Structure of the Atom : Complete Set of Questions

This set of questions contains all the possible concepts
which could be asked in the examination


Q.1    How hydrogen atom is different from atoms of all other elements?

All the atoms are made up of three subatomic particles: electrons, protons and neutrons. Hydrogen atom is made up of only one electron and one proton. It does not contain any neutron. So, it is different from atoms of all other elements.

Q.2    What is discharge tube?

A discharge tube is a long glass tube closed at both the ends. Two metal plates A and B are sealed at the ends and these plates are called as electrodes. A vacuum pump is attached to suck out the air or gas present inside the tube to reduce the pressure. Both the plates are connected with electrical power with high voltage. The plate with negative terminal is called cathode and with positive terminal is called anode.


Q.3    Name the subatomic particles present in an atom.

Atom is made up of smaller particles called subatomic particles. The sub-atomic particles are electrons, protons and neutrons.

Q.4    What is an electron? What is its relative mass and charge?

An electron is that subatomic particle which is negatively charge and has a mass about 1/1840 u of that of an atom of hydrogen.

Q.5    What is relative mass and charge of an electron?

The mass of electron is about {1 \over {1840}} of the mass of hydrogen .The absolute mass of an electron is 9 \times {10^{ - 28}} gram. The absolute charge on an electron is coulomb of negative charge which is smallest, carried by any particle. Thus, it is taken as unit of negative charge.

Q.6    Find out the number of electrons present in last shell of an atom having atomic number 15?

Electronic configuration of element with atomic number 15 will be 2, 8, 5. Hence, the number of valence electron present in an atom is 5.

Q.7    What is the basic differences between proton and neutron?

The basic difference between proton and neutron is the electric charge. Proton has a positive charge (1.6 × 10—19 coulomb) where as neutron has no charge i.e. electrically neutral.

Q.8    What is meant by atomic number of an element? Does the atomic number of an element change when its atoms form ions? Give one example each of diatomic and triatomic molecules.    

The atomic number of any element is the number of protons present in the nucleus of an atom of that element. The protons are present in the nucleus and they do not take part in the reactions. Therefore, the atomic number of an element which is nothing but the number of protons present in the nucleus of an atom of that element also remains same. 
Diatomic molecule is molecule made up of two atoms. For eg – oxygen (O2).
Triatomic molecule is molecule made up of three atoms.for eg – water (H2O), O3 (ozone) etc.

Q.9    Define the term ground state of an atom?

Ground state is the state of an atom, where all the electrons are in their lowest energy levels. After receiving energy, electrons can jump in higher energy levels which are known as excited state.

Q.10   How cathode rays are produced?

Cathode rays are produced in the discharge tube. The gas filled in the discharge tube, contains electrons. When high voltage is applied between electrodes, the electrical energy pushes out some of the electron from the atoms of the gas. These fast moving electrons form “Cathode rays”.


Q.11   What are the properties of cathode rays?

Some important properties of cathode rays are:

1. When an object such as a metal cross is placed in the path of the cathode rays, they generate a shadow of the object at the back of glass tube. So, cathode rays travel in straight lines.


2. When a light paddle wheel is placed in the path of the cathode rays such that cathode rays strike the blades of upper half, it starts to rotate. Hence, cathode rays are consists of particles.


3. When an electric field is applied on the cathode rays, they are deflected towards the positive plate of the electric field thus cathode rays carry negative charge.


4. They produce green fluorescence on the glass walls of the discharge tube. When cathode rays strike the atoms in the glass, they knock their valence electrons into a higher energy level and when the electrons fall back to their original energy level, they emit light. This process is called as fluorescence, causing the glass to glow, usually yellow-green.

5. Cathode rays produce heating effect. Thus, when cathode rays strike a metal foil, it becomes hot.

6. When electrons hit against the surface of hard metals like tungsten, molybdenum etc, some of the electrons strike on nucleus of metal atoms and get deflected because of positive charge of nucleus. This deflection results the energy of electron to decrease and then results in formation of X-rays. So, cathode rays produce X-rays.

Q.12   Who discovered the fundamental particles neutron, electron and proton?

(i) Neutron - Chadwick

(ii) Electrons - Thomson

(iii) Proton - Goldstein

Q.13   How are anode rays produced?

The presence of positively charged particles in an atom was shown by Goldstein in 1886. He took a discharge tube filled with H2 gas and applied high voltage between the anode and the cathode. He observed that some rays were coming from the side of anode and passed through the holes in the cathode and then strikes on the glass wall and they are called “anode rays”. They carry positive charge and hence called “positive rays”.


