Atomic and Nuclear Physics Online Test 12th Science Questions

The word atom in Greek means without division or indivisible.

An American Physicist Richard P. Feynman said that if the atom becomes the size of an apple, then the apple becomes the size of the earth.

The Bohr atom model is more successful than J. J. Thomson and Rutherford atom models. It explained many unsolved issues in those days and also gave better understanding of chemistry.

Later, scientists observed that even the atom is not the fundamental entity. It consists of electrons and nucleus. Around 1930, scientists discovered that nucleus is also made of proton and neutron. Further research discovered that even the proton and neutron are made up of fundamental entities known as quarks.

Gases at normal atmospheric pressure are poor conductors of electricity because they do not have free electrons for conduction. But by special arrangement, one can make a gas to conduct electricity.

Cathode rays possess energy and momentum and travel in a straight line with high speed of the order of 107m s -1. It can be deflected by application of electric and magnetic fields. The direction of deflection indicates that they are negatively charged particles.

When the cathode rays are allowed to fall on matter, they produce heat. They affect the photographic plates and also produce fluorescence when they fall on certain crystals and minerals.

When the cathode rays fall on a material of high atomic weight, x-rays are produced.

The speed of cathode rays is up to 1/ 10th of the speed of light.

Thomson’s experiment is considered as one among the landmark experiments for the birth of modern physics. In 1887, J. J. Thomson made remarkable improvement in the scope of study of gases in discharge tubes. In the presence of electric and magnetic fields, the cathode rays are deflected. By the variation of electric and magnetic fields mass normalized charge or the specific charge (charge per unit mass) of the cathode rays is measured.

Let m be the mass of the oil drop and q be its charge. Then the forces acting on the droplet are gravitational force, electric force, buoyant force, viscous force.

J. J. Thomson proposed a theoretical atom model which is based on static distribution of electric charges. Since this model fails to explain the stability of atom, one of his students E. Rutherford proposed the first dynamic model of an atom. Rutherford gave atom model which is based on results of an experiment done by his students (Geiger and Marsden). But this model also failed to explain the stability of the atom.

Neil’s Bohr who is also a student of Rutherford proposed an atomic model for hydrogen atom which is more successful than other two models. Neil’s Bohr atom model could explain the stability of the atom and also the origin of line spectrum.

The atoms are electrically neutral; this implies that the total positive charge in an atom is equal to the total negative charge. According to the Neil’s model, all the charges are assumed to be at rest. But from classical electrodynamics, no stable equilibrium points exist in electrostatic configuration (this is known as Earnshaw’s theorem) and hence such an atom cannot be stable. Further, it fails to explain the origin of spectral lines observed in the spectrum of hydrogen atom and other atoms.

In 1911, Geiger and Marsden did a remarkable experiment based on the advice of their teacher Rutherford, which is known as scattering of alpha particles by gold foil.

Rutherford proposed that an atom has a lot of empty space and contains a tiny matter known as nucleus whose size is of the order of 10 -14m.

The minimum distance between the center of the nucleus and the alpha particle just before it gets reflected back through 180° is defined as the distance of closest approach r0 (also known as contact distance).

The nucleus is positively charged and most of the mass of the atom is concentrated in nucleus. The nucleus is surrounded by negatively charged electrons. Since static charge distribution cannot be in a stable equilibrium, he suggested that the electrons are not at rest and they revolve around the nucleus in circular orbits like planets revolving around the sun.

Rutherford calculated the radius of the nucleus for different nuclei and found that it ranges from 10^-14 m to 10^-15m.

The impact parameter is defined as the perpendicular distance between the center of the gold nucleus and the direction of velocity vector of alpha particle when it is at a large distance.

When impact parameter increases the scattering angle decreases. Smaller the impact parameter, larger will be the deflection of alpha particles.

Rutherford atom model helps in the calculation of the diameter of the nucleus and also the size of the atom.

The Rutherford model fails to explain the distribution of electrons around the nucleus and also the stability of the atom.

According to classical electrodynamics any accelerated charge emits electromagnetic radiations. Due to emission of radiations, it loses its energy. Hence, it can no longer sustain the circular motion. The radius of the orbit, therefore, becomes smaller and smaller (undergoes spiral motion) and finally the electron should fall into the nucleus and the atoms should disintegrate. But this does not happen.

According to the electrodynamics model, emission of radiation must be continuous and must give continuous emission spectrum but experimentally we observe only line (discrete) emission spectrum for atoms.

