In the previous section, we saw the Bohr model electronic configuration of atoms. We also saw some solved examples. In this section, we will learn about Isotopes, Isobars and Isotones.
We have seen that the number of protons in an atom is unique. It is the atomic number Z, and it will not change. It is the identity of the element. We have also seen that, in addition to protons, the nucleus contains neutrons also. The number of neutrons may not be the same in all atoms of the same element. Consider an example:
• In fig.2.15(a), the atom has 1 proton, 1 electron, and 0 neutron. So it's mass number A = Z + No. Of neutrons = 1 + 0 = 1
• In fig(b), the atom has 1 proton, 1 electron, and 1 neutron. So it's mass number A = Z + No. Of neutrons = 1 + 1 = 2
• In fig(c), the atom has 1 proton, 1 electron, and 2 neutrons. So it's mass number A = Z + No. Of neutrons = 1 + 2 = 3
In all the three cases, the number of protons is the same. So the three atoms belong to the same element. Though they belong to the same element, the mass is different. So we need a method to distinguish between the three. For this, we use the term 'isotopes'. In the above fig.2.15, the three atoms are 'isotopes of the same element'.
In the fig., the element shown is hydrogen. Because Z = 1. So we say: The three atoms are 'isotopes of hydrogen'. The three isotopes of hydrogen are:
• Protium Z = 1, A = 1
• Deutirium Z =1, A = 2
• Tritium Z = 1, A = 3
(a) represents Protium, (b) represents Deuterium, and (c) represents Tritium
In the same way, the isotopes of carbon can be represented as:
(a) represents Carbon-12, (b) represents Carbon-13, and (c) represents Carbon-14
Uses of isotopes
Isotopes find application in many fields. Some examples are:
• Carbon-14 is used to determine the age of fossils and prehistoric objects
• Deuterium is used in atomic reactors
So we have seen the details about isotopes. It involves the combination of two items: (i) Atomic number Z (same), and (ii) Mass number A (different)
Other combinations are also possible. Consider the example given below:
(a) is Argon. It has Z = 18 and A = 40
(b) is Calcium. It has Z = 20 and A = 40
Note that both have A same. Such elements having same mass number but different atomic numbers are called isobars. The combination here is: (i) Atomic number Z (different), and (ii) Mass number A (same).
Another combination that is possible is, (i) Atomic number (different) and (ii) Number of neutrons (same). An example is given below:
(a) is Nitrogen. It has Z = 7 and A = 15. There fore number of neutrons = 15 -7 = 8
(b) is Carbon-14. It has Z = 6 and A = 14. There fore number of neutrons = 14 - 6 = 8
Number of neutrons is the same. Such elements having different atomic numbers but same number of neutrons are called isotones.
We will now see some solved examples
Solved example 2.6
Bohr models of atoms A, B, C and D (symbols are not real) are given below:
(a) Write the atomic number, mass number and electronic configuration of the atoms
(b) Among these, which are isotopes? why?
Solution:
■ (A) has 6 protons in it's nucleus. So the atomic number Z = 6
• It has 6 neutrons in the nucleus. So mass number = No. of protons + No. of neutrons = 6 + 6 = 12
• It has 2 electrons in the K-shell and 4 electrons in the L-shell. So the electronic configuration is 2,4
■ (B) has 7 protons in it's nucleus. So the atomic number Z = 7
• It has 8 neutrons in the nucleus. So mass number = No. of protons + No. of neutrons = 7 + 8 = 15
• It has 2 electrons in the K-shell and 5 electrons in the L-shell. So the electronic configuration is 2,5
■ (C) has 6 protons in it's nucleus. So the atomic number Z = 6
• It has 8 neutrons in the nucleus. So mass number = No. of protons + No. of neutrons = 6 + 8 = 14
• It has 2 electrons in the K-shell and 4 electrons in the L-shell. So the electronic configuration is 2,4
■ (D) has 8 protons in it's nucleus. So the atomic number Z = 8
• It has 8 neutrons in the nucleus. So mass number = No. of protons + No. of neutrons = 6 + 8 = 14
• It has 2 electrons in the K-shell and 6 electrons in the L-shell. So the electronic configuration is 2,6
■ Among the above four atoms, (A) and (C) are isotopes because they both have the same atomic number Z = 6, and different mass numbers. (A) has mass number 12, and (C) has mass number 14. It can be represented as:
Solved example 2.7
Symbols (not real symbols) of some atoms are given below
(a) Find the atomic number and mass number of these elements
(b) Which among these are isotopic pairs
(c) Draw the bohr model of atom Q
Solution:
a. (P) has atomic number Z = 8 and mass number A = 17
(Q) has atomic number Z = 18 and mass number A = 36
(R) has atomic number Z = 8 and mass number A = 16
b. (P) and (R) are isotopic pairs. because they have the same atomic number Z = 8, and different mass numbers. P has mass number = 17 and Q has mass number = 16
c.The Bohr model of atom Q is shown below:
In the next chapter we will see 'Chemical bonding'.