Q.14   What are the properties of Anode rays?

The different properties of anode rays are as follows: - 

1. They travel in straight lines. If an object is placed in their path, they cast a shadow at the back of glass tube.

2. They are made up of material particles. If a light paddle wheel is placed on an axle in their path, it begins to rotate.

3. Anode rays carry positive charge. They get deflected towards the negative plate of the electric field.

4.  Mass of the positively charged particles constituting the anode rays depends upon the nature of the gas. The mass is found to be nearly equal to the mass of the atom of the gas.

Q.15   What is a proton?

A proton is a positively charged particle which carries one unit positive charge and found in the nucleus of atoms of all the elements.

Charge on proton = +1; mass = 1 u

Hence it is represented by the symbol _1{P^1}

Q.16   Write detail account on Thomson’s model?

J.J. Thomson proposed his model of atom in 1903. According to Thomson’s model of the atom:

1. An atom consists of a sphere of positive charge with negative charged electrons embedded in it.

2. The positive and negative charges in an atom are equal in magnitude, due to which atom is electrically neutral. These equal and opposite charges balance each other.


Q.17   Give a detail account on discovery of nucleus.


Rutherford made the following observations:

  • Most of the α -particles (99.9%) passed straight through the gold foil without undergoing any deflection.
  • Some α -particles were deflected by small angles and a few were deflected through large angles.
  • Very few were deflected back, i.e. through an angle greater than 90°.


From these observations, Rutherford drew the following conclusions :

  • Most of the α-particles passed through the foil without any deflection, thus there must be sufficient empty space within the atom.
  • Since some α-particles were deflected through small angle or large angle and α-particles are positively charged particles, these could be deflected only by some positively charged body present within the atom. The α -particles deflected through large angles were those which passed very close to the positive body.
  • Since some α -particles are deflected back and α -particles are heavy particles, these could be deflected back only when they hit heavier body inside the atom.
  • Since very few α -particles deflected back, this shows that the heavy body present in the atom must be occupying a very small volume.

Q.18   Define the term scintillations?

Scintillations are bright flashes produced through the α -particles in Rutherford model of an atom. When these particles passed through a slit and strikes against the gold foil, they get scattered and produce bright flashes known as scintillations.

Q.19   What is Rutherford’s Nuclear Model of atom?

The main points of this model are as follows:

  • An atom is made up of two parts, nucleus and extra nuclear part. Nucleus is the center of the atom with positive charge. Extra nuclear part means the space around the nucleus in which the electrons are distributed.
  • The whole mass of the atom is located in the nucleus. Since the electrons have negligible mass, the mass of the atom is mainly due to protons.
  • The electrons revolve around the nucleus in well defined orbits.
  • An atom is electrically neutral because number of protons and electrons is equal.
  • Most of the atom is empty space.

Q.20   What are planetary electrons?

Rutherford compared his model of an atom with our solar system where the nucleus is like the sun and the electrons are like the planets. Thus, these electrons are also called planetary electrons.

Q.21  What was the reason behind the selection of gold foil by Rutherford in α -particle scattering ?

Gold is a highly malleable metal which can be hammered and converted into very thin sheets or foil. Thus, it is easier for the α -particle to pass through the gold foil rarely deviated by nucleus. As the thickness of the foil decrease, the possibility of correctness of experiment increases.

Q.22   Cathode rays originate from the cathode whereas anode rays do not. Explain?

Cathode rays consist of electrons with same mass and charge. These electrons are produced due to their knock out from the atoms of the gas inside. This shows that the cathode rays must be first originating from the cathodes 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 also 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.

Q.23   What are the major drawbacks of Rutherford’s model of atom?

A major drawback of Rutherford’s model of the atom is that it does not explain the stability of the atom. According to electromagnetic theory of physics, if charged particle undergoes accelerated motion, then it must radiate energy or lose energy continuously. It means the electrons revolving around the nucleus with accelerated motion, will also lose their energy and their speed will also go on decreasing and finally the electrons should fall into the nucleus and atom should collapse. But this does not happen and atom is quite stable.


Q.24   During a chemical reaction, number of electrons changes but number of proton remains constant .Why?

In a chemical reaction, exchange of electrons occurs. During a chemical reaction, electrons are lost, gained or shared between different atoms of different elements. Proton which is present in nucleus does not participate and its number which also represents atomic number of element remains constant.

