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:
• 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)
• 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
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
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
In the next section we will see molecules of Compounds.
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 |
• 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 |
• 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
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
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
In the next section we will see molecules of Compounds.
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