In the previous section, we saw the Bohr model electronic configuration of atoms. In this section, we will see some solved examples.
Solved example 2.3
Write the electronic configuration of the following elements and draw their Bohr model
(i) N with Z = 7 and A = 14 (ii) Mg with Z = 12 and A = 24 (iii) S with Z = 16 and A = 32
Solution:
(i) • Given Z = 7 and A = 14.
• So number of protons = number of electrons = 7
• Number of neutrons = A - Z = 14 - 7 = 7. (Number of neutrons must be marked in the nucleus when we draw the Bohr model)
• The electronic configuration can be determined by putting the arrangement of the 7 electrons in a tabular form as shown in fig. 2.10 below:
• From the table we can see that, the first 2 electrons fill up the K-shell, and the remaining 5 electrons take positions in the L-shell. So we get K = 2 and L = 5. So the electronic configuration is: 2,5
• Once we get the configuration, the Bohr model can be easily drawn. In the fig.2.10 above, the inner circle (K-shell) has 2 electrons, and the outer circle (L-shell) has 5 electrons
(ii) • Given Z = 12 and A = 24
• So number of protons = number of electrons = 12
• Number of neutrons = A - Z = 24 - 12 = 12. (Number of neutrons must be marked in the nucleus when we draw the Bohr model)
• The electronic configuration can be determined by putting the arrangement of the 12 electrons in a tabular form as shown in fig. 2.11 below:
• From the table we can see that, the first 2 electrons fill up the K-shell, next 8 electrons will fill up the L-shell. Maximum no. of electrons that L-shell can hold is 8. So the remaining 2 electrons take positions in the M-shell. So we get K = 2, L = 8 and M = 2. So the electronic configuration is: 2,8,2
• Once we get the configuration, the Bohr model can be easily drawn. In the fig.2.11 above, the inner circle (K-shell) has 2 electrons, the next outer circle (L-shell) has 8 electrons, and the outer most circle (M-shell) has 2 electrons.
(iii) • Given Z = 16 and A = 32
• So number of protons = number of electrons = 16
• Number of neutrons = A - Z = 32 - 16 = 16. (Number of neutrons must be marked in the nucleus when we draw the Bohr model)
• The electronic configuration can be determined by putting the arrangement of the 16 electrons in a tabular form as shown in fig. 2.12 below:
• From the table we can see that, the first 2 electrons fill up the K-shell, next 8 electrons will fill up the L-shell. Maximum no. of electrons that L-shell can hold is 8. So the remaining 6 electrons take positions in the M-shell. We get K = 2, L = 8 and M = 6. So the electronic configuration is: 2,8,6
• Once we get the configuration, the Bohr model can be easily drawn. In the fig.2.12 above, the inner circle (K-shell) has 2 electrons, the next outer circle (L-shell) has 8 electrons, and the outer most circle (M-shell) has 6 electrons.
Solved example 2.4
Bohr model of Al (Aluminium) is shown in fig.2.13 below. Analyse it and answer the following questions:
(i) Write the Atomic number and mass number of aluminium
(ii) Write the number of protons, neutrons and electrons in an aluminium atom
(iii) Write the electronic configuration of aluminium
Solution:
(i) In the Bohr model, it is written 13p. So the number of protons = 13. Also it is written 14n. So the number of neutrons = 14. Thus we get:
• Atomic number Z = No. of protons = 13
• Mass number A = No. of protons + No. of neutrons = 13 + 14 = 27
(ii) • Number of protons = 13. • No. of neutrons = 14
• No. of electrons in the k-shell = 2, No. of electrons in the L-shell = 8, and the no. of electrons in the M-shell = 3. Thus total no. of electrons = 2 + 8 + 3 = 13
(iii) We wrote the number of electrons in each shell in the previous step. From that, we get electronic configuration = 2,8,3
Solved example 2.5
The mass number of an atom = 31. It has 5 electrons in the M-shell.
(i) Write the electronic configuration of this atom
(ii) What is the atomic number of this atom ?
(iii) How many neutrons does this atom have?
(iv) Draw the Bohr model of this atom
Solution:
(i) Given that the M-shell contains 5 electrons. This is shown in fig.2.14(a) below:
• The M-shell can contain upto 2 × 32 = 18 electrons
• If the M-shell is the outermost shell, it can contain upto 8 electrons
• In our case it contains only 5 electrons.