In order to overcome the limitations of the Rutherford atom model in explaining the stability and also the line spectrum observed for a hydrogen atom, Neil’s Bohr made modifications of Rutherford atom model. He is the first person to give better theoretical model of the structure of an atom to explain the line spectrum of hydrogen atom.

Postulates of Bohr atom model: The electron in an atom moves around nucleus in circular orbits under the influence of Coulomb electrostatic force of attraction. This Coulomb force gives necessary centripetal force for the electron to undergo circular motion. Electrons in an atom revolve around the nucleus only in certain discrete orbits called stationary orbits where it does not radiate electromagnetic energy. Only those discrete orbits allowed are stable orbits.

The angular momentum of the electron in these stationary orbits are quantized that is, it can be written as integer or integral multiples of h/ 2π called as reduced Planck’s constant that is the integer n is called as principal quantum number of the orbit. l =n♄ where ♄= h /2π this condition is known as angular momentum quantization condition.

The de Broglie wavelength (λ) for an electron of mass m moving with velocity υ is  λ=h/m υ where h is called Planck’s constant.

Energy of orbits is not continuous but discrete. This is called the quantization of energy. An electron can jump from one orbit to another orbit by absorbing or emitting a photon whose energy is equal to the difference in energy (ΔE) between the two orbital levels.

The velocity of electron decreases as the principal quantum number increases. This curve is the rectangular hyperbola. This implies that the velocity of electron in ground state is maximum when compared to excited states.

On calculating wavelength or wave number difference between the faint and bright spectral lines, atomic mass of deuterium is measured to be twice that of atomic mass of hydrogen atom. Bohr atom model could not explain this isotopic shift. Thus by considering nuclear motion (although the movement of the nucleus is much smaller, it is observed) into account in the Bohr atom model, the wave number or wavelength difference between hydrogen atom and deuterium is theoretically calculated which perfectly agreed with the spectroscopic measured values.

The difference between hydrogen atom and deuterium is in the number of neutron. Hydrogen atom contains an electron and a proton, whereas deuterium has an electron, a proton and a neutron.

The energy required to excite an electron from lower energy state to any higher energy state is known as excitation energy. The excitation energy for an electron from ground state (n = 1) to first excited state (n = 2) is called first excitation energy.

The minimum energy required to remove an electron from an atom in the ground state is known as binding energy or ionization energy.

Ionization potential is defined as ionization energy per unit charge.

Thus, for a hydrogen atom (Z =1), the ionization potential is V = 13.6/ n²volt.

Materials in the solid, liquid and gaseous states emit electromagnetic radiations when they are heated up and these emitted radiations usually belong to continuous spectrum. For example, when white light is examined through a spectrometer, electromagnetic radiations of all wavelengths are observed which is a continuous spectrum.

Electrons in excited states have very small life time, these electrons jump back to ground state through spontaneous emission in a short duration of time (approximately 10 ^-8 s) by emitting the radiation with same wavelength (or frequency) corresponding to the colors it absorbed. This is called emission spectroscopy.

R is known as Rydberg constant whose value is 1.09737 x 10⁷m^-1 where m and n are positive integers such that m > n.

Lyman series: Put n = 1 and m = 2, 3, 4……. in the above equation. The wave number or wavelength of spectral lines of Lyman series which lies in ultra-violet region is v R m.

Balmer series: Put n = 2 and m = 3, 4, 5……. in the above equation. The wave number or wavelength of spectral lines of Balmer series which lies in visible region is v R m

The following are the drawbacks of Bohr atom model: Bohr atom model is valid only for hydrogen atom or hydrogen like-atoms but not for complex atoms. When the spectral lines are closely examined, individual lines of hydrogen spectrum is accompanied by a number of faint lines. These are often called fine structure. This is not explained by Bohr atom model. Bohr atom model fails to explain the intensity variations in the spectral lines. The distribution of electrons in atoms is not completely explained by Bohr atom model.

Atoms have a nucleus surrounded by electrons. The nucleus contains protons and neutrons. The neutrons are electrically neutral (q = 0) and the protons have positive charge (q = + e) equal in magnitude of the charge of the electron (q = –e).

The number of protons in the nucleus is called the atomic number and it is denoted by Z. The number of neutrons in the nucleus is called neutron number (N). The total number

of neutrons and protons in the nucleus is called the mass number and it is denoted by A. Hence, A = Z+N.

