The Atomic Nucleus Beryllium Atom Compacted nucleus: 4 protons 5 neutrons Since atom is electri- cally neutral, there must be 4 electrons. 4 electrons

Definitions A nucleon is a general term to denote a nuclear particle - that is, either a proton or a neutron. The atomic number Z of an element is equal to the number of protons in the nucleus of that element. The mass number A of an element is equal to the total number of nucleons (protons + neutrons). The mass number A of any element is equal to the sum of the atomic number Z and the number of neutrons N : A = N + Z

Symbol Notation A convenient way of describing an element is by giving its mass number and its atomic number, along with the chemical symbol for that element. For example, consider beryllium (Be):

Describe the nucleus of a lithium atom which has a mass number of 7 and an atomic number of 3. Lithium Atom N = A – Z = A = 7; Z = 3; N = ? Protons: Z = 3 neutrons: N = 4 Electrons: Same as Z

Isotopes of Elements Isotopes are atoms that have the same number of protons (Z 1 = Z 2 ), but a different number of neutrons (N). (A 1  A 2 ) Helium - 4 Helium - 3 Isotopes of helium

Nuclides Because of the existence of so many isotopes, the term element is sometimes confusing. The term nuclide is better. A nuclide is an atom that has a definite mass number A and Z-number. A list of nuclides will include isotopes. The following are best described as nuclides:

Radioactivity As the heavier atoms become more unstable, particles and photons are emitted from the nucleus and it is said to be radioactive. All elements with A > 82 are radioactive. Examples are: Alpha particles    particles (electrons)   particles (positrons) Gamma rays    

Nuclear Decay and Reactions Radioactive elements often go through a series of successive decays, until they form a stable nucleus. For example Uranium undergoes 14 separate decays before the stable isotope Lead is produced. When the ratio of N/z gets very large, the nucleus becomes unstable and often emits particles or photons

The Alpha Particle An alpha particle  is the nucleus of a helium atom consisting of two protons and two neutrons tightly bound. Charge = +2e - = 3.2 x C Mass = u Mass = u Relatively low speeds (  0.1c ) Not very penetrating Not very penetrating

The Beta Particle A beta-minus particle    is simply an electron that has been expelled from the nucleus. Charge = e - = -1.6 x C - High speeds (near c) - Mass = u Mass = u - Very penetrating Very penetrating-

The Gamma Photon A gamma ray  has very high electromagnetic radiation carrying energy away from the nucleus. Charge = Zero (0)  Mass = zero (0)  Speed = c (3 x 10 8 m/s)  Most penetrating radiation 

FISSION REACTIONS

Rules for Balancing 1.Balance Mass 2.Balance Charge Balancing Nuclear Reactions

Find the missing product from the neutron alpha bombardment of Aluminum - 27

Alpha Decay Alpha decay results in the loss of two protons and two neutrons from the nucleus. X is parent atom and Y is daughter atom The energy is carried away primarily by the K.E. of the alpha particle. Alpha decay: In alpha decay, an unstable nucleus produces a daughter nucleus and releases an  particle.

Write the reaction that occurs when radium- 226 decays by alpha emission.

Find the missing product from the neutron bombardment of Lithium-6

U-238 decays to produce (what) and an alpha particle?

Write the reaction that occurs when radium-226 decays by alpha emission. From tables, we find Z and A for nuclides. The daughter atom: Z = 86, A = 222 Radium-226 decays into radon-222.

Beta Decay Beta-minus   decay results when a neutron decays into a proton and an electron. Thus, the Z-number increases by one. X is parent atom and Y is daughter atom The energy is carried away primarily by the K.E. of the electron. -

Find the missing product from the Beta Decay carbon of 14

Nuclear Decay and Reactions Beta particles are electrons emitted by the nucleus. The nucleus contains no electrons, however, so where do these electrons come from? Beta decay occurs when a neutron is changed to a proton within the nucleus. Beta Decay In all reactions, charge must be conserved. That is, the charge before the reaction must equal the charge after the reaction.