• So we can conclude that, in the given atom, the M-shell is the outermost shell. Because, if it were an inner shell, it would have contained more than 5 electrons
• Two other conclusions can also be made:
♦ The inner K-shell is completely filled up
♦ The inner L-shell is also completely filled up
• The above conclusions are made on the fact that, occupation of M can begin only when K and L are completely filled up. So we get: 2 electrons in K, 8 electrons in L and 5 electrons in M
• Thus the electronic configuration is 2,8,5
(ii) The total number of electrons = 2 + 8 + 5 = 15. So the number of protons is also equal to 15. Thus atomic number Z = 15
(iii) Mass number A is given as 31. So number of neutrons = A - Z = 31 - 15 = 16
(iv) The Bohr model of the given atom is shown in fig.2.14(b) above
In the next section we will see 'Isotopes'.
Solved example 2.3
Write the electronic configuration of the following elements and draw their Bohr model
(i) N with Z = 7 and A = 14 (ii) Mg with Z = 12 and A = 24 (iii) S with Z = 16 and A = 32
Solution:
(i) • Given Z = 7 and A = 14.
• So number of protons = number of electrons = 7
• Number of neutrons = A - Z = 14 - 7 = 7. (Number of neutrons must be marked in the nucleus when we draw the Bohr model)
• The electronic configuration can be determined by putting the arrangement of the 7 electrons in a tabular form as shown in fig. 2.10 below:
 |
| Fig.2.10 |
• Once we get the configuration, the Bohr model can be easily drawn. In the fig.2.10 above, the inner circle (K-shell) has 2 electrons, and the outer circle (L-shell) has 5 electrons
(ii) • Given Z = 12 and A = 24
• So number of protons = number of electrons = 12
• Number of neutrons = A - Z = 24 - 12 = 12. (Number of neutrons must be marked in the nucleus when we draw the Bohr model)
• The electronic configuration can be determined by putting the arrangement of the 12 electrons in a tabular form as shown in fig. 2.11 below:
 |
| Fig.2.11 |
• Once we get the configuration, the Bohr model can be easily drawn. In the fig.2.11 above, the inner circle (K-shell) has 2 electrons, the next outer circle (L-shell) has 8 electrons, and the outer most circle (M-shell) has 2 electrons.
(iii) • Given Z = 16 and A = 32
• So number of protons = number of electrons = 16
• Number of neutrons = A - Z = 32 - 16 = 16. (Number of neutrons must be marked in the nucleus when we draw the Bohr model)
• The electronic configuration can be determined by putting the arrangement of the 16 electrons in a tabular form as shown in fig. 2.12 below:
 |
| Fig.2.12 |
• Once we get the configuration, the Bohr model can be easily drawn. In the fig.2.12 above, the inner circle (K-shell) has 2 electrons, the next outer circle (L-shell) has 8 electrons, and the outer most circle (M-shell) has 6 electrons.
Solved example 2.4
Bohr model of Al (Aluminium) is shown in fig.2.13 below. Analyse it and answer the following questions:
 |
| Fig.2.13 |
(ii) Write the number of protons, neutrons and electrons in an aluminium atom
(iii) Write the electronic configuration of aluminium
Solution:
(i) In the Bohr model, it is written 13p. So the number of protons = 13. Also it is written 14n. So the number of neutrons = 14. Thus we get:
• Atomic number Z = No. of protons = 13
• Mass number A = No. of protons + No. of neutrons = 13 + 14 = 27
(ii) • Number of protons = 13. • No. of neutrons = 14
• No. of electrons in the k-shell = 2, No. of electrons in the L-shell = 8, and the no. of electrons in the M-shell = 3. Thus total no. of electrons = 2 + 8 + 3 = 13
(iii) We wrote the number of electrons in each shell in the previous step. From that, we get electronic configuration = 2,8,3
Solved example 2.5
The mass number of an atom = 31. It has 5 electrons in the M-shell.
(i) Write the electronic configuration of this atom
(ii) What is the atomic number of this atom ?
(iii) How many neutrons does this atom have?
(iv) Draw the Bohr model of this atom
Solution:
(i) Given that the M-shell contains 5 electrons. This is shown in fig.2.14(a) below:
 |
| Fig.2.14 |
• If the M-shell is the outermost shell, it can contain upto 8 electrons
• In our case it contains only 5 electrons.
• So we can conclude that, in the given atom, the M-shell is the outermost shell. Because, if it were an inner shell, it would have contained more than 5 electrons
• Two other conclusions can also be made:
♦ The inner K-shell is completely filled up
♦ The inner L-shell is also completely filled up
• The above conclusions are made on the fact that, occupation of M can begin only when K and L are completely filled up. So we get: 2 electrons in K, 8 electrons in L and 5 electrons in M
• Thus the electronic configuration is 2,8,5
(ii) The total number of electrons = 2 + 8 + 5 = 15. So the number of protons is also equal to 15. Thus atomic number Z = 15
(iii) Mass number A is given as 31. So number of neutrons = A - Z = 31 - 15 = 16
(iv) The Bohr model of the given atom is shown in fig.2.14(b) above
In the next section we will see 'Isotopes'.
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