Upload
nelsrodrig
View
223
Download
0
Embed Size (px)
Citation preview
8/8/2019 Bsc Maths-II
1/13
CONSTITUENTS
OF
NUCLEI
8/8/2019 Bsc Maths-II
2/13
Group membersGroup members
JINITHA MARY 4105JINITHA MARY 4105 AMALA JOSE 4114 AMALA JOSE 4114
LIGI JOB 4135LIGI JOB 4135
8/8/2019 Bsc Maths-II
3/13
The ProtonThe Proton--ElectronElectron
HypothesisHypothesis As early as 1816,on the basis of small number of atomic weight of elementsthen known,Prout suggested that all atomic weights being whole numbers,
they are integral multiples of the atomic weight of hydrogen and so all
elements must be built up of hydrogen. But as it was found that atomic
weights of some elements are fractional,Prouts hypothesis was
discarded.Still,since many elements have atomic weights which are veryclose to whole numbers, it seemed that there is some basis for Prouts
hypothesis. His suggestion received new support during the early years of
the 20th century when the study of the radioactive elements led to the
discovery of the isotopes. It was concluded that the atomic weights of the
elements become fractional due to the existence of two or more isotopes of
the elements. This led to formulate his whole number rule, which is really amodified form of Prouts hypothesis. The analysis of positive rays from
different substances revealed that the lightest positively charged particle
ever found had the same mass as the hydrogen atom, and carried unit
charge equal in magnitude to the electronic charge, but of opposite sign.
8/8/2019 Bsc Maths-II
4/13
This particle was confirmed to be the nucleus of the hydrogen atom,and it was found to have a massvery close to one atomic massesunit. The combination of the whole number rule and the properties ofthe hydrogen nucleus led to the conclusion that atomic nuclei arecomposed of hydrogen nuclei.The hydrogen nucleus was called theproton.
The atomic nucleus-the central massive core of the atom-wasdiscovered by Rutherford in 1911 through his alpha scatteringexperiments. The experimental skill of Marsden confirmed its
existence .According to them each atom consists of a very smallnucleus having radius of the order of 10-14.The discovery of thenucleus led to the Bohr model of the atom. However, in thesedevelopments, the nucleus was regarded just a point having positivecharge, located at the centre of the atom and all the importantproperties were attributed to the electron revolving round thenucleus. The studies on radioactivity revealed the very significant
role the nucleus has in the theory of the atom. The fact that inradioactivity atomic nuclei emit alpha and beta particles havingcorpuscular nature indicated that the nucleus has some constituentsinside. The constituents should have properties to enable them to bebound together inside the nucleus. Thus the nucleus should have adefinite structure.
8/8/2019 Bsc Maths-II
5/13
ATOMIC RADIUS VARIATION IN THE PERIODICATOMIC RADIUS VARIATION IN THE PERIODIC
TABLETABLE
8/8/2019 Bsc Maths-II
6/13
The fact that radioactive nuclei emit alpha particles led to the
speculation that in each nucleus there must be sufficient number ofthose particles. This idea was forced to be ruled out due to the
existence of the hydrogen nucleus, namely the proton. Then came
the next immediate suggestion that all nuclei are made up of
protons. If this is accepted, the mass number A and the atomic
number Z of a nucleus should be identical.However,with the only
exception of hydrogen, they are quite different for atomic nuclei. Toovercome this difficulty, the proton-electron model of the nucleus
was suggested. In this model, it was postulated that the nucleus with
mass number A and atomic number Z ,contained a protons and(A-Z)
electrons. With this theory, the charge of the nucleus is
A(+e)+(A-Z)(-e)=Ze.Thus the theory could explain the mass numberand the charge of the nucleus.
8/8/2019 Bsc Maths-II
7/13
The proto-electron hypothesis seemed to give satisfactoryexplanation for the emission of alpha and beta particles inradioactiveity.Beta particles are identical with the electrons. Thepresence of electrons in the nucleus make it seem reasonable thatunder appropriate conditions one of them may be ejected. It is alsoapparently reasonable to assume that an alpha particle may beformed inside the nucleus by the combination of four protons andtwo electrons. The alpha particle may exist as such inside thenucleus or may be formed at the instant of emission.
Though the hypothesis seemed to have some satisfactory aspects, ithad to be abandoned due to certain contradictions. The detailedstudy of spectral lines of certain elements revealed that the atomicnucleus has an angular momentum or nuclear spin with which isassociated a magnetic moment. The proton-electron hypothesis
could not satisfactorily explain these special features of the nucleus.The hypothesis also failed to account for the energy of the electron ifit were present in the nucleus. Developments in wave mechanicsconclusively established that electrons cannot be present in thenucleus. The problem could be solved with the discovery of theneutron by Chadwick in 1932.
8/8/2019 Bsc Maths-II
8/13
The ProtonThe Proton--Neutron HypothesisNeutron Hypothesis The discovery of the neutral particle,neutron,having mass equal to
unity led to the assumption that every atomic nucleus consists of
protons and neutrons. This proton-neutron hypothesis was used for
the first time by Heisenberg in 1932 as the basis of a detailed theory
of the nucleus. According to it, since the protons and neutrons haveunit mass, the total number of particles in the nucleus-protons plus
neutrons-is equal to the mass number A of the nucleus. The number
of protons is given by the atomic number Z.Therefore the number of
neutrons in the nucleus or the neutron number N=(A-Z).In the case
of many elements, the nuclei are found to contain the same number
of protons but different number of neutrons ie,they have the sameatomic number Z,but different mass numbers A. They are identical in
chemical properties since the properties depend on the atomic
number Z.such nuclei are called isotopes.
8/8/2019 Bsc Maths-II
9/13
Example: Hydrogen has three isotopes-ordinary hydrogen with Z=1and A=1,heavy hydrogen or deuterium with Z=1 and A==2 and
tritium with Z=1 and A=3.Similarly carbon has six isotopes, all with
atomic number 6 and mass numbers ranging from 11 to 16. In
naturally occurring carbon, they are present in different percentages.
These percentages are referred to as natural abundances.Artificialltisotopes may be produced by nuclear reactions.
8/8/2019 Bsc Maths-II
10/13
HYDROGEN ISOTOPESHYDROGEN ISOTOPES
8/8/2019 Bsc Maths-II
11/13
Isotopes of carbon
8/8/2019 Bsc Maths-II
12/13
The proton-neutron hypothesis satisfies the observations regardingnuclear angular momenta.The hypothesis also gives satisfactoryexplanation for radioacivity.Electrons can never be present in thenucleus due to various reasons. Still in radioactivity, which isessentially a nuclear disintegration process, beta particles which areidentical with the electrons are emitted. The explanation for this isthat the beta particles are created only in the act of emission andnever before. The emission is regarded as the result of the changeof a neutron in the nucleus into a proton with the emission of anelectron and a new particle called the neutrino. Both theoretical andexperimental evidence offer strong support to this view. Similarly analpha particle can be formed by the combination of two protons andtwo neutrons. They may combine and exist as an alpha particleinside the nucleus or it may be formed just at the moment of
emission. The latter possibility is considered more likely.
8/8/2019 Bsc Maths-II
13/13
THANK YOU