Chapter 19 - Nuclear Chemistry Nuclear Stability and Modes of Decay

History and Discovery of Radioactivity

The Discovery of Radioactivity (1896) Antoine-Henri Bequerel designed experiment to determine whether phosphorescent minerals also gave off X-rays.

Bequerel discovered that certain minerals were constantly producing penetrating energy rays like X-rays not related to fluorescence Bequerel determined that the minerals contained uranium. uranic rays Production of uranic rays did not require exposure to outside energy. Energy was being produced from nothing!!!

The Curies Marie Curie (1867-1934) broke down these minerals and used an electroscope to detect uranic rays. She discovered the rays were emitted from specific elements. She also discovered new elements 1. radium named for its green phosphorescence 2. polonium named for her homeland She coined the name radioactivity

Other Properties of Radioactivity Can ionize matter (cause matter to become charged) (basis of Geiger Counter) High energy Can penetrate matter Can cause phosphorescent chemicals to glow (basis for the scintillation counter)

Electroscope Ionizing radiation +++ +++ When charged, the metal foils spread apart due to like charge repulsion. Ionizing radiation knocks electrons off the air molecules, which jump onto the foils and discharge them.

Rutherford (1871-1937) Discovered Three Types of Radiation ++++++++++++ β γ α --------------

Types of Radioactive Rays Alpha Rays (α) charge of +2 and mass of 4 amu essentially the nucleus of a helium atom Beta Rays (β) charge of -1 and negligible mass high-energy electrons Gamma Rays (γ) electromagnetic radiation, not α or β

Penetrating Ability of Radioactive Rays α β γ 0.01 mm 1 mm 100 mm Pieces of Lead

Nuclear Chemistry Nuclear reaction process that alters the number of neutrons and protons in the nucleus of an atom. Radionuclide an unstable nuclide that undergoes radioactive decay. Radioactive decay the spontaneous disintegration of unstable particles accompanied by the release of radiation.

Binding Energy and Nuclear Stability

What Causes Nuclei to Break Down? The particles in the nucleus are held together by a very strong attractive force between nucleons, the strong force, which acts only over very short distances.

Neutrons and Protons are Held Together by the Strong Force. Neutrons play an important role in stabilizing the nucleus. They add to the strong force, but don t repel each other like protons.

Mass Defect (Δm) He nucleus 2 neutrons + 2 protons = 6.69510 10 27 kg Mass of 4 He = 6.64465 10 27 kg Δm = 5.045 10 29 kg E = mc 2 BE = 4.54 x 10-12 J/atom BE = 1.13 x 10-12 J/nucleon x NA = 6.80 x 10 11 J/mol

Binding Energy

Unstable Nuclei and Modes of Radioactive Decay

Review of Nuclear Structure Every atom of an element has the same number of protons designated by the atomic number Z Atoms of the same element may have different numbers of neutrons called isotopes have different atomic masses Isotopes and Nuclides are represented symbolically: mass number atomic number A Z X symbol mass # = protons + neutrons

Radioactivity Unstable radioactive nuclei spontaneously decompose into smaller nuclei through radioactive decay. PARENT NUCLIDE > DAUGHTER NUCLIDE(S) PARTICLE(S) and/or ENERGY All nuclides with 84 or more protons are radioactive

Important Atomic Symbols Particle Symbol Nuclear Symbol proton p + neutron n 0 electron e - alpha beta positron α β, β β, β +

Transmutation Atoms of one element are changed into atoms of a different element. The number of protons in the nucleus changes. We describe the process with nuclear equations.

Nuclear Equations In nuclear equations, atomic numbers and mass numbers are conserved. conservation of nucleons 238 = 234 + 4 92 = 90 + 2 conservation of charge

Alpha Emission An α particle contains 2 protons and 2 neutrons. a helium nucleus The most ionizing, but least penetrating of radiation types

Alpha Emission Radium-222 decays by alpha emission Loss of an α particle means atomic number decreases by 2 mass number decreases by 4

Beta Emission A beta particle is like an electron moves much faster (has more energy) produced in the nucleus In β decay, a neutron changes into a proton

Beta Emission Thorium-234 decays by beta emission Loss of an β particle means atomic number increases by 1 mass number remains the same

Gamma Emission Gamma (γ) rays are high energy photons. No loss of particles from the nucleus No change in composition of the nucleus Occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange Least ionizing, but most penetrating

Positron Emission from the Positron has a charge of +1 and negligible mass Appears to result from a proton changing into a neutron

Positron Emission Sodium-22 decays by positron emission When an atom loses a positron from its nucleus, atomic number decreases by 1 mass number remains the same

Electron Capture An inner orbital electron is pulled into the nucleus No particle emission, but the atom changes

Ruthenium-92 undergoes electron capture Proton combines with electron to make a neutron Mass number stays the same Atomic number decreases by 1 The result is the same as positron emission!!

Patterns of Nuclear Stability The majority of elements in the universe are small (Z<40) and have even numbers of protons. Certain numbers appear to be magic with regard to number of neutrons or protons: 2,8,20,50,82,4,126 Stable nuclei are found in a band of stability surrounded by a band of instability.

Patterns of Nuclear Stability The number of stable nuclides having even of odd numbers of neutrons and protons: 157 50 53 even #Neutrons odd 4 even odd #Protons

The Valley of Stability and the N/Z Ratios (neutrons/protons) Valley of For Z =1-20, stable N/Z ratio = 1 For Z = 20-40, stable N/Z ratio 1.25 For Z = 40-80, stable N/Z ratio 1.5 For Z > 84, there are no stable nuclei

The Band of Stability Expanded

Predictability of Nuclear Decay Nuclei that lie above the band of stability are neutron rich. These nuclei tend to decay so that the final n/p ratio is closer to that found in the band of stability. 14 6 C 14 7 N + 0-1 e Nuclei that lie below the band of stability are proton rich. These nuclei tend to decay so that the final n/p ratio is closer to that found in the band of stability. 29 15 P 29 14 Si + 0 +1 e 7 4 Be 0-1 e + 7 3 Li

14 C 6 29 P 15 14 7N + 29 Si + 14 0 7 Be + -1e 4 0 e -1 0 e +1 7 Li 3

Predictability of Nuclear Decay

188 Re is formed by the decay of 188 W. Write a balanced equation describing the decay process. Why doesn t 188 W decay by electron capture or positron emission? 188 188 74 W 75 Re + 0-1 e n/p = 114/74 = 1.54 n/p = 113/75 = 1.51 188 74 W + 0-1 e 188 73 Ta 188 188 73 Ta 0 74 W + +1 e n/p = 115/73 = 1.58

Selective Types of Radioactive Emissions

Nuclear Decay Series

Nuclear Decay Series In nature, one radioactive nuclide often changes into another radioactive nuclide. All of the radioactive nuclides that are produced one after another until a stable nuclide is made is called a decay series.

What is the product formed when 238 U goes through one alpha decay followed by two beta emissions and then another alpha decay? α β β α 238 92 U 234 90 Th 234 91 Pa 234 92 U 230 90 Th

A Natural Radioactive Decay Series for U-238 U-238 Decay Series α β β α α α α β α β α β β α or α β α β β α β or other combinations