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What will be discussed in this chapter? fundamental particles of the atom Types of forces holding up the atom and its particles together Nuclear stability.

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Presentation on theme: "What will be discussed in this chapter? fundamental particles of the atom Types of forces holding up the atom and its particles together Nuclear stability."— Presentation transcript:


2 What will be discussed in this chapter? fundamental particles of the atom Types of forces holding up the atom and its particles together Nuclear stability Natural radioactivity and types of radioactive decay Artificial radioactivity Nuclear energy Health hazards

3 After 1932, physicists viewed all matter as consisting of only three constituent particles: electrons, protons, and neutrons.

4 Beginning in 1945, many new particles were discovered in experiments involving high-energy collisions between known particles. These new particles are characteristically very unstable, and have a very short half-lives, ranging between and s. So far, more than 300 of these unstable, temporary particles have been catalogued.

5 The current theory of elementary particles of atoms, the standard model, claims that all matter is believed to be constructed from only two families of particles: QUARKS and LEPTONS

6 LEPTONS Leptons (from the Greek word leptos meaning, small or light) are group of particles which participate in the weak interaction. Included in this group are : electrons (e), muons (μ), and taus (τ).

7 They interact only through weak and electromagnetic forces. There are six types of leptons. Each lepton has its own antiparticle. A neutrino is associated with each lepton. Have elementary structure which means that they dont seem to break down into smaller units. LEPTONS

8 Particle namesymb ol Anti-particlelifetime(s) Electrone-e- e + (positron)Stable Electron neutrino νeνe e Stable Muonμ-μ- μ+μ+ Unstable Nutrino muonνμνμ μStable Tauτ-τ- τ+τ+ Unsatble Neutrino tauντντ τstable TYPES OF LEPTONS

9 The neutrino tau hasnt been discovered yet but its presence is believed.

10 Electron is the lepton having the smallest mass.

11 QUARKS The unusual property of quarks is that they have fractional electronic charges. Associated with each quark is an antiquark of opposite charge. There are six types of quarks : up (u), down (d), strange (s), charmed (c), top (t), and bottom (b).

12 TYPES OF QUARKS Name of the particle symbolcharge Upu+ 2/3 Downd-1/3 Stranges-1/3 Charmedc+ 2/3 bottomb-1/3 topt+ 2/3

13 Protons and neutrons are formed as a combination of different types of quarks. PROTON-- two up [(+ 2/3) + (+ 2/3) ] & one down quark (-1/3)

14 NEUTRON-- two down [(- 1/3) + (-1/3) ] & one up quark (+2/3)

15 THE FUNDAMENTAL FORCES IN NATURE The strong nuclear forces The electromagnetic forces The weak nuclear forces The gravitational force The strength decreases downward.

16 THE STRONG NUCELAR FORCES These are the strongest forces holding the quarks in protons and neutrons together. They have the shortest range ( m), meaning that particles must be extremely close before their effects are felt. The quarks are considered to be held together by the color force. The strong force between nucleons may be considered to be a residual color force. color force

17 A property of quarks labeled color is an essential part of the quark model.color It has nothing whatever to do with real color provides distinct quantum states. THE STRONG NUCELAR FORCES

18 Analogous to the exchange of photons in the electromagnetic force between two charged particles, Gluons are the exchange particles for the color force between quarks. The color force involves the exchange of gluons. THE STRONG NUCELAR FORCES


20 WEAK NUCLEAR FORCES It is responsible for the radioactive decay of subatomic particles. Its strength is about times the strong forces. Its a short range force. If the nucleon number in a nucleus isnt too much, the attraction forces between the nucleons,the strong nuclear forces, counteract the repulsion forces, the weak nuclear forces, between the protons.

21 If the nucleon number is too much (Z >83), then the distance between the nucleons will be big and the weak nuclear forces will approach zero (because of the increase in the distance between the nucleons). Therefore, the electrical repulsion forces between the protons will be more effective making the nucleus unstable and the nucleus undergoes radioactive decays (e.g., beta decay).