Q.25   What is Bohr’s mode of atom?

Bohr’s model of the atom : Nucleus is at the centre
Electrons revolve round the nucleus in ‘fixed’ energy levels or electron shells
(only first four energy levels are shown in the above diagram)

The Bohr’s model of atom can be described as follows: -

1) An atom consists of electrons, protons and neutrons. Due to the presence of equal number of negative electrons and positive protons, the atom on the whole is electrically neutral.

2) The protons and neutrons are located in the center of an atom.

3) The electrons revolve in shells or orbits or shells .Orbits are represented either by the numbers 1, 2, 3, 4.....or by the letters K, L, M, N......

4) The maximum number of electrons accommodated in any shell is fixed. For eg, the first orbit or K shell can hold a maximum of 2 electrons; second orbit or L shell can hold a maximum of 8 electrons; third orbit or M shell can hold a maximum of 18 electrons and fourth orbit or N shell can hold a maximum of 32 electrons.

5) Each orbit or shell is associated with a fixed amount of energy, the shell nearest to the nucleus having minimum energy and the shell farthest from the nucleus having the maximum energy.

6) There is no change in the energy of electrons when it revolves in the same orbit and atom remains stable.

The change in the energy of an electron occurs when it jumps to a higher orbit or when it comes down to a lower energy level. When an electron gains energy, it jumps from a lower shell to a higher shell, and when an electron comes down from a higher shell to a lower shell, it loses energy.

Q.26   Why are Bohr’s orbits called stationary states?

According to Bohr’s theory, electrons revolves around the nucleus and they have fixed amount of energy. Thus, they are called as stationary states.

Q.27   What is a neutron?

Neutron is a fundamental particle which carries no charge, i.e. it is a neutral particle and has a mass equal to proton (i.e. amu).

Charge on neutron = 0; mass =1 u

It can be represented as {}_0^1n

Q.28   What are nucleons?

Total number of protons and neutrons present in nucleus of an atom is known as nucleon.

Q.29  How neutron has been discovered?

The presence of neutrons in the nucleus was confirmed by Chadwick in 1932. He did an experiment. He bombarded the nuclei of some light element with fast moving α - particles. He found some particles were ejected from the nucleus. These particles carry no charge and have a mass equal to proton. These particles were named as neutron.

Q.30   Tabulate the characteristics of electron, proton and neutron.

Q.31  What is atomic number?

Atomic number of an element is number of protons present in the nucleus of the atom of that element for e.g.

- Nucleus of Hydrogen atom contains one proton; hence its atomic no. is 1.

- Nucleus of C atom contain 6 protons; hence its atomic number is = 6.

All the atoms of same element have same number of protons in their nuclei .Atomic number distinguishes the atoms of different elements. In a neutral atom, the number of protons is equal to the number of electrons.

Thus, atomic number of an element = Number of electrons in one neutral atom

Q.32   What is mass number?

Mass number of an element is the sum of number of protons and neutrons present in the atom of the element.

Mass number = No. of protons + No. of neutrons.

For e.g. : Hydrogen atom has 1 proton but 0 neutron thus the mass number of H is 1.

The mass number of an element is denoted by the letter A. Protons and neutrons present in a nucleus, together known as nucleons.

Hence, Mass number = Number of nucleons. 

Q.33   What is relation between atomic number and mass number?

Mass number = no. of protons + no. of neutrons

As, the number of protons is equal to atomic number of the element.

Hence, Mass number = Atomic number + no. of neutrons.

Q.34   What is the difference between mass number and atomic mass?

The difference between mass number and atomic number is that mass number is always a whole number (because it is number of protons and neutrons) where as atomic mass is mass of protons and neutrons compared to C-12 atom taken as 12 for e.g. mass number of hydrogen is 1 but its atomic weight is 1.008 u.

Q.35   What is Bohr’s – Bury Scheme for distribution of electrons in different shell?

The main points of Bohr-Bury Scheme are as follows:

(i) The maximum number of electrons that can be present in the nth shell is equal to 2n2.

Shell                                                    Maximum number of electrons present

1st shell or K-shell (n = 1)                                           2 × 12 = 2

2nd shell or L-shell (n = 2)                                          2 × 22 = 8

3rd shell or M-shell (n = 3)                                          2 × 32 = 18

4th shell or N-shell (n = 4)                                           2 × 42 = 32


(ii) The outermost shell cannot have more than 8 electrons.

