NCERT Class 9 Science Structure of the Atom notes pdf

In the given section, we have provided you the NCERT Class 9 Science Structure of the Atom notes pdf. These notes have been prepared by the subject experts, who are very experienced in teaching science at secondary and senior secondary levels.

NCERT Class 9 Science Structure of the Atom Additional Questions

In the following section, you will get additional and conceptual question for NCERT Class 9 Science chapter named Structure of the Atom. These notes will strengthen your understanding of the chapter and equip you well for your upcoming exams.

Commonly Used Terms

Electron: it is that subatomic or fundamental particle which carries one unit negative charge and has a mass nearly 1/1840th that of hydrogen atom. (9.1 × 10-31 kg).

Proton: it is defined as that subatomic or fundamental particle which carries one unit positive charge and has mass nearly equal to that of hydrogen atom. (1.672× 10-27kg)

Neutron: it may be defined as that subatomic or fundamental particle which carries no charge, but has mass nearly equal to that of proton. (1.765 × 10-27 kg)

Electronic Configuration: The distribution or arrangement of electrons in different shells of the atom is called the electronic configuration of the element.

Valence electrons: The electrons present in the outermost shell of the atom of an element.

Valence shell: The outermost shell is called valence shell.

Valency: The combining power of an element, especially as measured by the number of hydrogen atoms it can displace or combine with.

What are cathode rays? Give their properties.

William Crooks in 1889 examined the conduction of electricity through gases at low pressure. For this purpose, he took a discharge tube (a long glass tube) about 60cm long and fitted two metal electrodes at two ends. Then a high voltage (10,000 volts) is applied between the electrodes.

He observed that some invisible rays are emitted from the cathode., which when pass through the hole in the anode and strike against the fluorescent coating glows. These rays are called cathode rays.

J.J. Thomson carried various experiments and concluded that the cathode rays posses the following properties:-

  1. They produce a sharp shadow of the solid object placed in their path. This shows that cathode rays travel in straight lines.
  2. If a light paddle wheel (that of mica) mounted on axle is placed in their path, the wheel begins to rotate. This shows that cathode rays are made up of material particles.
  3. When an electric field is applied on the cathode rays, they are deflected towards the positive plate of the electric field. This shows that cathode rays carry negative charge.
  4. When cathode rays strike a metal foil, the foil becomes hot. This shows that cathode rays produce heating effect.
  5. They affect the photographic plate.
  6. They posses penetrating effect.

These negatively charged particles which constitute cathode rays were called electrons. They possess a charge of negative nature of magnitude 1.60×10′> coulombs and a mass of 9.11x1028g or 1837part of a hydrogen atom.

Explain Rutherford’s alpha (α) ray scattering experiment. What observations did he made from the experiment?

Rutherford, in 1911, performed some scattering experiments in which he bombarded a thin gold foil (100mm thick) with a beam of alpha particles. The source of α-particles was radium, a radio active substance.

From these experiments he made following observations.

  1. Most of the α-particles passed through the foil without undergoing any deflection.
  2. Few alpha-particles underwent deflection through small angles.
  3. Very few were deflected back.

What did Rutherford conclude from alpha-ray scattering experiment?

Rutherford concluded from α ray scattering experiment that:

  1. Most of the space inside the atom is empty because most of the α-particles passed through the foil without undergoing any deflection.
  2. Alpha -particles, which carries two units of positive charge and are very heavy, could only be deflected by some positive charged body present inside the atom.
  3. The number of α-particles deflected back is very small, this shows that this heavy and positive charged body present inside the atom must be very small in volume.

Write about “Discovery of Neutron”.

Since each proton has one unit mass, therefore, the mass of an atom was calculated from the no. of protons present in the nucleus.

However, for any element (except hydrogen), this mass was found to be much less than the actual mass of atom. This led to the idea that there must be some other particles present in the nucleus and should be neutral but should have considerable mass.

Chadwick, in 1932, performed some experiments in which he bombarded some light elements like Beryllium and boron with fast moving α particles. He found that some new particles were emitted which carried no charge but had mass nearly equal to that of proton. This particle was termed as neutron.

Why are anode rays called canal Rays?

When Goldstein used the perforated cathode then he found that the anode rays were passing through the pores of perforated cathode so, he called these rays as canal rays.

Why do isotopes have same chemical properties?

Isotopes have same chemical properties because they have same atomic number and hence have same valence electron and we know that chemical properties depend upon the valence electrons.

Why fluorine forms F ion whereas oxygen forms O2- ion? 

Flourine has 7 electrons in its valence shell, so it requires 1 more electron to acquire stability. Therefore, it gains 1 electron and forms F ion.

Oxygen has 6 electrons in its valence shell. So, it gains 2 electrons to acquire stability and forms O2- ion.

