In the previous section, we saw the molecules of Elements. In this section we will see the molecules of Compounds. We know that a compound contains more than one element. So there will be more than one type of atoms. For example, in water, there will be atoms of hydrogen and oxygen.
The different types of atoms in a compound will not exist independently. The atoms will always be in groups. Groups cannot be formed in any manner. There are strict rules for the formation of groups:
• Each group will have atoms of different elements
• The number of atoms of each element will be the same in any given group
• In the case of compounds also, the name of such groups is Molecule.
• For example, in water, there will be atoms of two elements: hydrogen and oxygen
• In any given molecule of water, there will be two atoms of hydrogen and one atom of oxygen. This is shown in the fig.1.3 below:
We have seen that the symbol for the element sulphur is S. One atom of sulphur is also represented by the letter S. So if we see the letter S, it indicates one atom of sulphur. But no one atom of sulphur can exist independently. It is always found as groups of 8 sulphur atoms. We have seen that such groups are called molecules. So a group of 8 sulphur atoms is one molecule of sulphur. (sulphur is a polyatomic molecule). One atom of sulphur is represented as S. How will we represent one molecule of sulphur? For that we write the number 8 as a subscript. So one molecule of sulphur is represented as S8.
If there are more than one sulphur molecule, that number is written on the left side. So, if a sample of sulphur contains 3 molecules of sulphur, it is written as 3S8. Such a notation will also help us to calculate the total number of sulphur atoms in that sample. The calculation is as follows:
• S8 indicates that, one Sulphur molecule has 8 sulphur atoms
• 3S8 indicates 3 molecules of sulphur.
• So the total number of sulphur atoms in 3 molecules = 3 × 8 = 24
• If the sample contains 2 sulphur molecules, then the total number of sulphur atoms will be 2 × 8 = 16
Another example: The symbol for the element helium is He. When we write He, it represents one atom of helium. Each of the helium atoms are able to exist independently. So a molecule of helium will consist of only one helium atom. (Helium is a monoatomic molecule). So when we write He, it will represent one molecule of helium also.
If there are more than one helium molecule, that number is written on the left side. if a sample of helium contains 5 helium molecules, it is written as 5He. As we saw above in the case of sulphur, this type of representation will give us the number of atoms also:
• He indicates that, one helium molecule has 1 helium atom.
• 5He indicates 5 molecules of helium
• So the total number of helium atoms in 5 molecules = 5 × 1 = 5
• If we see 7He, it will indicate 7 molecules of helium, which is equal to 7 atoms of helium
One more example: We have seen that the symbol for the element hydrogen is H. One atom of hydrogen is also represented by the letter H. So if we see the letter H, it indicates one atom of hydrogen. But no one atom of hydrogen can exist independently. It is always found as groups of 2 hydrogen atoms. We have seen that such groups are called molecules. So a group of 2 hydrogen atoms is one molecule of hydrogen. (hydrogen is a diatomic molecule). One atom of hydrogen is represented as H. How will we represent one molecule of hydrogen? For that we write the number 2 as a subscript. So one molecule of hydrogen is represented as H2.
If there are more than one hydrogen molecule, that number is written on the left side. So, if a sample of hydrogen contains 3 molecules of hydrogen, it is written as 3H2. This will also give us the total number of hydrogen atoms in that sample. It can be calculated as follows:
• H2 indicates that, one hydrogen molecule has 2 hydrogen atoms
• 3H2 indicates 3 molecules of hydrogen
• So the total number of hydrogen atoms in 3 molecules = 3 × 2 = 6
• If the sample contains 5 hydrogen molecules, then the total number of hydrogen atoms will be 5 × 2 = 10
We will now see some solved examples:
Solved example 1.1
Write the number of atoms and molecules in each of the following:
(i) 3N2 (ii) O3 (iii) 6Cl2
Solution: (i) 3N2
• The subscript 2 indicates that there are two atoms in one molecule
• 3 on the left side indicates that there are a total of 3 molecules
• So the total number of atoms = 3 × 2 = 6
(ii) O3
• The subscript 3 indicates that there are 3 atoms in one molecule
• There is no number on the left side. It indicates 1 molecule
• So the total number of atoms = 1 × 3 = 3
(iii) 6Cl2
• The subscript 2 indicates that there are two atoms in one molecule
• 6 on the left side indicates that there are a total of 6 molecules
• So the total number of atoms = 6 × 2 = 12
So we have seen how the molecules of elements are represented. Now we will see the representation of the molecules of compounds. Consider the example of carbon dioxide. It is a compound, formed when carbon burns in oxygen. It can also form during the decomposition of calcium carbonate. But the ‘method of formation’ of carbon dioxide is not important for our present discussion. We want to know the elements present inside the carbon dioxide, and also quantity of each of those elements.
