In this experiment, a glass tube with two holes was used. One of the hotels acted as the outlet for the vacuum pump, and the other hole was the inlet for the gas pumping. The vacuum pump attached was added to maintain a 'partial vacuum' inside the chamber. Using electrodes, a high voltage supply is attached, and the anode is placed within the tube. Through this experiment, Thompson mentioned the science behind the following discoveries: 

• He mentioned that the structure of atoms is a positive circle, and the negative electrons were confined. 
• He called it the ‘plum pudding model’ since the pudding could be imagined as the atom and the electrons as the plum bits. 
• This model stated that atoms are electrically neutral and that the positive and negative charges were equal in magnitude. 

Thus, based on Thompson Model, an atom is stated to be neutral as: 

(i) An atom is composed of a positively charged sphere, and the electrons are embedded in it. 

(ii) The negative and positive charges are equal in magnitude. Hence, the atom is electrically neutral.

Through his gold foil and alpha particles experiment, Rutherford concluded that the centre of the atom, called the nucleus and is much smaller in size than the atom, is positively charged, and electrons which are negatively charged, circulate the nucleus in a circular pattern. However, there were drawbacks to this model. IIf the electrons circulated in a circular pattern, they would undergo acceleration and, as a result, radiate energy. This would lead them to lose some energy and ultimately fall into the nucleus's structure, thus stating that the atom is extremely unstable and the matter is not how it exists.

Bohr’s Model 

In order to overcome the drawbacks that were raised in Rutherford's model, Neils Bohr made a few postulates stating that only discrete orbits of electrons are allowed inside the atom, and these discrete orbits do not radiate any energy while revolving around the nucleus. He came up with the theory that orbits around the nucleus called energy levels.

Neutrons

In 1932, the presence of neutrons was discovered by J. Chadwick. He stated that it was a particle with a mass almost equal to a proton and contained no charge. Hence, the sum of protons and neutrons defines the mass of an atom. Neutrons are present in all atoms except Hydrogen.

Distribution of Electrons

According to Bohr and Bury, the distribution of electrons is given by the formula '2n', and" here stands for the orbit number. Meaning on the first shell, the maximum number of electrons is 2x12= 2, and on the second one, it would be 2x22= 8. They also stated that the orbits are filled stepwise, meaning the outer orbit will only be filled if the inner one is already filled. They also mentioned that the maximum number of electrons present on the outermost orbit is 8.

Valency

The electrons on atoms are present in different orbits. However, those in the outer orbit are called valence electrons. Now that they had already discovered that the maximum electrons on the outermost shell are 8, atoms with filled outermost shells are not chemically very active. Their valency is zero, meaning their combining capacity is zero.

The combining capacity is the atom’s tendency to combine with atoms of the same or different elements to form molecules and complete 8 electrons on the outermost shell and gain the octet. Atoms either gain or lose electrons to achieve this. This gives us the valency, which refers to the combining capacity of an atom, i.e. the number of electrons an atom gains or loses to achieve the octet is the valency number given to the particular atom.  For example, a sodium atom has 1 electron on its outermost orbit, which means it can lose one electron to another atom and form an octet. Thus, sodium’s valency is given as 1.

If the number of electrons on the outer orbit is closer to 8, the valency is determined accordingly. For example, fluorine has 7 electrons on its outermost orbit. The valency could be given as 7; however, it is easier for fluorine to gain an electron than lose 7. Thus, the valency of fluorine becomes 1.

Atomic Number and Mass Number

The atomic number is determined by the number of protons in the atom's nucleus. This is denoted by 'Z', and all atoms of the same element have the same atomic number. For example, Carbon's atomic mass is six since it has 6 protons in its nucleus.

As mentioned earlier, the mass of an atom is defined by the protons and neutrons present in the nucleus. Protons and neutrons together are called nucleons. For example, the mass of aluminium is 27 u (sum of 13 protons and 14 neutrons)

Isotopes

Isotopes refer to the atoms of the same element with the same atomic number but different mass numbers. For example, Hydrogen has three isotopes: protium (atomic number 1, mass number 1), deuterium (atomic number 1, mass number 2) and tritium (atomic number 1, mass number 3). These isotopes have the same atomic number but different mass numbers. Each isotope of an element can be called a pure substance. While the chemical properties of isotopes are similar, their physical properties vary.

Some elements consist of a mixture of different isotopes. In this case, the percentage of both isotopes is considered, and the average mass of both is considered the mass of the element. Let us take the example of Chlorine, which occurs in two isotopic forms which have the masses of 35u and 37u and are present in the ratio of 3:1. To calculate the mass of Chlorine, the average mass of both isotopes is calculated as per the percentage of presence, and the value is said to be the mass of the atom. In the case of Chlorine, it would be 35.5u.

