Chemistry of the Nucleus Nuclear Chemistry BRING A BAG OF SKITTLES/M&Ms TOMORROW; 1 per lab group Chemistry of the Nucleus

A typical star builds elements up to Iron & Nickel A typical star builds elements up to Iron & Nickel. Beyond a mass of 60, even regular stars don’t have enough energy to bind the nucleus together.

Larger elements are built in supernovae, in which there is plenty of energy to create the larger elements.

vs Types of Reactions Chemical Reactions Nuclear Reactions Involves valence electrons Elements rearranged to form new compounds. Law of conservation of mass obeyed. Little energy released. Nuclear Reactions Changes occur in the nucleus; involves protons, neutrons, and electrons. Transmutation occurs (one element turns into another) Law is not always obeyed. Mass is changed into energy. Lots of energy produced, according to Einstein’s famous equation, E = mc2 vs

History of Radioactivity Roentgen – discovered X-rays as a result of exposing photographic plates to the rays emitted from a cathode ray tube. Found that X-rays could travel through some substances.

History of Radioactivity Becquerel – studied phosphorescence (glowing) in uranium ore when he discovered that the ore also exposed photographic plates.

The Curies Further experimented with uranium ore. Isolated the uranium as the substance with the unique properties. First to call uranium “radioactive” – meaning a substance that gave off rays of energy from the nucleus.

Let’s review some terms… Atomic number Determines an element’s identity The number of protons AND the number of electrons Mass number = protons + neutrons Isotopes: atoms of the same element that have different number of neutrons. They have a different mass number but same atomic number.

ISOTOPES OF CARBON 6 Neutrons 8 Neutrons Carbon-12 Carbon-14 Mass Number = 12 Atomic Number = 6 Neutrons = Mass – Protons Carbon-14 Mass Number = 14 Atomic Number = 6 Neutrons = Mass - Protons 6 Neutrons 8 Neutrons Protons Neutrons

Writing Isotopes Super/Subscript 235 U 235 – mass #; 92 – atomic # 92 Hyphen notation U-235 235 – mass # ??? atomic # ??

Nuclear Chemistry Terminology Nuclear Mass Defect The difference in the calculated mass of an atom (based on C-12) and the measured mass. Mass is always smaller than expected. Lost mass has been converted to energy.

Types of Radiation Radioisotopes are isotopes that have unstable nuclei that emit radiation to become more stable. Nuclei that are not stable tend to undergo radioactive decay, a process that emits radiation and particles from the nucleus. The three most common types are alpha, beta, and gamma radiation.

What causes nuclear stability? Neutron to Proton Ratio If this ratio is close to 1:1 in a low mass (small) element, the isotope is likely to be stable. Ex: Carbon-12 (6 protons, 6 neutrons) Ex: Helium-4 (2 protons, 2 neutrons)

What causes nuclear stability? Neutron to Proton Ratio For larger elements, the stable ratio is 1.5:1 – 1.5 neutrons per proton. Ex: Xenon-132 (54 protons, 78 neutrons) Elements with atomic #s > 82 are unstable and radioactive

Natural vs. Artificial Radioactivity Most elements have some radioactive isotopes (radioisotopes) that occur naturally, but the percentage of radioactive isotopes is so low that we do not consider that element radioactive. Ex: the majority of carbon in our world is Carbon-12, which is stable. A small amount of radioactive isotopes are mixed in.

Natural vs. Artificial Radioactivity Other elements, such as radon, have a high percentage of radioisotopes, so we consider that element, in general, to be radioactive. All elements heavier than bismuth are naturally radioactive due to their instability. All elements heavier than uranium, number 92, are artificially created and are always radioactive. These are the transuranium elements usually formed by bombarding a heavy atom with neutrons or alpha praticles. We have created elements up to about 118 protons.

Types of Radioactive Decay Alpha Decay An atom loses an alpha particle (2 protons, 2 neutrons—a helium nucleus) The loss of two protons decreases the atomic number by 2, and the atomic mass decreases by 4. A new element is formed. Alpha radiation isn’t powerful and can be stopped by paper alone. 2

Types of Radioactive Decay Beta Decay An atom emits a beta particle (an electron) from the nucleus. Result of a neutron converting to a proton and electron, and the electron being ejected. The resulting proton increases the atomic number by 1 and a new element is formed. Note that the mass number is unchanged.

Types of Radioactive Decay Gamma Radiation High energy photons (electromagnetic energy) ejected from the nucleus. Usually accompanies other nuclear emissions, and represents the energy lost when the nucleus becomes more stable. Does not change the atomic number or mass. Difficult to stop, requires several cm of lead or several feet of concrete.

Gamma Decay Reactions Write the gamma decay for Iodine-125.

Relative Strengths of Radiation How will the particle penetrate?

Artificial Nuclear Changes: Neutron Bombardment A neutron enters the nucleus at a high speed and is captured, usually creates a radioactive isotope. Used to create synthetic isotopes useful in medicine and research. Increases the mass number by one, but the atomic number remains the same. Example 4020Ca + 10n  4120Ca

Electron Capture Capture by the nucleus of an electron from the electron cloud. The electron combines with a proton to convert into a neutron. The mass number remains the same and the atomic number decreases by one since a proton was converted. Example: 8137Rb + 0-1e 8136Kr

Positron Emission The radioactive decay process in which a positron is emitted from the nucleus. A positron is a particle with the same mass as an electron but opposite in charge. The number of protons decreases but the mass number remains the same.

Summary Type of radioactive change Particle emitted Change in mass number Change in atomic number Alpha decay Alpha particle Decrease by 4 Decrease by 2 Beta decay Beta particle (electron) No change Increase by 1 Positron emission Positron Decrease by 1 Electron capture n/a Gamma emission Gamma ray