We have seen that the number of protons in an atom is unique. It is the atomic number Z, and it will not change. It is the identity of the element. We have also seen that, in addition to protons, the nucleus contains neutrons also. The number of neutrons may not be the same in all atoms of the same element. Consider an example:
 |
| Fig.2.15 |
• In fig(b), the atom has 1 proton, 1 electron, and 1 neutron. So it's mass number A = Z + No. Of neutrons = 1 + 1 = 2
• In fig(c), the atom has 1 proton, 1 electron, and 2 neutrons. So it's mass number A = Z + No. Of neutrons = 1 + 2 = 3
In all the three cases, the number of protons is the same. So the three atoms belong to the same element. Though they belong to the same element, the mass is different. So we need a method to distinguish between the three. For this, we use the term 'isotopes'. In the above fig.2.15, the three atoms are 'isotopes of the same element'.
In the fig., the element shown is hydrogen. Because Z = 1. So we say: The three atoms are 'isotopes of hydrogen'. The three isotopes of hydrogen are:
• Protium Z = 1, A = 1
• Deutirium Z =1, A = 2
• Tritium Z = 1, A = 3
Representation of Isotopes
We have seen that the elements are represented by their symbols. We have also seen that the Z is written on left side bottom, and A on left side top. This same method can be used to represent isotopes also. The isotopes of hydrogen are represented as shown below:
In the same way, the isotopes of carbon can be represented as:
Uses of isotopes
Isotopes find application in many fields. Some examples are:
• Carbon-14 is used to determine the age of fossils and prehistoric objects
• Deuterium is used in atomic reactors
So we have seen the details about isotopes. It involves the combination of two items: (i) Atomic number Z (same), and (ii) Mass number A (different)
Other combinations are also possible. Consider the example given below:
(b) is Calcium. It has Z = 20 and A = 40
Note that both have A same. Such elements having same mass number but different atomic numbers are called isobars. The combination here is: (i) Atomic number Z (different), and (ii) Mass number A (same).
Another combination that is possible is, (i) Atomic number (different) and (ii) Number of neutrons (same). An example is given below:
(b) is Carbon-14. It has Z = 6 and A = 14. There fore number of neutrons = 14 - 6 = 8
Number of neutrons is the same. Such elements having different atomic numbers but same number of neutrons are called isotones.
We will now see some solved examples
Solved example 2.6
Bohr models of atoms A, B, C and D (symbols are not real) are given below:
(a) Write the atomic number, mass number and electronic configuration of the atoms
(b) Among these, which are isotopes? why?
Solution:
■ (A) has 6 protons in it's nucleus. So the atomic number Z = 6
• It has 6 neutrons in the nucleus. So mass number = No. of protons + No. of neutrons = 6 + 6 = 12
• It has 2 electrons in the K-shell and 4 electrons in the L-shell. So the electronic configuration is 2,4
■ (B) has 7 protons in it's nucleus. So the atomic number Z = 7
• It has 8 neutrons in the nucleus. So mass number = No. of protons + No. of neutrons = 7 + 8 = 15
• It has 2 electrons in the K-shell and 5 electrons in the L-shell. So the electronic configuration is 2,5
■ (C) has 6 protons in it's nucleus. So the atomic number Z = 6
• It has 8 neutrons in the nucleus. So mass number = No. of protons + No. of neutrons = 6 + 8 = 14
• It has 2 electrons in the K-shell and 4 electrons in the L-shell. So the electronic configuration is 2,4
■ (D) has 8 protons in it's nucleus. So the atomic number Z = 8
• It has 8 neutrons in the nucleus. So mass number = No. of protons + No. of neutrons = 6 + 8 = 14
• It has 2 electrons in the K-shell and 6 electrons in the L-shell. So the electronic configuration is 2,6
■ Among the above four atoms, (A) and (C) are isotopes because they both have the same atomic number Z = 6, and different mass numbers. (A) has mass number 12, and (C) has mass number 14. It can be represented as:
Solved example 2.7
Symbols (not real symbols) of some atoms are given below
(a) Find the atomic number and mass number of these elements
(b) Which among these are isotopic pairs
(c) Draw the bohr model of atom Q
Solution:
a. (P) has atomic number Z = 8 and mass number A = 17
(Q) has atomic number Z = 18 and mass number A = 36
(R) has atomic number Z = 8 and mass number A = 16
b. (P) and (R) are isotopic pairs. because they have the same atomic number Z = 8, and different mass numbers. P has mass number = 17 and Q has mass number = 16
c.The Bohr model of atom Q is shown below:
In the next chapter we will see 'Chemical bonding'.
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