In one molecule of carbon dioxide:
• Number of carbon atoms = 1
• Number of oxygen atoms = 2
The above numbers will be true for every molecule of carbon dioxide. That is., what ever be the method by which we obtain carbon dioxide, or from which ever source, we obtain the carbon dioxide, every molecule of it will contain one atom of carbon and two atoms of oxygen. We can write in in the form of ratio:
• In every carbon dioxide molecule, the ratio of carbon to oxygen is 1:2.
If this ratio is not maintained, we will not get carbon dioxide. For example, if 2 atoms of carbon combine with 3 atoms of oxygen, the ratio of carbon to oxygen is 2:3. It will not be carbon dioxide.
As this ratio is always maintained, we can represent carbondioxide as CO2. It indicates one atom of carbon and 2 atoms of oxygen. This type of representation is called the Chemical formulae of the compound.
Another example: Any one water molecule will have two atoms of hydrogen and one atom of oxygen. So the chemical formula of water is H2O
• A chemical formula can have natural numbers (1, 2, 3 etc.,) in two forms: Regular form and subscript form
• Subscripts are found on the right of each element. It indicates the number of atoms of that element in the molecule. If there is only one atom of an element, it’s subscript ‘1’ is not written
• Numbers in the Regular form is always seen on the extreme left side of the chemical formula. it indicates the total number of molecules. If the total number of molecules is 1, it is not written. Such numbers in the regular form will never be seen in side a chemical formula. it will be only seen on the extreme left side
With this knowledge, we can analyse any given chemical formula, and write the number of atoms of each element present in it. The following solved examples will demonstrate the procedure
Solved example 1.2:
Determine the number of atoms of each element from the following chemical formulae:
(i) H2SO4 (ii) 5H2O (iii) ZnCl2 (iv) 7NH3
Solution: (i) H2SO4
■ There is 'no number' on the extreme left. This indicates '1'. So the given chemical formula indicates one molecule.
• The first element is hydrogen (indicated by H).
♦ It's subscript is 2. So there are 2 atoms of hydrogen
♦ Total number of hydrogen atoms = Total number of molecules × number of atoms per molecule = 1 × 2 = 2
• The second element is sulphur (indicated by S)
♦ There is 'no subscript'. This indicates '1' atom
♦ Total number of sulphur atoms = Total number of molecules × number of atoms per molecule = 1 × 1 = 1
• The final element is oxygen (indicated by O).
♦ It's subscript is 4. So there are 4 atoms of oxygen
♦ Total number of oxygen atoms = Total number of molecules × number of atoms per molecule = 1 × 4 = 4
(ii) 5H2O
■ There is '5' on the extreme left. So the given chemical formula indicates 5 molecules.
• The first element is hydrogen (indicated by H).
♦ It's subscript is 2. So there are 2 atoms of hydrogen
♦ Total number of hydrogen atoms = Total number of molecules × number of atoms per molecule = 5 × 2 = 10
• The second element is oxygen (indicated by O)
♦ There is 'no subscript'. This indicates '1' atom
♦ Total number of Oxygen atoms = Total number of molecules × number of atoms per molecule = 5 × 1 = 5
(iii) ZnCl2
■ There is 'no number' on the extreme left. This indicates '1'. So the given chemical formula indicates one molecule.
• The first element is zinc (indicated by Zn)
♦ There is 'no subscript'. This indicates '1' atom
♦ Total number of Zinc atoms = Total number of molecules × number of atoms per molecule = 1 × 1 = 1
• The second element is chlorine (indicated by Cl).
♦ It's subscript is 2. So there are 2 atoms of chlorine
♦ Total number of chlorine atoms = Total number of molecules × number of atoms per molecule = 1 × 2 = 2
(iv) 7NH3
■ There is '7' on the extreme left. So the given chemical formula indicates 7 molecules
• The first element is nitrogen (indicated by N)
♦ There is 'no subscript'. This indicates '1' atom
♦ Total number of nitrogen atoms = Total number of molecules × number of atoms per molecule = 7 × 1 = 7
• The second element is hydrogen (indicated by H).
♦ It's subscript is 3. So there are 3 atoms of hydrogen
♦ Total number of hydrogen atoms = Total number of molecules × number of atoms per molecule = 7 × 3 = 21
In the next section we will see Chemical Equations.