The two constituents of nucleus namely neutrons and protons, are collectively called nucleons.

The mass of a proton is 1.6726×10^-27kg which is roughly 1836 times the mass of the electron. The mass of a neutron is slightly greater than the mass of the proton and it is equal to 1.6749×10^-27 kg.

To specify the nucleus of any element, we use the following general notation _Z ^AX where X is the chemical symbol of the element, A is the mass number and Z is the atomic number.

The nucleus is made up of positively charged protons and electrically neutral neutrons, the overall charge of the nucleus are positive and it has the value of +Ze. But the atom is electrically neutral which implies that the number of electrons in the atom is equal to the number of protons in the nucleus.

In nature, there are atoms of a particular element whose nuclei have same number of protons but different number of neutrons. These kinds of atoms are called isotopes. In other words, isotopes are atoms of the same element having same atomic number Z, but different mass number A.

The chemical properties of any atom are determined only by electrons, the isotopes of any element have same electronic structure and same chemical properties. So the isotopes of the same element are placed in the same location in the periodic table.

Isobars are the atoms of different elements having the same mass number A, but different atomic number Z. In other words, isobars are the atoms of different chemical element which has same number of nucleon.

Unlike isotopes, isobars are chemically different elements. They have different physical and chemical properties.

Isotones are the atoms of different elements having same number of neutrons. 512B and 613C are examples of isotones which 7 neutrons.

The mass of nuclei is very small when it expressed in SI units (about 10-25 kg or less). Therefore, it is more convenient to express it in terms of another unit namely, the atomic mass unit (u).

One atomic mass unit (u) is defined as the 1/12th of the mass of the isotope of carbon ₆ ¹²C the most abundant naturally occurring isotope of carbon.

Experimentally the atomic mass is determined by the instrument called Bainbridge mass spectrometer. If we determine the atomic mass of the element without considering the effect of its isotopes, we get the mass averaged over different isotopes weighted by their abundances.

The nuclei are found to be approximately spherical in shape. It is experimentally found that radius of nuclei for Z > 10, satisfies the following empirical formula R = R_o A^1/3

The empirical formula R = Ro A1/3shows that the nuclear density is independent of the mass number A.

In other words, all the nuclei (Z > 10) have the same density and it is an important characteristic of the nuclei. It is experimentally found out that the mass of any nucleus is always less than the sum of the mass of its individual constituents.

The experimental mass of carbon-12 nucleus is less than the total mass of its individual constituents by Δm = 0.09888 u . This difference in mass Δm is called mass defect. In general, if M, m_p, and m_n are mass of the nucleus ( _A ^ZX ), the mass of a proton and the mass of a neutron respectively, then the mass defect is given by

Albert Einstein with the help of famous mass-energy relation (E = mc ) 2 . According to this relation, the mass can be converted into energy and energy can be converted into mass.

In the case of the carbon-12 nucleus when 6 protons and 6 neutrons combine to form carbon-12 nucleus, mass equal to mass defect disappears and the corresponding energy is released. This is called the binding energy of the nucleus (BE) and is equal to (Δm)c2 . In fact, to separate the carbon-12 nucleus into individual constituents, we must supply the energy equal to binding energy of the nucleus.

Using Einstein’s mass-energy equivalence, the energy equivalent of one atomic mass unit 1υ = 1.66 * 10^-27 x (3x 10⁸)² = 14.94x 10^-11 J ≏931 Me V

The average binding energy per nucleon is the energy required to separate single nucleon from the particular nucleus.

There must be a strong attractive force between protons to overcome the repulsive Coulomb’s force. This attractive force which holds the nucleus together is called strong nuclear force.

A few properties of strong nuclear force are: The strong nuclear force is of very short range, acting only up to a distance of a few Fermi. But inside the nucleus, the repulsive Coulomb force or attractive gravitational forces between two protons are much weaker than the strong nuclear force between two protons. Similarly, the gravitational force between two neutrons is also much weaker than strong nuclear force between the neutrons. So nuclear force is the strongest force in nature. The strong nuclear force is attractive and acts with an equal strength between proton-proton, proton-neutron, and neutron – neutron. Strong nuclear force does not act on the electrons. So it does not alter the chemical properties of the atom.

In the binding energy curve, the stability of the nucleus that has Z > 82 starts to decrease and these nuclei are called unstable nuclei. Some of the unstable nuclei decay naturally by emitting some kind of particles to form a stable nucleus.

