Structure Of The Atom

---

4.1 Thomson’s model of an atom-

•JJ Thomson was the first one to propose a model of a structure of an atom.
•He compared atom with Christmas pudding and watermelon. Therefore this model is also known as plum pudding model

•He proposed that:
– The atom consists of a positively charged sphere and negatively charged. electrons are embedded in it like seeds are spread in red eatable part of the watermelon.
-The positive and negative charges are equal in charges.

---

4.2 Rutherford’s model of an atom-

•This model was proposed by Ernest Rutherford.
•Experiment- fast-moving Alpha particles you are made to fall on thin gold foil.

•Following observation was made after the experiment :
1- Most of the Alpha particles passed straight from the gold foil because most of the space inside the atom is empty.
2- Some of the Alpha particles were deflected by the small angles because the positive charge of an atom occupies very little space.
3- Out of every 12000 particles appeared to rebound this is because a very small fraction of Alpha particles were deflected by 180, Indicating that all the positive charge and mass of gold atom were concentrated in very small the volume within the atom.

•Features of Rutherford Model of an atom –

1- The positively charged centre in an atom is called nucleus where all the mass of an atom resides.
2- Electron revolve around the nucleus in a circular path.
3- Size of nucleus is very small as compared to that of the atom.

•Drawbacks of Rutherford model of an atom.

The revolution of an electron around the nucleus in a circular path is not stable and undergo acceleration. During acceleration, charged particle radiate energy and finally, electron fall into the nucleus.

---

4.3- Bohr’s model of an atom-

•Given by -Neil Bohr

• Postulates of Bohr’s model of an atom –

1- Only special orbits are there inside the atom in which electrons revolve.
2- These discrete orbits are called energy levels.
3- Electrons revolve in these discrete orbits without radiating energy. These energy levels are named as k, L, M, N or by the number N = 1 2 3 4.
( n=1) k shell
(n=2) L shell
(n=3)M shell
(n=4)N shell …….so on

---

4.4-Neutrons-

•Given by-J. Chadwick.
•Particle which do not have a charge but have mass nearly equal to that of the proton.
•Represented by ‘n’.

---

4.5Atomic number and Mass number-

Atomic number-• It is defined as a number of proton of an atom.
• Atomic number is denoted by Z.

Example –

Atomic number of that of hydrogen atom =1 because only one proton is. present in the nucleus.
Atomic number of oxygen atom =8 because protons are present in the nucleus.

Atomic mass-

• It is defined as a sum of the total number of protons and neutrons present in a nucleus (nucleons).
• It is denoted by A.
• Atomic mass = number of protons + number of neutrons
Example- Mass of carbon=6u+6u=12u.

Mass number. A
Symbol of an element. X
Atomic number. Z

---

4.6How are electrons distributed in different orbits?

Here are rules for writing electrons at different levels:
•Formula for the maximum number of electrons in the Shell=2n²
Where n=orbit as
( n=1) k shell=2 electrons
(n=2) L shell=8 electrons
(n=3)M shell=18electrons
(n=4)N shell =32 electrons…….so on

•Maximum number of electrons that can be accommodated in outermost shell is 8.
•Electrons are not filled in the outer shell until inner shells are filled.

---

4.7. Valency-

• Valence Electrons-electrons present in the outermost shell of an atom.
• Outermost shell of an atom contains a maximum of 8 electrons and this atom are said to be stable atom except for Helium atom which has two electrons in its outermost shell.
• Combining capacity of the atom is defined as the tendency to react and form molecules with an atom of the same or different elements.
• Every atom have the tendency to make itself stable. In order to complete the octet, atoms gain/ loss or share an electron.
• Example- Lithium has 1 electron in its outermost shell. Lithium loses one electron. Valency of lithium Is 1.

---

4.8 Isotopes-

Atoms of the same element having the same atomic number but a different mass number.
•The chemical properties of isotopes are similar but their physical properties and different.

•Application-

1- An isotope of Uranium- fuel in a nuclear reactor.
2- An isotope of Cobalt -cancer.
3- An isotope of iodine -treatment of goitre.
Example-Hydrogen atom has three isotopes:
– Protium
– ²H deuterium
– ³H tritium

---

4.9 Isobars –

The atom of different elements with different atomic number and the same mass number.
Example-

Calcium -Atomic no.=20
Atomic mass=40
Argon-Atomic no.=18
Atomic mass=40
So calcium and argon are isobars.

---

NCERT Solutions

---

Page: 47

---

1. What are the canal rays?

Answer:

The positively charged radiations are called canal rays.

2. If an atom contains one electron and one proton, will it carry any charge or not?

Answer:
No, it will not contain any charge because p sum of positive charge and the negative charge will make net charge zero. So it will carry a neutral charge.

---

Page 49

---

1. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer:

Atom is neutral as whole because according to Thomson’s model of an atom, negative and positive charges are equal in magnitude in an atom. So their net charge will be zero or electrically neutral.

2. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Answer:

According to Rutherford’s model of an atom, an atom consists of positively a charged particle which we call protons.

3. Draw a sketch of Bohr’s model of an atom with three shells.

Answer:

4. What do you think would be the observation if the ∝– particle scattering experiment is carried out using a foil of a metal other than
gold?
Answer:

If the foil of metal is used rather than gold oil then observation of the experiment remain the same.

---

Page 49

---

1. Name the three subatomic particles of an atom.
Answer:

Three subatomic particles of an atom-
• Positively charged- Protons
• Negatively charged-electrons
• Neutrally charged -neutron.

2. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Answer:

The atomic mass of helium atom = 4u in which no. Of protons =2

We know that,
Atomic mass = number of protons + number of neutrons
4 = 2 + number of neutrons
Number of neutrons = 4 – 2
= 2 neutrons

---

Page: 50

---

1. Write the distribution of electrons in Carbon and Sodium atoms.
Answer:

•Carbon atoms: atomic number = 6
distribution is K = 2, L = 4. Or 2,4
•Sodium atoms: atomic number = 11
distribution is K =2, L =8, M = 1 or 2,8,1

2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer:

If K and L shells are filled with an electron in an atom then the total number of electrons k shell =2,l shell =8
2+8 = 10 electrons.

---

Page: 52

---

1. How will you find the valency of chlorine, sulphur and magnesium?
Answer:

•Chlorine(Cl)-
atomic number – 17
So, K – 2, L – 8, M – 7
Valency =-1( since only 1 electron is required to complete octet)

• sulphur:
atomic number = 16
atomic number- 16
So, K – 2, L – 8, M – 6
valency= -2 (since only electron is required to complete octet).

•Magnesium:
atomic number = 12
So, K – 2, L – 8, M – 2
Valency=+2

---

Page: 52

---

1. If the number of electrons in an atom is 8 and number of protons is also 8, then
(i) What is the atomic number of the atom? and
(ii) What is the charge on the atom?
Answer:

(i) The atomic number of an atom=8.As the number of protons in that atom is equal to the atomic number of an atom.
(i)It does not possess any charge because the charge of electrons and protons cancel out each other.

2. With the help of given Table, find out the mass number of oxygen and sulphur atom.

Table: Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells.

Exercise

---

1. Compare the properties of electrons, protons and neutrons.
Answer:

Electrons	Protons	Neutrons
They are Negatively charged	They are Positively charged	No charge.
Revolve around the nucleus	Present inside the nucleus	Located inside the nucleus of an atom
negligible Mass	Mass is of 1 a.m.u	1 a.m.u

2. What are the limitations of J.J.Thomson’s model of the atom?
Answer:

The following are the limitations of J.J. Thomson’s model of an atom.
-Failed to explain alpha particle experiment.
-no experimental shreds of evidence

3. What are the limitations of Rutherford’s model of the atom?
Answer:

The revolution of an electron around the nucleus in a circular path is not stable and undergo acceleration. During acceleration, charged particle radiate energy and finally electron fall into the nucleus.

4. Describe Bohr’s model of the atom.
Answer:

Given by -Neil Bohr
1- Only special orbits are there inside the atom in which electrons revolve.
2- These discrete orbits are called energy levels.
3- Electrons revolve in these discrete orbits without radiating energy. These
energy levels are named as k, L, M, N or by the number N = 1 2 3 4.
( n=1) k shell
(n=2) L shell
(n=3)M shell
(n=4)N shell …….so on

5. Compare all the proposed models of an atom given in this chapter.

Answer:

Thomson Model	Rutherford Model	Bohr model of atom
The atom consists of a positively charged sphere and negatively charged. Electrons are embedded in it like seeds are spread in red eatable part of the watermelon.	1-The positively charged centre in an atom is called nucleus where all the mass of an atom resides.	Only special orbits are there inside the atom in which electrons revolve.
The positive and negative charges are equal in charges.	Electron revolve around the nucleus in a circular path.	These discrete orbits are called energy levels.
	Size of the nucleus is very small as compared to that of the atom.	Electrons revolve in these discrete orbits without radiating energy. These energy levels are named as k,L,M,N or by the number [ N = 1 2 3 4. ( n=1) k shell (n=2) L shell (n=3)M shell (n=4)N shell …….so on

6-Summarise the rules for the writing of distribution of electrons in various shells for the first eighteen elements.

Answer:

Here are rules for writing electrons at different levels:

•Formula for the maximum number of electrons in the Shell=2n^2
Where n=orbit as
( n=1) k shell=2 electrons
(n=2) L shell=8 electrons
(n=3)M shell=18electrons
(n=4)N shell =32 electrons…….so on.
•Maximum number of electrons that can be accommodated in outermost shell is 8.

•Electrons are not filled in the outer shell until inner shells are filled.