22 Nuclear Stability The stability of an atom is the balance of the repulsive and attractive forces within the nucleus.

23 Nuclear Stability If the attractive strong forces prevail, the nucleus is stable. Stable nuclei dont undergo radioactive decay. If the repulsive weak forces outweigh the attraction forces, the nucleus is unstable and undergoes radioactive reactions spontaneously. Such nuclei are called radioactive nuclei.

24 Think about it… How might a higher number of neutrons change the balance between the repulsive and attractive forces in a nucleus? How might a lower number of neutrons affect this same balance?

25 Light nuclei are most stable if they contain an equal number of protons and neutrons, if N=Z. Nuclear Stability

26 Heavy nuclei are more stable if the number of neutrons exceed the number of protons (Remember that, as the number of protons increases, the strength of the Coulomb (repulsion) forces increase, which tends to break the nucleus apart). As a result, more neutrons are needed to keep the nucleus stable since neutrons experience only the attractive nuclear forces. Nuclear Stability

27 Therefore, the ratio of n 0 /p + determines the stability of a nucleus. For the stable nuclei, this ratio is close to 1. This ratio is 1 for the atoms with atomic number smaller than 20 (though this has exceptions for some isotopes). As atomic number increases, stable atoms have ratios greater than one, can reach 1.5.

28 Elements having more than 83 protons do not have stable nuclei. The isotopes of all of these atoms are radioactive. Nuclear Stability

29 The shaded cluster is the band of stability. The stable nuclei are present in the band of stability. The solid line represents a neutron-to- proton ratio of 1:1.

30 The nuclei which arent in the band of stability are radioactive and try to enter the band of stability as a result of some radioactive decays. Nuclear Stability

31 Nuclei to the right of the band of stability dont have enough neutrons to remain stable. Nuclei to the left of the band have too many neutrons to remain stable. Nuclear Stability n/p <1 n/p >1

32 Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles.

33 Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture.

34 Stable Nuclei There are no stable nuclei with an atomic number greater than 83. These nuclei tend to decay by alpha emission.

35 Chemical Reactions vs Nuclear Reactions Chemical ReactionsNuclear Reactions Electrons react outside nucleus. Protons and neutrons react inside nucleus. The same number of each kind of atom appear in the reactants and products. Elements transmute into other kind of elements. Isotopes react the same.Isotopes react differently. Mass reactants = mass products. Mass changes are detectable. Energy changes equal ~10 3 kJ. Energy changes equal ~10 8 kJ/mol. Rate of reaction is affected by temperature, pressure, concentration and use of a catalyst Rate of reaction is not affected by temperature, pressure, concentration and use of a catalyst

36 Radioactive nuclei are generally classified into two groups: 1) unstable nuclei found in nature, which give rise to what is callednatural radioactivity. 2) nuclei produced in the laboratory through nuclear reactions, which exhibit artificial radioactivity.

37 NATURAL RADIOACTIVITY In natural radioactivity, the unstable nuclei undergo the radioactive reactions spontaneously until they reach a stable configuration. These transformations are accompanied by releases of energy. These radiations are alpha radiation(ışıma) beta radiation positron emission(yayma) electron capture

38 TYPES OF NATURAL RADIOACTIVE DECAYS 1. Alpha Decay (2p +,2n o ) Nucleus emits an alpha particletwo protons and two neutrons Alpha particle is a helium nucleus. U Th He

39 1. Alpha Decay For an atom which emits rays; p + number decreases by two, Atomic number decreases by two, n 0 number decreases by two, Mass number decreases by four.

40 1. Alpha Decay

41 Properties of α- rays They have a fogging effect on the photographic films. Charge: α - particle carry positive charge. Its nuclear charge is +2. Mass: Mass of each α – particle is 4 times that of a proton or H- atom. Penetration power: α - rays have very small penetration power.They are stopped by a sheet of paper. Effect of human body: α - rays can be stopped by the skin but very damaging to the skin due to ionization power. Artificial radioactivity: α - rays can produce artificial radioactivity in certain nuclei. Ionization capability : They have strong ionizing power because they remove electrons from the atoms of gas through which they pass. With gained electrons, they become He gas. Velocity: Their velocity range is 3 x 10 7 m/s to 3 x 10 6 m/s. They deflect towards the (-) side in the magnetic field.