The elements of atomic number Z > 82 and isotopes of lighter nuclei belong to naturally-occurring radioactive nuclei. Each of these radioactive nuclei decays to another nucleus by the emission of ₂ ⁴He nucleus (α-decay) or electron or positron (β-decay) or gamma rays ( γ -decay).

Emission of highly penetrating radiations such as α, β and γ rays by an element is called radioactivity and the substances which emit these radiations are called radioactive elements. These radioactive elements can be heavy elements (Z > 82), isotopes of lighter and heavy elements and these isotopes are called radioisotopes.

Radioisotopes have a variety of applications such as carbon dating, cancer treatment, etc. When radioactive nucleus undergoes decay, the mass of the system decreases – that is, the mass of the initial nucleus before decay is always greater than the sum of the mass of the final nucleus and that of the emitted particle. When this difference in mass Δm < 0 , it appears as the energy according to Einstein’s relation E = Δm c2 .

The phenomenon of radioactivity was first discovered by Henri Becquerel in 1896. Later, Marie Curie and her husband Pierre Curie did a series of experiments in detail to understand the phenomenon of radioactivity.

In India, Saha Institute of Nuclear Physics (SINP), Kolkata is the premier institute pursuing active research in nuclear physics.

In beta decay, a radioactive nucleus emits either electron or positron. If electron (e–) is emitted, it is called β- decay and if positron (e+) is emitted, it is called β+ decay. The positron is an anti-particle of an electron whose mass is same as that of electron and charge is opposite to that of electron – that is, +e. Both positron and electron are referred to as beta particles.

A very interesting application of alpha decay is in smoke detectors which prevent us from any hazardous fire.

After a detailed theoretical and experimental study, in 1931 W.Pauli proposed a third particle which must be present in beta decay to carry away missing energy and momentum.

Fermi later named this particle the neutrino (little neutral one) since it has no charge, have very little mass.

For many years, the neutrino (symbol ν, Greek nu) was hypothetical and could not be verified experimentally. Finally, the neutrino was detected experimentally in 1956 by Fredrick Reines and Clyde Cowan. Later Reines received Nobel Prize in physics in the year 1995 for his discovery.

The neutrino has the following properties· It has zero charge,· It has an antiparticle called anti-neutrino,· Recent experiments showed that the neutrino has very tiny mass, · It interacts very weakly with the matter. Therefore, it is very difficult to detect. In fact, in every second, trillions of neutrinos coming from the sun are passing through our body without any interaction.

In α and β decay, the daughter nucleus is in the excited state most of the time. The typical life time of excited state is approximately 10-11s. So this excited state nucleus immediately returns to the ground state or lower energy state by emitting highly energetic photons called γ rays.

When the atom is in the excited state, it returns to the ground state by emitting photons of energy in the order of few eV. But when the excited state nucleus returns to its ground state, it emits a highly energetic photon (γ rays) of energy in the order of MeV

In a radioactive sample which contains a vast number of the radioactive nuclei and not all the radioactive nucleus in sample decay at the same time and this decay is basically a random process. It implies that we cannot predict which nucleus is going to decay or rather we can determine like probabilistic basis (like tossing a coin). We can calculate approximately how many nuclei in a sample are decayed over a period of time. At any instant t, the number of decays per unit time called rate of decay dN / dt ¬ is proportional to the number of nuclei (N) at the same instant.

The proportionality constant λ is called decay constant which is different for different radioactive sample and the negative sign in the equation implies that the N is decreasing with time. By rewriting the equation we get dN =−λ N dt, Here dN represents the number of nuclei decaying in the time interval dt.

We can define the half-life T1/2 as the time required for the number of atoms initially present to reduce to one half of the initial amount. The half-life is the important characteristic of every radioactive sample. Some radioactive nuclei are known to have half-life as long as 10¹⁴ years and some nucleus have very shorter life time. The shorter half-life sample will have higher activity and it is more ‘radioactive’ which is more harmful.

When the radioactive nucleus undergoes the decay, the nucleus which disintegrates first has zero life time and the nucleus which decays last has an infinite lifetime. The actual life time for each nucleus varies from zero to infinity. Therefore, it is meaningful to define average life or mean life time t, that the nucleus survives before it decays.

The mean life time of the nucleus is the ratio of sum or integration of life times of all nuclei to the total number nuclei present initially.