A redistribution of the energy within the nucleus results in gamma decay. Gamma Decay The γ ray is a high-energy photon. Neither the mass number nor the atomic number is changed in gamma decay. Gamma radiation often accompanies alpha and beta decay.

Write the reaction that occurs when Thorium 234 decays by beta emission and gamma radiation is emitted.

Nuclear Decay and Reactions The three types of radiation are summarized in the table below.

Nuclear Decay and Reactions To create a controlled chain reaction and make use of the energy produced, the neutrons need to interact with the fissionable uranium at the right rate. Nuclear Reactors Most of the neutrons released by the fission of U atoms are moving at high speeds These are called fast neutrons. In addition, naturally occurring uranium consists of less than one percent U and more than 99 percent U

Nuclear Decay and Reactions Nuclear Reactors Thus, most neutrons released by the fission of U are unable to cause the fission of another U atom When a U nucleus absorbs a fast neutron, it does not undergo fission, but becomes a new isotope, U The absorption of neutrons by U keeps most of the neutrons from reaching the fissionable U atoms

Nuclear Decay and Reactions To control the reaction, the uranium is broken up into small pieces and placed in a moderator, a material that can slow down, or moderate, the fast neutrons. When a neutron collides with a light atom it transfers momentum and energy to the atom. In this way, the neutron loses energy. In this way, the neutron loses energy. Nuclear Reactors The moderator thus slows many fast neutrons to speeds at which they can be absorbed more easily by U than by U

Nuclear Decay and Reactions The type of nuclear reactor used in the United States, the pressurized water reactor, contains about 200 metric tons of uranium sealed in hundreds of metal rods. The rods are immersed in water, as shown in the figure. Nuclear Reactors

Atomic Mass Unit, u One atomic mass unit (1 u) is equal to one- twelfth of the mass of the most abundant form of the carbon atom--carbon-12. Atomic mass unit: 1 u = x kg Common atomic masses: Proton: u Neutron: u Electron: uHydrogen: u

The average atomic mass of Boron-11 is u. What is the mass of the nucleus of one boron atom in kg? Electron: u = The mass of the nucleus is the atomic mass less the mass of Z = 5 electrons: Mass = u – 5( u) 1 boron nucleus = u m = 1.83 x kg

Mass and Energy Recall Einstein’s equivalency formula for m and E: The energy of a mass of 1 u can be found: E = (1 u)c 2 = (1.66 x kg)(3 x 10 8 m/s) 2 E = 1.49 x J Or E = MeV When converting amu to energy:

What is the rest mass energy of a proton ( u)? E = mc 2 = ( u)(931.5 MeV/u) Proton: E = MeV Similar conversions show other rest mass energies: Electron: E = MeV Neutron: E = MeV

The Mass Defect The mass defect is the difference between the rest mass of a nucleus and the sum of the rest masses of its constituent nucleons. The whole is less than the sum of the parts! Consider the carbon-12 atom ( u): Nuclear mass = Mass of atom – Electron masses = u – 6( u) = u The nucleus of the carbon-12 atom has this mass. (Continued...)

The Binding Energy The binding energy E B of a nucleus is the energy required to separate a nucleus into its constituent parts. E B = m D c 2 where c 2 = MeV/u The binding energy for the carbon-12 example is: E B = ( E B = ( u)(931.5 MeV/u) E B = 92.2 MeVBinding E B for C-12:

Binding Energy per Nucleon An important way of comparing the nuclei of atoms is finding their binding energy per nucleon: Binding energy per nucleon For our C-12 example A = 12 and:

Mass Defect (Continued) Mass of carbon-12 nucleus: Proton: u Neutron: u The nucleus contains 6 protons and 6 neutrons: 6 p = 6( u) = u 6 n = 6( u) = u Total mass of parts: = u Mass defect m D = u – u m D = u

Where Elements On the Periodic Table Came From (and what the sun tells us)

Origin Of Universe Our Sun is Our Best Indicator Our Sun is small – largest sun plotted is 100 times larger

Conditions at the Sun’s core are extreme temperature is 15.6 million Kelvin pressure is 250 billion atmospheres The Sun’s energy produced by nuclear fusion reactions.

