Chapter Thirteen NUCLEI

1 INTRODUCTIONIn the previous Chapter , we have learnt that in every atom, the positivecharge and mass are densely concentrated at the centre of the atomforming its nucleus . The overall dimensions of a nucleus are much smallerthan those of an atom. Experiments on scattering of a-particlesdemonstrated that the radius of a nucleus was smaller than the radiusof an atom by a factor of about 104. This means the volume of a nucleusis about 10 12 times the volume of the atom. In other words, an atom isalmost empty. If an atom is enlarged to the size of a classroom, the nucleuswould be of the size of pinhead. Nevertheless, the nucleus contains most(more than ) of the mass of an the nucleus have a structure, just as the atom does?

2 If so, whatare the constituents of the nucleus ? How are these held together? In thischapter, we shall look for answers to such questions. We shall discussvarious properties of NUCLEI such as their size, mass and stability, andalso associated nuclear phenomena such as radioactivity, fission and ATOMIC MASSES AND COMPOSITION OF NUCLEUSThe mass of an atom is very small, compared to a kilogram; for example,the mass of a carbon atom, 12C, is 10 26 kg. Kilogram is nota very convenient unit to measure such small quantities. Therefore, aChapter ThirteenNUCLEIR ationalised 2023-24307 Nucleidifferent mass unit is used for expressing atomic masses. This unit is theatomic mass unit (u), defined as 1/12th of the mass of the carbon (12C)atom.

3 According to this definition12 mass of one C atom1u = 12 10kg12 = 10kg = ( )The atomic masses of various elements expressed in atomic massunit (u) are close to being integral multiples of the mass of a hydrogenatom. There are, however, many striking exceptions to this rule. Forexample, the atomic mass of chlorine atom is measurement of atomic masses is carried out with a massspectrometer, The measurement of atomic masses reveals the existenceof different types of atoms of the same element, which exhibit the samechemical properties, but differ in mass. Such atomic species of the sameelement differing in mass are called isotopes. (In Greek, isotope meansthe same place, they occur in the same place in the periodic table ofelements.)

4 It was found that practically every element consists of a mixtureof several isotopes. The relative abundance of different isotopes differsfrom element to element. Chlorine, for example, has two isotopes havingmasses u and u, which are nearly integral multiples of themass of a hydrogen atom. The relative abundances of these isotopes and per cent, respectively. Thus, the average mass of a chlorineatom is obtained by the weighted average of the masses of the twoisotopes, which works out to be= + = uwhich agrees with the atomic mass of the lightest element, hydrogen has three isotopes having u, u, and u. The nucleus of the lightest atom ofhydrogen, which has a relative abundance of , is called theproton.

5 The mass of a proton u 10kgpm == ( )This is equal to the mass of the hydrogen atom (= ), minusthe mass of a single electron (me = u). The other two isotopes ofhydrogen are called deuterium and tritium. Tritium NUCLEI , beingunstable, do not occur naturally and are produced artificially positive charge in the nucleus is that of the protons. A protoncarries one unit of fundamental charge and is stable. It was earlier thoughtthat the nucleus may contain electrons, but this was ruled out later usingarguments based on quantum theory. All the electrons of an atom areoutside the nucleus . We know that the number of these electrons outsidethe nucleus of the atom is Z, the atomic number.

6 The total charge of theRationalised 2023-24 Physics308atomic electrons is thus ( Ze), and since the atom is neutral, the chargeof the nucleus is (+Ze). The number of protons in the nucleus of the atomis, therefore, exactly Z, the atomic of NeutronSince the NUCLEI of deuterium and tritium are isotopes of hydrogen, theymust contain only one proton each. But the masses of the NUCLEI ofhydrogen, deuterium and tritium are in the ratio of 1:2:3. Therefore, thenuclei of deuterium and tritium must contain, in addition to a proton,some neutral matter. The amount of neutral matter present in the nucleiof these isotopes, expressed in units of mass of a proton, is approximatelyequal to one and two, respectively.

7 This fact indicates that the NUCLEI ofatoms contain, in addition to protons, neutral matter in multiples of abasic unit. This hypothesis was verified in 1932 by James Chadwickwho observed emission of neutral radiation when beryllium NUCLEI werebombarded with alpha-particles (a-particles are helium NUCLEI , to bediscussed in a later section). It was found that this neutral radiationcould knock out protons from light NUCLEI such as those of helium, carbonand nitrogen. The only neutral radiation known at that time was photons(electromagnetic radiation). Application of the principles of conservationof energy and momentum showed that if the neutral radiation consistedof photons, the energy of photons would have to be much higher than isavailable from the bombardment of beryllium NUCLEI with clue to this puzzle, which Chadwick satisfactorily solved, was toassume that the neutral radiation consists of a new type of neutralparticles called neutrons.

8 From conservation of energy and momentum,he was able to determine the mass of new particle as very nearly thesame as mass of proton .The mass of a neutron is now known to a high degree of accuracy. It ismn = u = 10 27 kg( )Chadwick was awarded the 1935 Nobel Prize in Physics for hisdiscovery of the free neutron, unlike a free proton, is unstable. It decays into aproton, an electron and a antineutrino (another elementary particle), andhas a mean life of about 1000s. It is, however, stable inside the composition of a nucleus can now be described using the followingterms and symbols:Z - atomic number = number of protons[ (a)]N - neutron number = number of neutrons[ (b)]A - mass number = Z + N = total number of protons and neutrons[ (c)]One also uses the term nucleon for a proton or a neutron.

9 Thus thenumber of nucleons in an atom is its mass number species or nuclides are shown by the notation XAZ where X isthe chemical symbol of the species. For example, the nucleus of gold isdenoted by 19779Au. It contains 197 nucleons, of which 79 are protonsand the rest118 are 2023-24309 NucleiThe composition of isotopes of an element can now be readilyexplained. The NUCLEI of isotopes of a given element contain the samenumber of protons, but differ from each other in their number of , 21H, which is an isotope of hydrogen, contains one protonand one neutron. Its other isotope tritium, 31H, contains one proton andtwo neutrons. The element gold has 32 isotopes, ranging from A =173 toA = 204.

10 We have already mentioned that chemical properties of elementsdepend on their electronic structure. As the atoms of isotopes haveidentical electronic structure they have identical chemical behaviour andare placed in the same location in the periodic nuclides with same mass number A are called isobars. Forexample, the nuclides 31H and 32He are isobars. Nuclides with sameneutron number N but different atomic number Z, for example 19880 Hgand 19779Au, are called SIZE OF THE NUCLEUSAs we have seen in Chapter 12, Rutherford was the pioneer whopostulated and established the existence of the atomic nucleus . AtRutherford s suggestion, Geiger and Marsden performed their classicexperiment: on the scattering of a-particles from thin gold foils.