Chapter 1 Nuclear Chemistry CHEM 396 by Dr. Ahmad Hamaed Fall 2014

Nuclear Chemistry Nuclear chemistry, by definition, is the study of the structure of atomic nuclei and the changes they undergo. Why it is important? Has a role in shaping world politics. Due to its applications in the production of electricity and in the diagnosis and treatment of diseases. Therefore, nuclear chemistry has profound effects upon the world in which we live.

Discovery of Radioactivity (1895-1898) 1. Roentgen found that invisible rays were emitted when electrons bombarded the surface of certain materials. 2. Becquerel accidently discovered that phosphorescent uranium salts produced spontaneous emissions that darkened photographic plates. 3. Marie Curie -Isolated the components (uranium atoms) emitting the rays. - Radioactivity→ Process by which particles give off rays. - Identified 2 new elements, Polonium and Radium on the basis of their radioactivity This discovery contradicted Dalton’s theory of indivisible atoms!

Matter and Energy Nuclear science began with Albert Einstein at the beginning of the last century, who recognized that matter and energy were equivalent; E = mc2. Energy which is defined as the capacity to do work or to provide heat, had an equivalency with matter- the mass of the physical universe. If matter could be converted to energy in a practical manner, a very small amount of matter would generate enormous amounts of energy.

Definitions Electron: The least massive electrically-charged particle, hence absolutely stable. It has an electric charge of -1. Alpha particle: A positively charged helium isotope. Beta particle: High speed electron Positron: high energy photon (+ve charge), it is an antiparticle of an electron (e+). Gamma rays: high energy photon (zero charge).

Definitions Electrons, protons, and neutrons are all fermions. Isotopes are atoms of the same element that have different numbers of neutrons. Proton: The most common hadron, a baryon with electric charge +1 equal and opposite to that of the electron. Protons have a basic structure of two up quarks and one down quark. The nucleus of a hydrogen atom is a proton. A nucleus with electric charge Z contains Z protons.

Definitions Quantum: The smallest discrete amount of any quantity (plural: quanta). Nucleus: A collection of protons and neutrons that forms the core of an atom (plural: nuclei). Nucleons: Subatomic particles in the nucleus, e.g., protons and neutrons. Radio nucleotides: Radioactive nuclei; unstable nuclei that spontaneously emit particles and electromagnetic radiation. Radio isotopes: Atoms containing radioactive nuclei.

Definitions Fermion: Any particle that has odd-half-integer (1/2, 3/2, …) intrinsic angular momentum (spin). All particles are either fermions or bosons. Fermion obey the Pauli Exclusion Principle, which states that no two fermions can exist in the same state at the same time. Pauli Exclusion Principle: The principle that no two particles in the same quantum state may exist in the same place at the same time. Particles that obey this principle are called fermions; particles that do not obey the Pauli Exclusion Principle are called bosons.

Definitions Quark: A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons. Quarks have electric charge of either +2/3 (up, top) or -1/3 (down, bottom) in units where the proton charge is 1. Hadron: A hadron is a composite particle made of quarks held together by the strong force. Hadrons are categorized into two families: baryons (such as protons and neutrons, made of three quarks) and mesons (see definition below). Meson: A hadron made of an even number of quark-antiquark pair. The basic structure of most meson is one quark and one antiquark. Mesons are bosons, meaning that they have integral spin, i.e., 0, 1, or -1, as they have an even number of quarks. Neutron: A baryon with electric charge zero; it is a fermion with a basic structure of two down quarks and one up quark (held together by gluons).

Nuclear Reactions vs. Normal Chemical Changes Nuclear reactions involve the nucleus The nucleus opens, and protons and neutrons are rearranged The opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energy. “Normal” Chemical Reactions involve electrons, not protons and neutrons

A Comparison of Chemical and Nuclear Reactions Characteristics of Chemical and Nuclear Reactions Chemical Reactions Nuclear Reactions 1- Occurs when bonds are broken and formed. 1- Occurs when nuclei emits particles and/ or rays 2- Atoms remain unchanged, though they may be rearranged. 2- Atoms are often converted into atoms of another element. 3- Involve only valence electrons. 3- May involve protons, neutrons, and electrons. 4- Associated with small energy changes. 4- Associated with large energy changes. 5- Reaction rate is influenced by temperature, pressure, concentration, and catalysts. 5- Reaction rate is not normally affected by temperature, pressure, concentration, and catalysts.

Review of the Structure of the Atom

Isotopes/ Radio-isotopes Recall that Isotopes are atoms of the same element that have different numbers of neutrons. Isotopes of atoms with unstable nuclei are called Radioisotopes. These unstable nuclei emit radiation to attain a more stable atomic configurations in a process called radioactive decay. During radioactive decay, unstable atoms lose energy by emitting one of several types of radiation; Alpha, Beta, Gamma,….

What are the types of radiation emitted by a radioactive sources? Alpha (α) – a positively charged helium nuclei (nuclear charge +2). Beta (β) – a fast moving electron Gamma (γ) – pure energy; called a ray rather than a particle

Properties of Alpha, Beta, and Gamma Radiation Composition Alpha particles Beta particles High-energy electromagnetic radiation Description of radiation Helium nuclei Electron Photon Charge +2 -1 Mass 6.64 x10-24 kG 9.11 x10-28kG Approximate energy 5 MeV 0.05 to 1 MeV 1MeV Relative penetrating power Blocked by paper Blocked by metal foil Not completely blocked by lead or concrete

The Effect of an Electric Field on the Three Types of Radiation

Other Nuclear Particles Neutron Positron – a positive electron Proton – usually referred to as hydrogen-1

Alpha Decay

Beta Decay

Types of Radioactive Decay: Gamma Rays

Summary of Radioactive Decay Processes

Nuclear Stability

Radioactivity and Nuclide Stability Number of Stable Nuclides Neutrons Protons 157 even 52 odd 50 5 “Magic numbers” are analogous to the noble gas electronic configurations occur at: 2, 8, 20, 28, 40, 50, 82, 126, 184. The most stable isotopes have “magic numbers” of both protons and neutrons.

Stable Nuclear configurations Some configurations of protons and neutrons are particularly stable just like some configurations of electrons, e.g., (2, 10, 18, 36, 54, 86) Proton numbers: 2, 8, 20, 28, 50, 82 Neutron numbers: 2, 8, 20, 28, 50, 82, 126 Nuclei with even numbers of protons and neutrons are more stable than those with odd numbers. For Pb; A= 208 both the number of protons (Z=82) and the number of neutrons are “magic” (N = 126, P = 82) Which means this is a double closed shell nucleus and it takes more than 2 MeV to raise it to its first excited state.

Stable n:p ratios Neutrons are needed to hold protons together in the nucleus by the strong force. At low atomic numbers, the ratio of neutrons to protons (n/p) in stable isotopes is generally very close to 1. As the atomic number increases the n/p ratio increases, so does the neutron/proton ratio as seen in Hg with atomic number 80 ( n= 120 & p=80) → n/p = 1.5:1. Pb (p=80 & n= 126) → n/p = 1.53:1

Neutron to Proton Ration for Group 15 Elements

Nuclear Stability: Band of Stability

The Zone of Stability

n:p Ratio and Particle Emission 80 (Hg) 50 (Sn) 40 (Zr) 7 (N) Atomic number 1.5 1.4 1.25 1 Ration n/p for Stable Isotopes Atomic number Z > 83, Emission of α particle Atomic number Z ≤ 83, Emission of β particle High n/p ratio ( about 1.5 or more), then emission of β particle Low n/p ratio ( less than 1.5), then emission of positron or electron capture.