The application of isotopes is all around us. For instance, an isotope of uranium is used as a fuel in nuclear reactors, and an isotope of cobalt contributes to the treatment of cancer. An isotope of iodine is beneficial for the treatment of goitre.

Isobars

Atoms of the same element with the same mass number but different atomic numbers are called isobars. For example, the mass number for calcium and argon is 40, but the atomic number for calcium is 20, and for argon it is 18.

Page 49

Question 1. Based on Thomson’s model of an atom, explain how the atom is neutral as a whole.

Answer: According to Thomson’s model of an atom

(i) An atom consists of a positively charged sphere, and the electrons are embedded in it,

(ii) The negative and positive charges are equal in magnitude. So the atom is electrically neutral.

Question 2. Based on Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?

Answer: As per Rutherford's model of an atom, the positively charged protons are present in the nucleus of an atom.

Question 3. Sketch of Bohr’s model of an atom with three shells.

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

Answer: On using any metal foil, the observations of the a-particle scattering experiment would remain the same as all atoms would have the same structure.

Page 49

Question 1. Name the three subatomic particles of an atom.

Answer: The subatomic particles of an atom are

Protons: positively charged

Electrons: Negatively Charged

Neutrons: Neutrally Charged

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

Answer: 2 neutrons

Page 50 

Question 1. Write the distribution of electrons in carbon and sodium atoms.

Answer:

Carbon:

Atomic Number: 6

No. of Protons: 6

No. of Electrons= No. of Protons= 6

Distribution of Electrons: K   L

2    4

Sodium: 

Atomic Number: 11

No. of Protons: 11

No. of Electrons= No. of Protons= 11

Distribution of Electrons: K   L    M

2   8    1

Question 2. If an atom's K and L shells are full, what would be the total number of electrons in the atom?

Answer: K shell can hold 2 electrons, and L shell can hold 8 electrons. When both the shells are full, there will be (8 + 2) 10 electrons in the atom.

Page 52

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

Answer:

Chlorine:

Atomic Number: 17

Protons= Electrons= 17

Distribution of Electrons: K   L   M

2   8   7

Hence, it needs 1 electron to complete the octet structure. Thus, the valency is -1 (needs to gain 1 electron)

Sulphur:

Atomic Number: 16

Protons= Electrons= 16

Distribution of Electrons: K   L   M

2    8   6

Hence, it needs 1 electron to complete the octet structure. Thus, the valency is -2 (needs to gain 2 electrons)

Magnesium

Atomic Number: 12

Protons= Electrons= 12

Distribution of Electrons: K   L   M

2   8    2

Hence, it needs to donate 2 electrons. Thus, the valency is +2 (needs to donate 2 electrons)

Page 52

Question 1. If the number of electrons in an atom is eight and the 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:

No. of electrons= no. of Protons= Atomic Number= 8

(i) 8

(ii) As electrons=protons, charge is neutral

Question 2. With the help of given Table 4.1, find out the mass number of oxygen and sulphur atoms.

Answer: 

Oxygen: 

No. of Protons= 8

No. of neutrons= 8

Atomic No. = 8

Atomic Mass= P+N= 16

Sulphur: 

No. of Protons= 16

No. of neutrons= 16

Atomic No. = 16

Atomic Mass= P+N= 32

Question 1. For the symbols, H, D and T tabulate three subatomic particles found in each of them

Answer:

Question 2. Write the electronic configuration of any one pair of isotopes and isobar.

Answer. Isotopes: Atoms of the same element have the same atomic number but different mass numbers.

NCERT Textbook Exercise 

Question 1. Compare the properties of electrons, protons and neutrons.

Answer:

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

Answer: According to J.J. Thomson’s model of an atom, the electrons are embedded all over in the positively charged spheres. But experiments done by other scientists showed that protons are present only in the centre of the atom, and electrons are distributed around it.

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

Answer: According to Rutherford’s model of an atom the electrons revolve in a circular orbit around the nucleus. Any such particle that revolves would undergo acceleration and radiate energy. The revolving electron would lose its energy and finally fall into the nucleus; the atom would be highly unstable. But we know that atoms are quite stable.

Question 4. Describe Bohr’s model of the atom.

Answer: Bohr’s model of the atom

(1) Atoms have a nucleus in the centre.

(2) Electrons revolve around the nucleus.

(3) Certain special orbits known as discrete orbits of electrons are allowed inside the atom.

(4) While revolving in discrete orbits, the electrons do not radiate energy.

(5) These orbits or shells are called energy levels.

(6) These orbits or shells are represented by the letters K, L, M, N or the numbers n = 1, 2, 3, 4

Question 5. Compare all the proposed Bohr’s models of an atom given in this chapter.