Why did Rutherford use gold foil for his experiment?

Rutherford used gold foil for his experiment because gold is the most malleable metal and he wanted the thinnest layer possible to conduct his experiment.

How can you justify that cathode rays originate from the cathode whereas anode rays do not?

Any gas taken in the discharge tube, cathode rays consist of electrons with some mass and charge.

These electrons are produced due to their knock out from the atoms of the gas inside. This shows that cathode rays must be first originating from the cathode which are hitting the atoms of the gas to knock out electrons from them.

Anode rays consist of positively charged particles with mass nearly equal to the mass of the atoms of the gas. These are again produced due to knock out of electrons from the atoms of the gas by cathode rays converting the atoms into positive ions.

Thus these positive ions are produced in the space between cathode and anode and do not originate from the anode.

NCERT Class 9 Science Structure of the Atom Questions & Answers

In the following section, you will get NCERT Class 9 Science Structure of the Atom Questions & Answers. These Class 9 Structure of the Atom Solutions will help you a lot in your exam preparation.

What are anode rays or canal rays or positive rays? Give their properties?

Gold Stein in 1886 performed a discharge tube experiment in which he took a perforated cathode and a gas at low pressure.

On passing high voltage between electrodes, it was found that some rays were coming from anode side which passed through the holes of cathode and produced a green glowing light on opposite wall coated with zinc sulphide. These rays were called anode rays or canal rays or positive rays.

These rays were believed to be produced as a result of knocking out of the electrons from the gaseous atoms by the bombardment of high speed electrons of the cathode rays on them and producing positively charged particles. These positively charged particles move towards the cathode in the form of anode rays.

Thus, anode rays are not emitted from anode but are produced in the space between anode and cathode.

Properties of Anode rays

Anode rays were found to possess the following properties:

  1. They travel in straight lines. However, their speed is much less than that of cathode rays.
  2. They are made up of material particles.
  3. They are positively charged particles as indicated by the deflection of these rays towards the negative plate of the electric field.
  4. They possess less penetrating power as compared to cathode rays.
  5. These anode rays were consisting of material particles and were given the name of protons. Each among them carries a charge of positive nature of magnitude 1. 60×109 coulombs and mass of 1.67×1024 g or 1.67 x 10-2’kg equal to the mass of hydrogen atom.

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

Atom will not carry any charge. This is because the charge present on the electron will be balanced by the equal amount of charge but of opposite nature carried by proton. So, the net charge on atom will be zero.

On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole?

The total charge on the electrons is equal to the total positive charge on the sphere. Hence, the atom as whole is electrically neutral.

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


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


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

If the α-particle scattering experiment is carried out using a foil of any heavy metal, same observations will be achieved.

But with the foil of a light metal, eg; that of lithium, the massive alpha-particles may push the nucleus and may not be deflected back.

Name three subatomic particles of an atom?

The three subatomic particles of an atom are; (i) Electron (ii) Proton (iii) Neutron.

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

Atomic mass of helium = 4u (given)

No. of protons = atomic no. = 2 (given)

No. of neutrons = atomic mass – atomic no. = 4 – 2 = 2 neutrons.

Write the distribution of electrons in carbon and sodium atoms?

No. of electrons in carbon atom = 6

Electronic distribution in carbon atom = K2, L4

No. of electrons in sodium atom = 11

Electronic distribution in sodium atom = K2, L8, M1.

If K & L shells of an atom are full, then what would be the total no. of electrons in the atom?

The maximum number of electrons in K shell = 2x (1)2 =2 (because no. of electrons = 2n2)

The maximum number of electrons in L shell = 2 × (2)2 = 2 × 4 = 8

Thus, Total number of electrons = 2+8 = 10

How will you find the valency of chlorine, Sulphur and magnesium?

No. of electrons in chlorine atom = 17

Electronic distribution in chlorine atom = K2, L8, M7

Chlorine atom is short of one electron to complete its octet. Therefore, its valency = 1

No. of electrons in Sulphur atom = 16

Electronic distribution in Sulphur atom = K2, L8, M6

Sulphur atom is short of 2 electrons to complete its octet. Therefore, its valency = 2

No. of electrons in magnesium atom = 12

Electronic distribution in magnesium atom = K2, L8, M2

Magnesium atom loses two electrons to complete its octet. The valency of magnesium = 2

If 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?

No. of electrons = 8 (given)

No. of protons = 8 (given)

Atomic no. of the atom = No. of proton or no. of electrons = 8

Since protons and electrons are equal in number. So, the net charge on atom will be zero.

With the help of table 10.1 find out the mass number of oxygen and Sulphur atom?

