Potassium, Manganese and
Oxygen are present in this compound.
(ii)
It also tells us that there is one atom of
Potassium, one atom of Manganese and four atoms of Oxygen are present in one
molecule of Potassium Permanganate.
(iii) It represents only one molecule of
Potassium Permanganate.
(iv)
The definite mass represented by it is 39
+ 55 + 4 x 16 = 158 gm
(v)
This tells us that one molecule of KMnO4
contains 39 gm of Potassium, 55 gm of Manganese and 64 gm of Oxygen
Name and formula of
some common compounds