7- Define valency by taking examples of silicon and oxygen.
Answer:

Combining the capacity of the atom is defined as valency. Every atom has the tendency to make itself stable. In order to complete the octet, atoms gain/ loss or share an electron.
Example
1-silicon: atomic number = 14
Electronic configuration =2, 8, 4
to fill the M shell 4 electrons are required
Therefore its valency is 4.
2-oxygen: atomic number =8
Electronic configuration = 2, 6
to fill the M shell 2 electrons are required
Therefore its valency is 2

8. Explain with examples:
(i) Atomic number (ii) Mass number,
(iii) Isotopes and (iv) Isobars.
Give any two uses of isotopes.
Answer:

(i) Atomic number- It is defined as a number of proton of an atom.
Example-

•Atomic number of that of hydrogen atom =1 because only one proton is. present in the nucleus.
•Atomic number of oxygen atom =8 because protons are present in the nucleus.

(ii) Mass number- It is defined as a sum of the total number of protons and neutrons present in a nucleus (nucleons).
Example-

Mass of carbon=6u+6u=12u.

(iii) Isotopes -Atoms of the same element having the same atomic number but a different mass number.
Example-

A hydrogen atom has three isotopes:
-Protium
-²H deuterium
-³H tritium

(iv) Isobars-Atom of different elements with different atomic number and same mass number.
Example-

calcium -Atomic no.=20
Atomic mass=40
Argon-Atomic no.=18
Atomic mass=40
So calcium and argon are isobars.

Give any two uses of isotopes.
1- An isotope of Uranium as fuel in a nuclear reactor.
2- An isotope of Cobalt for cancer.

---

9- Na+ has completely filled K and L shells. Explain.
Answer:

Atomic no. of sodium=11
The electronic configuration is K=2
L-=8
M=1
Now Na+ means one electron will lose from outermost orbit and new
the configuration will be K=2
L-=8
Which is a filled state. Hence it is very stable.

10 -If bromine atom is available in the form of, say, two isotopes ₃₅Br⁷⁹ (49.7%) and ₃₅Br⁸¹  (50.3%), calculate the average atomic
mass of Bromine atom.
Answer:

It is given that The atomic masses of two isotopic atoms
₃₅Br⁷⁹ = (49.7%)
₃₅Br⁸¹= (50.3%).
total mass
= (79 x 49.7 / 100) + (81 x 50.3 / 100)
= 39.263 + 40.743
= 80.006 u

11- The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes  ₈X¹⁶ and ₈X¹⁸ in the sample?
Answer:

Let us assume,
the percentage of

₈X¹⁶ =‘y’
₈X¹⁸ = ‘100-y’.
So,
16 y / 100 + 18 (100-y) /100=16.2u
16y +1800-18y/100=16.2 u
16y – 18y + 1800 /100=16.2 u
1800 – 2y=1620u
y= 90%
Hence, the percentage of ₈X¹⁶ = 90%
₈X¹⁸ = 100-y = 100- 90=10%

12. If Z=3, what would be the valency of the element? Also, name the element.
Answer:

The atomic number of an element (Z) = 3
So,electronic configuration = 2,1
Therefore,its valency = 1
Element is Lithium.

13. Composition of the nuclei of two atomic species X and Y are given as under
                 X. Y
Protons = 6. 6
Neutrons = 6. 8
Give the mass numbers of X and Y. What is the relation between the two species?
Answer:

For X, Mass number of X:=Protons + neutrons
No .of Protons = 6.
No .of Neutrons = 6
Mass number of X = 6+6
= 12
For Y, Mass number of X:=Protons + neutrons
No .of Protons = 6.
No .of Neutrons = 8
The mass number of Y = 6+8
= 14
They differ in the number of neutrons But same atomic no. So they are isotopes.

14. For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore,it is neutral.
(c) The mass of an electron is about 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Answer:

(a) False
(b) False
(c) True
(d) False

Put tick against correct choice and cross (x) against wrong choice in questions 15, 16 and 17.

15-Rutherford’s alpha – particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron

Answer:

(a) Atomic nucleus

16-Isotopes of an element have
(a) The same physical properties
(b) Different chemical properties
(c) Different number of neutrons
(d) Different atomic numbers.

Answer:

(c) different number of neutrons

17. Number of valence electrons in Cl– ion are:
(a) 16
(b) 8
(c) 17
(d) 18

Answer:

(b) 8

18-Which one of the following is a correct electronic configuration of
Sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1

Answer:

(d) 2, 8, 1

19-Complete the following table.

Atomic number	Mass number	Number of neutrons	Number of Protons	Number of electrons	Name of the atomic species
9 16 – – –	– 32 24 2 1	10 – – – 0	– – 12 1 1	– – – – 0	–Sulphur – – –

Answer:

As we know that
Atomic number of atom =Number of proton
Mass number of atom= Number of neutrons + atomic number

Atomic number	Mass number	Number of neutrons	Number of Protons	Number of electrons	Name of the atomic species
9 16 12 1 1	19 32 24 2 1	10 16 12 1 0	9 16 12 1 1	9 16 12 1 0	Fluorine Sulphur Magnesium Deuterium Hydrogen