Answer:

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

Answer: The rules for writing of distribution of electrons in various shells for the first eighteen elements are:

(i) The maximum number of electrons present in a shell is given by the formula-2 n2

\( \because \) n = orbit number i.e., 1, 2, 3

\( \because \) Maximum number of electrons in different shells are:

K shell n = 1 2n2 => 2(1)2 = 2

L shell n = 2 2n2 => 2(2)2 = 8

M shell n = 3 2n2 => 2(3)2 = 18

N shell n = 4 2n2 => 2(4)2 = 32

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.

(iii) Electrons are not accommodated in a given shell unless the inner shells are filled. (Shells are filled step-wise).

Question 7. Define valency by taking examples of silicon and oxygen.

Answer: Valency is the combining capacity of an atom.

Atomic number of oxygen = 8 Atomic number of silicon = 14 K L M

Electronic configuration of oxygen = 2 6 –

Electronic configuration of silicon =2 8 4

In the atoms of oxygen the valence electrons are 6 (i.e., electrons in the outermost shell). To fill the orbit, 2 electrons are required. In the atom of silicon, the valence electrons are 4. To fill this orbit 4 electrons are required.

Hence, the combining capacity of Oxygen is two, and silicon is 4.

i.e., Valency of oxygen = 2

Valency of silicon = 4

Question 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: The atomic number of an element is equal to the number of protons in the nucleus of its atom. e.g., Oxygen has 6 protons hence atomic no. = 6.

(ii) Mass number: The mass number of an atom is equal to the number of protons and neutrons in its nucleus.

Nucleons = number of protons + number of neutrons Example: Protons + Neutrons = Nucleus = Mass number  6 + 6 = 12

(iii) Isotopes: Isotopes are atoms of the same element with different mass numbers but the same atomic number.

(iv) Isobars: Atoms have the same mass number but different atomic numbers.

Both calcium and argon have the same mass number but different atomic numbers.

Two uses of isotopes are:

(i) An isotope of iodine is used to treat goitre.

(ii) An isotope of uranium is used as a fuel in nuclear reactors.

Question 9. Na+ has filled K and L shells. Explain.

Answer: Sodium atom (Na), has atomic number =11

Number of protons =11

Number of electrons = 11

Electronic configuration of Na = K L M – 2 8 1

Sodium atoms (Na) lose 1 electron to become stable and form Na+ ions. Hence it has filled K and L shells.

Question 10. If a bromine atom is available in the form of, say, two isotopes 7935Br (49.7%) and 8135Br (50.3%), calculate the average atomic mass of the bromine atom.

Answer:

Question 11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 168X and 188X in the sample?

Answer: Let the percentage of 168X be x, and the percentage of 168X be 100 – x.

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

Answer: Z = 3, (i.e, atomic number —> z)

\( \therefore \) Electronic configuration = 2, 1

Valency = 1

The name of the element is lithium.

Question 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 number of X and Y. What is the relation between the two species?

Answer: Mass number of X = Protons + Neutrons

= 6 + 6 = 12

Mass number of Y = Protons + Neutrons = 6 + 8 = 14

As the atomic number is the same i.e., = 6.

[atomic number = number of protons].

Both X and Y are isotopes of the same element.

Question 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 combining an electron and a proton. Therefore, it is neutral.

(c) The mass of an electron is about 1/2000 times that of a 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 a tick against the correct choice and cross (x) against the wrong choice in questions 15, 16 and 17.

Question 15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of

(a) Atomic nucleus (c) Proton

(b)Electron (d)neutron

Answer: (a) Atomic nucleus

Question 16. Isotopes of an element have

(a) the same physical properties (c) different number of neutrons

(b) a different number of neutrons (d) different atomic numbers.

Answer: (c) different number of neutrons

Question 17. Number of valence electrons in Ct ion are :

(a) 16 (b) 8

(c) 17 (d) 18

Answer: (b) 8

Question 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

Question 19. Complete the following table.

Answer:

The fundamental structure of an atom consists of the nucleus and protons, electrons and neutrons.

Protons	Electrons	Neutrons
They are positively charged	They are negatively charged	They are neutral
They get attracted to -ve charges	They get attracted to +ve charges	Since they are neutral, they do not get attracted.
They are present inside the nucleus	They are present outside the nucleus	They are present in the nucleus of the atom.

The nucleus is made up of protons and neutrons, the sum of which dentes the atomic number of the element. Atoms that are neutral consist of an equal number of protons and electrons. Over the years, through years of experiments and discoveries, several scientists proposed various models to understand the structure of an atom. In this process, several concepts such as atomic number, atom mass, orbits, isotopes, isobars and valency were discovered. To learn more and have a thorough understanding of the science behind the structure of a nucleus through self-study, download the MSVGo app and discover a fun way to learn. Talk to other users and hear what our champs say about this easy to use NCERT solutions app that brings vast knowledge from your books to your fingertips. Tensed for exams and worried about how to prepare? Prepare for exams and be exam-ready with MSVGo right by your side always!