(iii) Electrons do not enter into a new shell unless the inner shells are completely filled. In other words, the shells are filled in a step-wise manner.

(iv) The penultimate shell (i.e. second last shell) cannot accommodate more than 18 electrons.

(v) The anti penultimate shell (i.e.third last shell) can have a maximum of 32 electrons.

Q.36   What do you mean by valence electrons?

The outermost shell of any atom is known as valence shell and the electrons present in this shell are known as valence electrons.

For e.g. Carbon (atomic number Z= 6) => electronic configuration = >   K - 2 ,  L - 4

Thus, L shell is the valence shell and the 4 electrons present in this shell are valence electrons.

Q.37   Define the term valency.

The number of electrons gained, lost or shared by the atom of an element to complete its valence shell with 8 electrons is called the valency of that element.

For e.g.

Sodium (11) = > electronic configuration = > 2, 8, 1

By losing 1 electron its octet will be complete so its valency is 1.

Q.38   What is variable valency?

The term variable valency is used for those elements which show more than one valency.

Valency = Number of electrons in the valence shell


= (8 — Number of valence electrons)

For e.g., Phosphorus (Z = 15) has electronic configuration 2, 8, 5.

So, valency of Phosphorus is 5 as well as 8 – 5 = 3.

Q.39   What is electrovalency? Give examples.

The number of electrons lost or gained by one atom of an element to acquire nearest noble gas configuration, in the process of formation of any ionic / electrovalent compound is known as electrovalency. The element which lose electrons convert into positive ions, so they have positive electrovalency where as the elements which gain electrons convert into negative ions, known as negative electrovalency.

e.g. 1, Valency of Magnesium : -

Atomic number of Magnesium is 12 and its electronic configuration is 2, 8, 2. It has 2 valence electrons .Magnesium atom lose these 2 valence electrons and forms a magnesium ion, Mg+2 to achieve the noble gas configuration. Thus, the valency of magnesium is (+2).

e.g.2, Valency of oxygen :

Atomic number of oxygen is 8 and its electronic configuration is 2, 6. It has 6 valence electrons so it needs 2 more electrons and forms oxygen ion, 0-2 to achieve the noble gas configuration. Thus, the valency of oxygen is (-2).

Q.40   What is covalency? Give examples.

The number of electrons contributed by one atom for sharing to acquire the nearest noble gas electron configuration is known as covalency.

For e.g., Covalency of Nitrogen : -

Atomic number of N is 7 so its electronic configuration is 2, 5. It has 5 valence electrons. Since one nitrogen atom shares 3 electrons to achieve the nearest noble gas configuration. Thus the covalency of N is 3.In the formation of Nitrogen molecule, N2; each N atom shares its 3 electrons with other atom.

Q.41   What are the different isotopes of oxygen?

There are three isotopes of oxygen. They are-

Q.42   What are the general features of isotope?

The general features of isotopes are as follows: 

1. The isotopes of an element have same atomic number (i.e. same number of protons in the   nucleus and same number of electrons in the extra nuclear part)

2. The isotopes of an element have different mass numbers (i.e. different in the number of neutrons present in the nucleus)

3. Isotopes have same electronic configuration hence share similar chemical properties.

4. Isotopes of an element have different masses, so they have different physical properties like melting point, boiling point, density etc.

5. Due to difference in the nuclear structure (i.e., number of neutrons), they have different nuclear properties, e.g., C-14 isotope is radioactive whereas C-12 isotope is non-radioactive.

Q.43   What are radioisotopes?

The radioactive isotopes are called as radioisotopes. For e.g. C-14

Q.44   Define the term “Half-life”.

Half –Life is a feature of unstable radioactive elements which disintegrate with time and emits alpha and beta particles or Half-life (t1⁄2) is the amount of time required for a quantity to degenerate to half of its value as compared to the starting of the time period. The rate of decay is depends on the amount of substances. For eg the half-life of Cobalt 27C60 is 5 years. If we start with 100 gm of cobalt, then after 5 years only 50 gram would be left.

Q.45   What are the different applications of isotopes?

The isotope having larger number of neutrons is generally unstable. It emits α, β, and γ-radiations spontaneously. Such isotopes are called radioisotopes. These radioisotopes possess some special properties which make them very useful in a number of fields. Some applications of the isotopes are given below:

1. As nuclear fuel -In the nuclear reactor, an isotope of uranium (U-235) is used as a nuclear fuel.

2. In medical field- Some radioisotopes are widely used for treatment as well as diagnosis of fatal diseases like cancer, tumour etc.