Sun’s Basic Forces 1. Gravity Significant only when masses are large Attractive only 2. Electro-magnetic force Like charges repel; opposites attract Much stronger than gravity for electrons and protons 3. Strong force Operates only at very short distances “felt” only by atomic nuclei

Conditions for Fusion (Temperature) 1. Particles must be hot enough (Temperature) 2. Particles must be in sufficient number (High Density) 3. Particles must be well contained (Confinement Time) Spin 3:20 – 8:20

Why Doesn’t Sun Explode? Gravity – Keeps the nuclear reaction under control. Savage Sun 7:30-12:00

Nuclear Fusion 1.The process that unites small mass nuclei into larger mass nuclei 2.Extremely large amounts of energy released 3.More efficient at producing energy than fission (BREAK APART)

Nuclear reactions do create new elements because they are reactions that involve the nucleus of an atom, called transmutation.

Transmutation Transmutation: conversion of atoms of one element into atoms of another. Alchemists have attempted this for hundreds of years (but not through nuclear chemistry) First artificial transmutation: Ernest Rutherford (1919) turned nitrogen into oxygen-17

Proton-proton chain Fusion Video 7:26-10:30

1. Early: nuclear fusion turns Hydrogen into Helium 2. Late stages: massive star… Helium converted into a. heavier elements (carbon, oxygen, …, iron) Helium converted into a. heavier elements (carbon, oxygen, …, iron) In the Normal Life of a Star When a small star has consumed its fuel, Iron is the final product.

Other Elements 1. Big Bang produced most of the hydrogen & helium today. 2. But other elements (naturally occurring elements as heavy as Uranium) It is believe that these elements were formed in the cores of stars (long after the big bang happened).

Can We Harvest the Sun? If fusion is a long way off….. dHo5fPAnc&list=PL0C99D9DFBBEEB6D6&index=12

90% of the atoms in the universe are hydrogen90% of the atoms in the universe are hydrogenatoms. 5% of all atoms are helium5% of all atoms are helium All of the other elements taken together make up 1%All of the other elements taken together make up 1% of the universe of the universe Li, Be, and B are mysteriously rare.Li, Be, and B are mysteriously rare. Elements of even atomic number are more abundantElements of even atomic number are more abundant than those with odd atomic numbers. There is a general decline in abundance from oxygenThere is a general decline in abundance from oxygen to lead. to lead. However, there is a very pronounced maximum inHowever, there is a very pronounced maximum in relative abundance around iron. There are no stable elements with mass numbersThere are no stable elements with mass numbers greater than about 210 amu. MYSTERY OF THE HEAVENS

Hydrogen fusion continues for 99% of the lifetime of a star Hydrogen fusion continues for 99% of the lifetime of a star When the Hydrogen is essentially depleted, the star collapses,When the Hydrogen is essentially depleted, the star collapses, and core temp is > 100 million Celsius. In this helium-burning’ cycle, it is the carbon nucleus first formedIn this helium-burning’ cycle, it is the carbon nucleus first formed The direct formation of carbon from helium means that theThe direct formation of carbon from helium means that the process has circumvented lithium, beryllium, and boron. ANSWERS FROM CHEMISTRY

RNote that the slope of the curve is much steeper in the fusion region than the fission region (fusion returns much more energy per nucleon than fission

Advantages of Fusion 1. Produces large amounts of energy 2. Fuels are plentiful 3. Inherently safe since any malfunction results in a shutdown of the fusion reaction 4. No atmospheric pollution leading to acid rain or greenhouse effect 5. No long-term storage of radioactive waste needed