The different types of atoms in a compound will not exist independently. The atoms will always be in groups. Groups cannot be formed in any manner. There are strict rules for the formation of groups:
• Each group will have atoms of different elements
• The number of atoms of each element will be the same in any given group
• In the case of compounds also, the name of such groups is Molecule.
• For example, in water, there will be atoms of two elements: hydrogen and oxygen
• In any given molecule of water, there will be two atoms of hydrogen and one atom of oxygen. This is shown in the fig.1.3 below:
 |
| Fig.1.3 Molecule of Water |
Representing Atoms and Molecules
The method of representing atoms and molecules can be demonstrated with the help of some examples:We have seen that the symbol for the element sulphur is S. One atom of sulphur is also represented by the letter S. So if we see the letter S, it indicates one atom of sulphur. But no one atom of sulphur can exist independently. It is always found as groups of 8 sulphur atoms. We have seen that such groups are called molecules. So a group of 8 sulphur atoms is one molecule of sulphur. (sulphur is a polyatomic molecule). One atom of sulphur is represented as S. How will we represent one molecule of sulphur? For that we write the number 8 as a subscript. So one molecule of sulphur is represented as S8.
If there are more than one sulphur molecule, that number is written on the left side. So, if a sample of sulphur contains 3 molecules of sulphur, it is written as 3S8. Such a notation will also help us to calculate the total number of sulphur atoms in that sample. The calculation is as follows:
• S8 indicates that, one Sulphur molecule has 8 sulphur atoms
• 3S8 indicates 3 molecules of sulphur.
• So the total number of sulphur atoms in 3 molecules = 3 × 8 = 24
• If the sample contains 2 sulphur molecules, then the total number of sulphur atoms will be 2 × 8 = 16
Another example: The symbol for the element helium is He. When we write He, it represents one atom of helium. Each of the helium atoms are able to exist independently. So a molecule of helium will consist of only one helium atom. (Helium is a monoatomic molecule). So when we write He, it will represent one molecule of helium also.
If there are more than one helium molecule, that number is written on the left side. if a sample of helium contains 5 helium molecules, it is written as 5He. As we saw above in the case of sulphur, this type of representation will give us the number of atoms also:
• He indicates that, one helium molecule has 1 helium atom.
• 5He indicates 5 molecules of helium
• So the total number of helium atoms in 5 molecules = 5 × 1 = 5
• If we see 7He, it will indicate 7 molecules of helium, which is equal to 7 atoms of helium
One more example: We have seen that the symbol for the element hydrogen is H. One atom of hydrogen is also represented by the letter H. So if we see the letter H, it indicates one atom of hydrogen. But no one atom of hydrogen can exist independently. It is always found as groups of 2 hydrogen atoms. We have seen that such groups are called molecules. So a group of 2 hydrogen atoms is one molecule of hydrogen. (hydrogen is a diatomic molecule). One atom of hydrogen is represented as H. How will we represent one molecule of hydrogen? For that we write the number 2 as a subscript. So one molecule of hydrogen is represented as H2.
If there are more than one hydrogen molecule, that number is written on the left side. So, if a sample of hydrogen contains 3 molecules of hydrogen, it is written as 3H2. This will also give us the total number of hydrogen atoms in that sample. It can be calculated as follows:
• H2 indicates that, one hydrogen molecule has 2 hydrogen atoms
• 3H2 indicates 3 molecules of hydrogen
• So the total number of hydrogen atoms in 3 molecules = 3 × 2 = 6
• If the sample contains 5 hydrogen molecules, then the total number of hydrogen atoms will be 5 × 2 = 10
We will now see some solved examples:
Solved example 1.1
Write the number of atoms and molecules in each of the following:
(i) 3N2 (ii) O3 (iii) 6Cl2
Solution: (i) 3N2
• The subscript 2 indicates that there are two atoms in one molecule
• 3 on the left side indicates that there are a total of 3 molecules
• So the total number of atoms = 3 × 2 = 6
(ii) O3
• The subscript 3 indicates that there are 3 atoms in one molecule
• There is no number on the left side. It indicates 1 molecule
• So the total number of atoms = 1 × 3 = 3
(iii) 6Cl2
• The subscript 2 indicates that there are two atoms in one molecule
• 6 on the left side indicates that there are a total of 6 molecules
• So the total number of atoms = 6 × 2 = 12
So we have seen how the molecules of elements are represented. Now we will see the representation of the molecules of compounds. Consider the example of carbon dioxide. It is a compound, formed when carbon burns in oxygen. It can also form during the decomposition of calcium carbonate. But the ‘method of formation’ of carbon dioxide is not important for our present discussion. We want to know the elements present inside the carbon dioxide, and also quantity of each of those elements.