(1) No. of protons in oxygen atom = 8

No. of neutrons in oxygen atom = 8

Mass number of Oxygen = 8 + 8 = 16

(2) No. of protons in Sulphur atom = 16

No. of neutrons in Sulphur atom = 16

Mass number of Sulphur = 16 + 16 = 32

For the symbol H, D & T, tabulate three sub-atomic particles found in each of them?

AtomAtomic NoAtomic MassNo of electronsNo of protonsNo of neutrons
H11111-1 =0
D12112-1 =1

Write the electronic configuration of any one pair of isotopes and isobars?

The two isotopes are: 17C35 , 17C37

Electronic configuration of above mentioned isotopes: K2, L8, M7.

The two isobars are: 18Ar40, 20Ca40

Electronic configuration of Argon = K2, L8, M8

Electronic configuration of calcium =K2, L8, M8, N2.

Compare the properties of electrons, protons and neutrons?

electron1/1837th that of Hydrogen atomNegativein extra nuclear part
protonequal to the mass of Hydrogen atomPositiveinside the nucleus
neutronequal to the mass of protonNeutralinside the nucleus

What is Thomson’s atomic model? What are its limitations?

Thomson in 1904, proposed that an atom behaves as a sphere of positive electric charge in which electrons are embedded. The number of electrons is sufficient to neutralize the positive charge.

This model is compared with water melon, where the positive charge is like the red edible part and the negative charge like the seeds of the water melon, therefore it is sometimes also called as water melon model.

Limitations of Thomson’s atomic model

Although Thomson’s model explained that atoms are electrically neutral, but this model couldn’t explain the results of scattering experiments carried out by Rutherford.

Describe Rutherford’s model of atom. What are its limitations?

In the light of scattering experiments, Rutherford proposed the model of atomic structure.

According to Rutherford’s atomic model:

  1. There is a positively charged centre in an atom and nearly, all the mass of an atom resides in this centre. He called this region of the atom as nucleus.
  2. Rutherford’s model proposed that the negatively charged electrons surround the nucleus of an atom.
  3. In order to explain why electrons do not fall into the nucleus due to mutual electrostatic force of attraction. He proposed that the electrons surrounding the nucleus revolve around it with very high speed in circular paths. The centrifugal force arising from this circular motion balances the force of electrostatic attraction between nucleus and electrons. He named these circular paths as orbits.
  4. The size of the nucleus is very small as compared to the size of an atom.

Rutherford’s model is also called as planetary model of atom.

Limitations of Rutherford’s Model of atom.

Although Rutherford’s atomic model was based on experimental observations. it failed to explain certain things:

(1) Rutherford proposed that the electrons revolve around the nucleus in fixed paths called orbits.

According to classical electromagnetic theory accelerated charged particles emit energy and hence an electron revolving around the nucleus should emit energy and due to continuous loss of energy orbit of electron must decrease continuously.

Consequently electron will fall into the nucleus and this shows that atom is unstable. But in actual practice this does not happen.

(2) Another drawback of Rutherford’s model was also that he did not say anything about the arrangement of electrons in an atom which made his theory incomplete.

Describe Bohr’s model of atom?

To overcome the drawbacks of Rutherford’s model of atom, Neils Bohr in 1913 proposed a new model of atom, known as Bohr’s model of atom.

The main points of this model are as follows:

  1. An atom consists of a small, heavy positively charged nucleus in the centre and electrons revolve around it in circular orbits.
  2. Out of large number of circular orbits possible around the nucleus, the electron revolves only in those orbits which have fixed value of energy.
    Hence these orbits are called as energy levels. The different energy levels are numbered as 1,2,3,4 etc or designated as K,L,M,N,O etc starting from energy level closet to nucleus.
  3. While revolving in discrete orbits, the electrons don’t radiate (give out) energy.
  4. Energy is lost or gained by an electron only when it jumps from one orbit to other. The energy lost or gained is equal to the difference of energy of two energy levels involved.

Summarize the rules for writing of distribution of electrons in various shells for first eighteen elements?

While distributing the electrons in different energy levels, following rules are to be followed:

Rule 1: The maximum number of electrons present in an energy level is given by 2n2, where ‘n’ is the energy level no. i.e. n= 1,2,3…… so, we have

ShellMax no of electrons
first shell or K shell2 x 12 = 2
2nd shell or L shell2 x 22 = 8
3rd shell or M shell2 x 32 = 18

Rule 2: The outermost shell can’t have more than 8 electrons even if the first rule is violated, e.g. M shell can accommodate 18 electrons but as soon as it has acquired 8 electrons, the filing of 4″ shell starts.

Rule 3: Electrons aren’t accommodated in a given energy level (shells), unless lower energy level are filled.

Define the valency by taking examples of silicon and oxygen?

Valency of an atom may be defined as the number of electrons lost, gained or shared by that with other atom or atoms to achieve stable noble gas electronic configuration.