42 2. Beta Decay Beta decay is loss of a -particle. -particle is an electron having high speed e 0101 or I Xe e 0101 n 1010 p e a neutron is converted to a proton. -

43 For an atom which emits - rays; *P + number increases by 1 *Atomic number increases by 1 *n o number decreases by 1 *Mass number doesnt change 2. Beta Decay

44 Properties of β- rays Nature: β - rays consist of fast moving electrons. Charge: β – rays have negative charge. That is - 1. Penatrating power: β – rays have 100 times greater penatrating power than alpha rays. They are stopped by a thin sheet of any metal. For example; aluminum Velocity: Their velocity range is 9 x 10 7 m/sec to 27 x 10 7 m/sec. Ionization power: Ionization power of β - rays is very small. They have a fogging effect on the photographic film. β – rays deflect towards the (+) side in the magnetic field.

45 3. Gamma emission ( γ ) Nuclei seeking lower energy states emit electromagnetic radiation, which is in the gamma ray region of the electromagnetic spectrum. Rays are emitted in conjunction with another type of decay (alpha or beta). An atom sometimes may remain in an excited nuclear state after radioactive decays. decay removes the excess energy and leaves back the atom in the ground state. Gamma Decay Additional animations:

46 3. Gamma emission ( γ )

47 Properties of γ - rays Nature: γ - rays are not particles. They are electromagnetic radiations with high energy. Charge: γ – rays have no charge. They are neutral. They dont deflect in the magnetic field. Mass: γ – rays have no mass. Velocity: γ - rays travel with the velocity of light that is 3 x 10 8 m/sec. Penetration power: Penetration power of γ – rays is very large. It is about hundred times larger than that of β -rays. They are stopped by a thick layer of lead.


49 WARNING!!!!! If an atom emits number of - s which are twice the number of s,then the isotope of the atom is formed.

50 FAJANS RULE:If an atom emits an, it forms an atom that is 2 behind than the atom which emitted the in the periodic table. If an atom emits a,it forms an atom which is 1 forward than the atom that emitted the in the periodic table.

51 4. Positron Emission It is the loss of a positron. Positron is positive electron. It is a particle having the same mass as but opposite charge of an electron. e 0 +1 C 11 6 B e 0101 Positron is formed when a proton converts to a neutron.

52 6. Electron Capture Addition of an electron to a proton in the nucleus. As a result, a proton is transformed into a neutron. Since the nucleus captures an orbital electron from K shell, it is a natural radioactivity. p e 0101 n 1010 Be 7474 e Li

53 6. Electron Capture Happens to nuclei with a low neutron:proton ratio A proton becomes a neutron causing a shift up and to the left. Always results in gamma radiation


55 5. Neutron Capture During neutron capture, isotope of parent nucleus is produced. This is not a natural radioactive event. n 1010 Kr Kr n γ

56 Which one(s) of the following statements is/are true for atom having following reaction in its nucleus ? 1 1 p 0 1 n β I. Its mass number increases by 1. II. Its isotope is formed. III. Its netron number decreases by 1. IV. Its atomic number decreases by 1. V. Its number of protons increases by 1. Solution: In the reaction given above, one proton is converted into one neutron.Thus, atomic number decreases by 1. So, IV is true EXAMPLE: 1

57 EXAMPLE: 2 Find X and Y in following reactions. I K Ar + X II Hg + Y Au Solution: X is +1 0 β I. II. Y is -1 0 β

58 EXAMPLE: 3 Find atomic number and mass number of Y in the following reaction X + β - + α Y + γ + 2β + Solution: Y

59 EXAMPLE: 4 A, B, C and D elements form compounds AC, A 2 D and BD. If AC and A 2 D are radioactive and BD is not radioactive compound, find whether the following compounds are radioactive or not. I. A 2 II. A 2 C III. C 2 D IV. BC Solution: A 2 and A 2 C are radioactive compounds. C 2 D and BC are radioactive or not.

60 For example 92 U 238 will go through 8 alpha emissions and 6 beta emissions (not all in order) before becoming 82 Pb 206 The steps a nuclei follows in becoming stable is called a radioactive series. The series for 92 U 238 is shown below as an example. Large radioactive nuclei can not stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).



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