Chapter 1.1 - Chemical Formula of a Compound

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:

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

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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

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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

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• 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

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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

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In the next section we will see Chemical Equations.

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Chapter 1 - Elements and Compounds

In this section we will discuss about Elements and Compounds. We see different substances all around us. Some examples are: water, sugar, iron, gold, cement etc., From a chemical point of view, substances can be classified into two categories: (i) Elements and (ii) Compounds. Elements are substances that are in a 'pure state'. They consist of only one substance. For example, a pure gold piece will contain gold only. A pure iron piece will contain iron only. Gold and iron are Elements. We can say: Elements are the 'basics'.

Compounds are combinations of basic Elements. Take the example of water. Even if we take water in it's purest form, It is not a single element. It is a combination of two elements: Hydrogen and Oxygen. Similarly, sugar is a compound, which is a combination of element like carbon, hydrogen, oxygen etc., Cement is a compound, which is a combination of elements like calcium, alluminium, silicon etc.,

A compound can be 'decomposed' by subjecting it to a chemical reaction. When it decomposes, we will be able to separate the various elements that made up the compound.

But an element cannot be further decomposed. For example, a compound may contain iron as one of it's components. When the compound is subjected to a suitable chemical reaction, we will get back the iron. We cannot proceed further, with the intention of getting the 'components of iron'. Because, there are no components for iron. Iron, which is an element, is in it's purest form.

■ Pure substances which cannot be further decomposed through chemical processes are called Elements.
■ Compounds are substances formed from two or more elements through chemical reactions.

We called elements to be ‘pure’. It may be noted that we cannot say compounds to be ‘not pure’. It is just that compounds are combinations of two or more pure substances.

Names of Elements
Elements are named after Places, Countries, Colour, Names of Scientists, Planets, satellites etc., Some examples are:
• Rutherfordium is named after the scientist Rutherford
• Neptunium is named after the planet Neptune

Symbols of elements
In chemistry, we will have to refer to the various elements very often. So it is convenient to use ‘symbols’ rather than their ‘full names’. Definite symbols are assigned to each elements. We must have a good knowledge about the symbols of commonly used elements in chemistry.

First letter of the English name is used as symbol for many elements. This letter should be in the ‘upper case’ (Capital letter) form. Some examples are:
• The symbol for Carbon is C
• The symbol for Oxygen is O
• The symbol for Sulphur is S

But if we strictly follow the above procedure, we will be able to assign symbols for only 26 elements. Because there are only 26 letters in the English alphabet. Also, some of the elements may have the same first letter. So, for some elements, a second letter is also used. This second letter may be the second letter in the name, or a prominent letter in the name. When used in the symbol, this second letter must be in the ‘lower case’ (small letter) form. Some examples are:
• The symbol for Calcium is Ca. Here a is the second letter in the name
• The symbol for Bromine is Br. Here r is the second letter in the name
• The symbol for Chlorine is Cl. Here l is a prominent letter in the name

Some elements that are well known to us, may have an entirely different symbol. That is., their symbol may seem to have no relation to their name. Such symbols are derived from their Latin names. Some examples are:
• Iron has the symbol Fe. it is derived from the Latin name for iron, which is ‘Ferrum’
•Potassium has the symbol K. It is derived from the Latin name for potassium, which is ‘Kalium’

Atoms and Molecules
• We have seen that elements are the ‘basics’. Consider a small piece of iron. 
• We can break it into small pieces. 
• We can break one of these small pieces into still smaller pieces. 
• Again we can take one of these smaller pieces and break it into still smaller pieces. This is shown in fig.1.1 below:

Fig.1.1

• If we can continue like this, we will get the smallest piece which retains all the characteristics of iron
• Such a smallest piece which retains all the characteristics of the element is called the atom 
• After reaching the ‘stage of the atom’, if we break it further, the pieces so formed will not have the characteristics of the element.

In the above experiment, we considered the breaking of a small piece of an element. What if we break a small piece of a compound?
We know that a compound consists of more than one element. If we continue breaking a piece of a compound, in the end, we will get more than one type of atoms. 
• Let the compound consist of 3 elements: Element 1, Element 2 and Element 3. 
• After continued breaking, we will get 3 types of atoms. 
• One type will be of Element 1. Another will be of Element 2, and, the third type will be of Element 3. 
• Each of these atoms will have all the characteristics of the corresponding element.

We now know what an atom is. 
• In the case of the elements like helium, the atoms can exist independently.This is shown in fig.1.2(a)

Fig.1.2

• The atoms of neon can also exist independently. This is shown in fig.1.2(b). But for some elements, their atoms can exist only as groups. Here are some examples: 
• In the case of hydrogen, two hydrogen atoms will combine together to form a group. In a sample of hydrogen, there will be many such groups. In fact, there will only be groups. Each of these groups will contain 2 hydrogen atoms. This is shown in fig.1.2(c). In the sample, we will not find any independent hydrogen atom
• In a sample of oxygen, their will only be groups. Each of these groups will contain 2 oxygen atoms (fig.1.2.d) In the sample, we will not find any independent oxygen atoms
• In a sample of phosphorus, their will only be groups. Each of these groups will contain 4 phosphorus atoms (fig.1.2.e) In the sample, we will not find any independent phosphorus atoms

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We will naturally wonder how the 2 atoms of hydrogen are kept together. Similarly, how the 4 atoms of phosphorus are kept together. In our day to day life, if we want to keep two or more objects together, we use methods like applying glue, or tying them together with a rope or using adhesive tape, etc., We will learn about the ‘forces that keep the atoms together’ in later chapters. At present, all we need to know is that atoms of some elements exist independently, and atoms of others exist in groups

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The groups of atoms are called Molecules. Molecules are the 'smallest particle of an element' which can exist independently. So we have a technical name for ‘groups’. The name is: ‘Molecules’. We can say:
• Hydrogen atoms cannot exist independently. They can exist only as ‘hydrogen molecules’. Each hydrogen molecule will contain two hydrogen atoms. The group shown in fig.1.2(c) is a hydrogen molecule
• Oxygen atoms cannot exist independently. They can exist only as ‘oxygen molecules’. Each oxygen molecule will contain two oxygen atoms. The group shown in fig.1.2(d) is an oxygen molecule
• Phosphorus atoms cannot exist independently. They can exist only as ‘Phosphorus molecules’. Each phosphorus molecule will contain four phosphorus atoms. The group shown in fig.1.2(e) is a phosphorus molecule

To give a general form for all elements, the term molecule is given to single atoms also, if they are able to exist independently. So we can say:
• Helium atoms can exist independently. Each helium atom (fig.1.2.a) is also a molecule of Helium
• Neon atoms can exist independently. Each neon atom (fig.1.2.b) is also a molecule of Neon.

So we see that, a molecule of an element can consist of one, or more than one of it’s atoms. Thus the following terms are used in chemistry:
• Molecules with only one atom are called Monoatomic molecules 
• Molecules with two atoms are called Diatomic molecules
• Molecules with more than two atoms are called Polyatomic molecules

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In the next section we will see molecules of Compounds.

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