In one molecule of carbon dioxide:
• Number of carbon atoms = 1
• Number of oxygen atoms = 2
The above numbers will be true for every molecule of carbon dioxide. That is., what ever be the method by which we obtain carbon dioxide, or from which ever source, we obtain the carbon dioxide, every molecule of it will contain one atom of carbon and two atoms of oxygen. We can write in in the form of ratio:
• In every carbon dioxide molecule, the ratio of carbon to oxygen is 1:2.
If this ratio is not maintained, we will not get carbon dioxide. For example, if 2 atoms of carbon combine with 3 atoms of oxygen, the ratio of carbon to oxygen is 2:3. It will not be carbon dioxide.
As this ratio is always maintained, we can represent carbondioxide as CO2. It indicates one atom of carbon and 2 atoms of oxygen. This type of representation is called the Chemical formulae of the compound.
Another example: Any one water molecule will have two atoms of hydrogen and one atom of oxygen. So the chemical formula of water is H2O
• A chemical formula can have natural numbers (1, 2, 3 etc.,) in two forms: Regular form and subscript form
• Subscripts are found on the right of each element. It indicates the number of atoms of that element in the molecule. If there is only one atom of an element, it’s subscript ‘1’ is not written
• Numbers in the Regular form is always seen on the extreme left side of the chemical formula. it indicates the total number of molecules. If the total number of molecules is 1, it is not written. Such numbers in the regular form will never be seen in side a chemical formula. it will be only seen on the extreme left side
With this knowledge, we can analyse any given chemical formula, and write the number of atoms of each element present in it. The following solved examples will demonstrate the procedure
Solved example 1.2:
Determine the number of atoms of each element from the following chemical formulae:
(i) H2SO4 (ii) 5H2O (iii) ZnCl2 (iv) 7NH3
Solution: (i) H2SO4
■ There is 'no number' on the extreme left. This indicates '1'. So the given chemical formula indicates one molecule.
• The first element is hydrogen (indicated by H).
♦ It's subscript is 2. So there are 2 atoms of hydrogen
♦ Total number of hydrogen atoms = Total number of molecules × number of atoms per molecule = 1 × 2 = 2
• The second element is sulphur (indicated by S)
♦ There is 'no subscript'. This indicates '1' atom
♦ Total number of sulphur atoms = Total number of molecules × number of atoms per molecule = 1 × 1 = 1
• The final element is oxygen (indicated by O).
♦ It's subscript is 4. So there are 4 atoms of oxygen
♦ Total number of oxygen atoms = Total number of molecules × number of atoms per molecule = 1 × 4 = 4
(ii) 5H2O
■ There is '5' on the extreme left. So the given chemical formula indicates 5 molecules.
• The first element is hydrogen (indicated by H).
♦ It's subscript is 2. So there are 2 atoms of hydrogen
♦ Total number of hydrogen atoms = Total number of molecules × number of atoms per molecule = 5 × 2 = 10
• The second element is oxygen (indicated by O)
♦ There is 'no subscript'. This indicates '1' atom
♦ Total number of Oxygen atoms = Total number of molecules × number of atoms per molecule = 5 × 1 = 5
(iii) ZnCl2
■ There is 'no number' on the extreme left. This indicates '1'. So the given chemical formula indicates one molecule.
• The first element is zinc (indicated by Zn)
♦ There is 'no subscript'. This indicates '1' atom
♦ Total number of Zinc atoms = Total number of molecules × number of atoms per molecule = 1 × 1 = 1
• The second element is chlorine (indicated by Cl).
♦ It's subscript is 2. So there are 2 atoms of chlorine
♦ Total number of chlorine atoms = Total number of molecules × number of atoms per molecule = 1 × 2 = 2
(iv) 7NH3
■ There is '7' on the extreme left. So the given chemical formula indicates 7 molecules
• The first element is nitrogen (indicated by N)
♦ There is 'no subscript'. This indicates '1' atom
♦ Total number of nitrogen atoms = Total number of molecules × number of atoms per molecule = 7 × 1 = 7
• The second element is hydrogen (indicated by H).
♦ It's subscript is 3. So there are 3 atoms of hydrogen
♦ Total number of hydrogen atoms = Total number of molecules × number of atoms per molecule = 7 × 3 = 21
In the next section we will see Chemical Equations.
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