The valency of Silicon and Oxygen is as follows:

(i) Silicon has 14 electrons and its electronic configuration is: K=2, L= 8, M=4.

Since silicon has a tendency to share its outermost four electrons to get stable electronic configuration.

Therefore, the valency of silicon is 4.

(ii) Oxygen has 8 electrons and its electronic configuration is: K=2, L= 6.

Since oxygen is short of two electrons, it will gain two electrons and will complete its octet. Hence its valency is 2.

Explain with examples; (i) Atomic number (ii) Atomic mass (iii) Isotopes and (iv) Isobars? Give any two uses of isotopes?

Atomic number: Atomic number may be defined as the total number of protons present in the nucleus of an atom. It is denoted by letter ‘Z’, e.g. Hydrogen has Z = 1 & carbon has Z = 6

Mass number: Mass number may be defined as the total number of protons and neutrons present in the

nucleus of an atom. It is represented by letter “A”, e.g. Helium has A = 4u & Aluminum has A = 23u

Isotopes: Isotopes may be defined as the atoms of an element having same atomic number but different mass numbers, e.g. Hydrogen has 3 isotopes. i.e., 1H1 (Protium), 1H2 (Deuterium), 1H3 (Tritium).

Uses of isotopes:

  1. Isotopes of Uranium are used as fuel in nuclear reactors.
  2. An isotope of Cobalt is used for the treatment of cancer.
  3. Some isotopes are used to check the leakage of gases in gas pipelines.
  4. An isotope of Iodine is used for the treatment of goiter.
  5. An isotope of Phosphorus is used to cure blood cancer.
  6. An isotope of Sodium is used to check the presence of blood clot.

Isobars: Isobars may be defined as the atoms of different elements having same mass numbers but different atomic number, e.g. A pair of isobars is 18Ar40, 20Ca40.

Na+ has completely filled K & L shells. Explain?

Sodium has electron number = 11

Distribution of electrons in sodium = K=2, L= 8, M=1.

Sodium has a tendency to lose one electron to get stable noble gas electronic configuration. So, after losing one electron, sodium completes its octet. So, Na+ has completely filled K & L shells (K=2, L=8).

If Bromine atom is available in the form of, say, two isotopes Br(49.7%) and Br(50.3%), calculate the average atomic mass of bromine atom?

Percentage of Br isotope with mass number 79 = 49.7

Percentage of Br isotope with mass number 81 = 50.3

Average atomic mass of Br= [(49.7 x 79) + (50.3 x 81)] ÷ 100 = 80.086u.

The average atomic mass of a sample of a clement X is 16.2u. What are the percentage of isotopes; 8X16 and 8X18 in the sample?

Suppose percentage of 8X16 = y

thus percentage of 8X18 = (100-y)

Average atomic mass = [(16 x y)+ 18 (100-y)] ÷ 100 = 16.2 u.

Upon solving, we get y = 90%

Therefore, percentage of 8X16 = y = 90%

and percentage of 8X18 = (100-y) = 100-90 = 10%

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

Here Z = 3, Electronic distribution = K=2, L= 1

From the distribution of electrons, it is evident that the element has a tendency to lose its outermost electron.

Therefore valency of the element is 1.

As the atomic no of element is 3 the name of the element is Lithium.

Composition of the nuclei of two atomic species x & y are given as under:


Give the mass numbers of x and y. What is the relation between the two species.

Atomic SpeciesNo of protonsNo of neutronsAtomic noAtomic mass
X6666+6= 12u
Y6866+8= 14u

Since the two atoms are having the same atomic number but different mass numbers, so they are isotopes.

For the following statements, write (T) for true and (F) for false.

J.J. Thomson proposed that the nucleus of an atom contains only nucleons.

False, It was not proposed by JJ Thomson.

A neutron is formed by an electron and a proton combining together. Therefore it is neutral.

False. A neutron is an independent sub-atomic particle.

The mass of an electron is about 1/2000 times that of proton.

True, It is a fact known from experiments.

Isotope of Iodine is used for making tincture iodine, which is used as a medicine.

False, As tincture of iodine is a solution of ordinary Iodine in alcohol.

Rutherford’s alpha-particle scattering experiment was responsible for the discovery of;

  1. Atomic Nucleus
  2. Electron
  3. Proton
  4. Neutron

Atomic Nucleus

Isotopes of an element have;

  1. the same physical properties
  2. different chemical properties
  3. different number of neutrons
  4. different atomic numbers.

different number of neutrons

Number of valence electrons in Cl ion are:

  1. 16
  2. 08
  3. 17
  4. 18


Which one of the following is a correct electronic configuration of sodium?

  1. 2,8
  2. 8,2,1
  3. 2,1,8
  4. 2,8,1.


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