The interesting application of beta decay is radioactive dating or carbon dating. Using this technique, the age of an ancient object can be calculated. All living organisms absorb carbon dioxide (CO₂) from air to synthesize organic molecules. In this absorbed CO₂, the major part is ₆¹²C and very small fraction (1.3x 10^-12) is radioactive ₆¹⁴C whose half-life is 5730 years.

Carbon-14 in the atmosphere is always decaying but at the same time, cosmic rays from outer space are continuously bombarding the atoms in the atmosphere which produces 614C. So the continuous production and decay of 6 ¹⁴C in the atmosphere keep the ratio of ₆ ¹⁴C to ₆ ¹²C always constant. Since our human body, tree or any living organism continuously absorbs CO₂ from the atmosphere, the ratio of ₆ ¹⁴C to ₆ ¹²C in the living organism is also nearly constant.

Keezhadi (கீழடி), a small hamlet, has become one of the very important archeological places of Tamil Nadu. It is located in Sivagangai district. A lot of artifacts (gold coins, pottery, beads, iron tools, jewelry and charcoal, etc.) have been unearthed in Keezhadi which have given substantial evidence that an ancient urban civilization had thrived on the banks of river Vaigai. To determine the age of those materials, the charcoal of 200 g sent for carbon dating.

In 1930, two German physicists Bothe and Becker found that when beryllium was bombarded with α particle highly penetrating radiation was emitted. This radiation was capable of penetrating the thick layer of lead and was unaffected by the electric and magnetic fields. Initially, it was thought as γ radiation.

In the year 1932, James Chadwick discovered that those radiations found in the Bothe and Becker experiment are not EM waves but they are particles of mass little greater than the mass of the proton and had no charge. He called them as neutrons.

Neutrons are stable inside the nucleus. But outside the nucleus they are unstable. If the neutron comes out of the nucleus (free neutron), it decays with emission of proton.

Neutrons are classified according to their kinetic energy as slow neutrons (0 to 1000 eV), fast neutrons (0.5 MeV to 10 MeV).

The neutrons with average energy of about 0.025 eV in thermal equilibrium are called thermal neutron, because at 298K, the thermal energy kT 0.025eV. Slow and fast neutrons play a vital role in nuclear reactors.

In 1939, German scientists Otto Hahn and F. Strassman discovered that when uranium nucleus is bombarded with a neutron, it breaks up into two smaller nuclei of comparable masses with the release of energy.

The process of breaking up of the nucleus of a heavier atom into two smaller nuclei with the release of a large amount of energy is called nuclear fission. The fission is accompanied by the release of neutrons.

The energy that is released in the nuclear fission is of many orders of magnitude greater than the energy released in chemical reactions. Uranium undergoes fission reaction in 90 different ways.

When one 92235U nucleus undergoes fission the energy released might be small. But from each fission reaction, three neutrons are released. These three neutrons cause further fission in another three 92235U nuclei which in turn produce nine neutrons. These nine neutrons initiate fission in another 27 92 235U nuclei and so on. This is called a chain reaction and the number of neutrons goes on increasing almost in geometric progression.

There are two kinds of chain reactions: uncontrolled chain reaction and controlled chain reaction.

In an uncontrolled chain reaction, the number of neutrons multiplies indefinitely and the entire amount of energy released in a fraction of second. The atom bomb is an example of nuclear fission in which uncontrolled chain reaction occurs. Atom bombs produce massive destruction for mankind.

If the chain reaction is controllable then we can harvest an enormous amount of energy for our needs. It is achieved in a controlled chain reaction. In the controlled chain reaction, the average number of neutron released in each stage is kept as one such that it is possible to store the released energy. In nuclear reactors, the controlled chain reaction is achieved and the produced energy is used for power generation or for research purpose.

Nuclear reactor is a system in which the nuclear fission takes place in a self-sustained controlled manner and the energy produced is used either for research purpose or for power generation.

The first nuclear reactor was built in the year 1942 at Chicago, USA by physicist Enrico Fermi.

The main parts of a nuclear reactor are fuel, moderator and control rods. In addition to this, there is a cooling system which is connected with power generation set up.

Fuel: The fuel is fissionable material usually uranium or plutonium. In addition to this, a neutron source is required to initiate the chain reaction for the first time. A mixture of beryllium with plutonium or polonium is used as the neutron source. During fission only fast neutrons are emitted but the probability of initiating fission by it in another nucleus is very low. Therefore, slow neutrons are preferred for sustained nuclear reactions.