A few examples are given below:

(i) Cobalt-60 is used in the treatment of cancer. The high energy -rays emitted by Co-60 kills the malignant cells of the cancer.

(ii) Phosphorus – 32 is used in the treatment of leukemia i.e. blood cancer.

(iii) Iodine-131 is used in the diagnosis and treatment of thyroid disorders (i.e. disease called goiter).

(iv) Some radioisotopes are used as isotope, called tracer, is injected into the body. Radioactive imaging is then used to detect accumulation of the isotope and therefore detect tumours and blood clots before they become dangerous. For e.g., sodium-24 is used to detect the blood clot and arsenic-74 is used to detect the tumour.

3. In carbon dating- Carbon dating is a technique of finding the age of fossils ( i.e. old samples dead animals) at archeological sites. Plants fixatmospheric carbon during photosynthesis, so the level of 14C in plants and animals when they die approximately equals the level of 14C in the atmosphere at that time. Although, it decreases thereafter because of radioactive decay, allowing the date of death or fixation to be determined.

4. In industry- Radioisotopes are used to detect the leakage in the underground oil pipes, gas pipes or water pipes. In such a case, radioisotope is allowed to flow through the pipe. At the point of leakage, a large amount of radiation will be emitted and this can be detected with an instrument called Geiger-Muller counter.

Q.46   What are isotopes? Give example.

Isotopes are the atoms of the same element with same atomic number but different mass numbers.

For e.g., The isotopes of Hydrogen are -

Q.47   Which isotope is used as nuclear fuel?

Uranium-235 is used as nuclear fuel.

Q.48   What is the reason behind fractional atomic mass?

The reason behind the fractional atomic masses of elements are the different isotopes of an element present in nature. Most of the elements have more than one natural isotope having different masses. Since the atomic mass of an element is the average relative mass of all the natural isotopes of that element.

For e.g., Chlorine in found to exist as two isotopes i.e. Chlorine 35 and chlorine 37 in ratio of 75% and 25% respectively.

This means that isotope of mass 35 u will contribute 75% whereas isotope of mass 37 u will contribute 25% to average atomic mass of Chlorine.

Average atomic mass of Chlorine = 35 \times {{75} \over {100}} + 37 \times {{25} \over {100}}

{{2625} \over {100}} + {{925} \over {100}}

26.25 + 9.25


Thus, average atomic mass of Chlorine is 35.5 u i.e. in fraction.

Q.49   What are isobars? Give examples.

Isobars are the atoms of different elements having different atomic number but same mass number. Isobars have different number of protons but the total number of nucleons (protons + neutrons) is same.

For e.g., Argon (Ar) and Calcium (Ca).

Q.50   Tabulate the properties of Cathode rays and Anode rays.

Q.51   What are α - particles?

α-particles are doubly charged helium ions (He+2) each having two protons and two neutrons bound together into a particle identical to helium nucleus.

α - Particles = > He+2.

Q.52   What are isotones?

Some atoms of different element have different atomic number and different mass numbers but they have same number of neutrons. These atoms are known as isotones.

For e.g., _6^{12}C and _8^{16}O.

Both C and O have same number of neutrons i.e. 8.

Q.53   What are isoelectronics ?

The species (atoms or ions) having same number of electrons are called isoelectronics.

For e.g., Na+ and Mg+2.

Both Na and Mg contains 10 electrons.

Q.54   What are nucleons?

Total number of protons and neutrons present in nucleus of an atom is known as nucleon.

Q.55   Why Bohr’s orbits are called stationary states?

According to Bohr’s theory, electrons revolve around the nucleus and they have fixed amount of   energy. Thus they are called as stationary states.

Q.56   What is an orbit?

Orbit is the path of the electron around the nucleus.

Q.57   What is mean by electronic configuration of elements?

The systematic arrangement of electrons in different orbits or shells of an atom of element is known as electronic configuration of elements.

Q.58   Why some atoms shows radioactivity?

When the number of neutrons exceeds the number of protons in the nucleus of atom, it becomes unstable and shows radioactivity. For eg , 6C12 has 6 protons and 6 neutrons so it stable while 6C14 has 6 protons and 8 neutrons is unstable and shows radioactivity.


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