Moderators: The moderator is a material used to convert fast neutrons into slow neutrons.

Usually the moderators are chosen in such a way that it must be very light nucleus having mass comparable to that of neutrons. Hence, these light nuclei undergo collision with fast neutrons and the speed of the neutron is reduced.

Most of the reactors use water, heavy water (D2O) and graphite as moderators. The blocks of uranium stacked together with blocks of graphite (the moderator) to form a large pile.

Control rods: The control rods are used to adjust the reaction rate. During each fission, on an average 2.5 neutrons are emitted and in order to have the controlled chain reactions only one neutron is allowed to cause another fission and the remaining neutrons are absorbed by the control rods.

Usually cadmium or boron acts as control rod material and these rods are inserted into the uranium blocks.

Depending on the insertion depth of control rod into the uranium, the average number of neutrons produced per fission is set to be equal to one or greater than one. If the average number of neutrons produced per fission is equal to one, then reactor is said to be in critical state. In fact, all the nuclear reactors are maintained in critical state by suitable adjustment of control rods. If it is greater than one, then reactor is said to be in super-critical and it may explode sooner or may cause massive destruction.

Cooling system: The cooling system removes the heat generated in the reactor core. Ordinary water, heavy water and liquid sodium are used as coolant since they have very high specific heat capacity and have large boiling point under high pressure.

India has 22 nuclear reactors in operation. Nuclear reactors are constructed in two places in Tamilnadu, Kalpakkam and Koodankulam. Even though nuclear reactors are aimed to cater to our energy need, in practice nuclear reactors now are able to provide only 2% of energy requirement of India.

When two or more light nuclei (A<20) combine to form a heavier nucleus, then it is called nuclear fusion. In the nuclear fusion, the mass of the resultant nucleus is less than the sum of the masses of original light nuclei. The mass difference appears as energy. The nuclear fusion never occurs at room temperature unlike nuclear fission. It is because when two light nuclei come closer to combine, it is strongly repelled by the coulomb repulsive force.

To overcome this repulsion, the two light nuclei must have enough kinetic energy to move closer to each other such that the nuclear force becomes effective. This can be achieved if the temperature is very much greater than the value 107 K. When the surrounding temperature reaches around 107K, lighter nuclei start fusing to form heavier nuclei and this resulting reaction is called thermonuclear fusion reaction.

The natural place where nuclear fusion occurs is the core of the stars, since its temperature is of the order of 107K. In fact, the energy generation in every star is only through thermonuclear fusion. Most of the stars including our Sun fuse hydrogen into helium and some stars even fuse helium into heavier elements.

The early stage of a star is in the form of cloud and dust. Due to their own gravitational pull, these clouds fall inward. As a result, its gravitational potential energy is converted to kinetic energy and finally into heat. When the temperature is high enough to initiate the thermonuclear fusion, they start to release enormous energy which tends to stabilize the star and prevents it from further collapse.

The sun’s interior temperature is around 1.5×10⁷K. The sun is converting 6×10¹¹kg hydrogen into helium every second and it has enough hydrogen such that these fusion lasts for another 5 billion years. When the hydrogen is burnt out, the sun will enter into new phase called red giant where helium will fuse to become carbon. During this stage, sun will expand greatly in size and all its planets will be engulfed in it.

According to Hans Bethe, the sun is powered by proton-proton cycle of fusion reaction. This cycle consists of three steps.

In 1964, physicist Murray Gellman and George Zweig theoretically proposed that protons and neutrons are not fundamental particles in fact they are made up of quarks. These quarks are now considered elementary particles of nature.

Electrons are fundamental or elementary particles because they are not made up of anything.

In the year 1968, the quarks were discovered experimentally by Stanford Linear Accelerator Center (SLAC), USA.

There are six quarks namely, up, down, charm, strange, top and bottom and their antiparticles. All these quarks have fractional charges. For example, charge of up quark is + 2 /3 e and that of down quark is − 1/3 e.

According to quark model proton is made up of two up quarks and one down quark and neutron is made up of one up quark and two down quarks.

It is known that there exists gravitational force between two masses and it is universal in nature. Our planets are bound to the sun through gravitational force of the sun.

Gravitational, electromagnetic, strong and weak forces are called fundamental forces of nature. It is very interesting to realize that, even for our day-to-day life we require these four fundamental forces.