Unstable Nuclei & Radioactive Decay

Radioactivity Nucleus of an element spontaneously emits subatomic particles & electromagnetic waves. Nucleus of an element spontaneously emits subatomic particles & electromagnetic waves. Nucleus changes into a different element when it does this. Nucleus changes into a different element when it does this. Original nucleus is called “unstable.” Original nucleus is called “unstable.” Process is called “decay” or “transmutation.” Process is called “decay” or “transmutation.”

Rutherford & Radioactivity 1898 – Rutherford began experiments with radioactivity – Rutherford began experiments with radioactivity – discovered alpha and beta “rays” from uranium – discovered alpha and beta “rays” from uranium.

Types of Radiation

Radioactivity – types

Characteristics of Radiation See Table O NameSymbol Mass (amu) Relative Charge Alpha particle  or 4 He or 4  4+2 Beta particle  or 0 e or 0  or  - 0 Gamma radiation  or 0  00 Positron emission 0  or 0 e or 

2 Neutrons & 2 Protons. Charge = +2 Mass = 4

Beta Particle – fast moving electron.

Radioactive atom: Change occurs in nucleus.

Shielding

Can we predict exactly when an atom will decay? NO! NO! For large #’s of atoms, we CAN predict how many will decay on average in a given amount of time. For large #’s of atoms, we CAN predict how many will decay on average in a given amount of time.

Which elements are radioactive? All elements past Bismuth in the periodic table. All elements past Bismuth in the periodic table. If the atomic number is  83, it’s radioactive! If the atomic number is  83, it’s radioactive! Other elements may have radioactive isotopes. Other elements may have radioactive isotopes. Stability depends on neutron/proton ratio. Stability depends on neutron/proton ratio. applet

What’s Going on in the Nucleus? Electrostatic repulsions between protons. Want to fly apart. Electrostatic repulsions between protons. Want to fly apart. But protons & neutrons all attracted to each other by nuclear strong force. But protons & neutrons all attracted to each other by nuclear strong force. So having neutrons helps hold a nucleus together. So having neutrons helps hold a nucleus together.

Strong Force Works best if the nucleus isn’t too large. Works best if the nucleus isn’t too large. As the nucleus gets larger, need to have more neutrons to help counteract the electrostatic repulsion between the protons. As the nucleus gets larger, need to have more neutrons to help counteract the electrostatic repulsion between the protons. Eventually, the nucleus is too large to be stable. Eventually, the nucleus is too large to be stable.

Balancing Act Balance exists between electrostatic repulsive force & nuclear strong force. Balance exists between electrostatic repulsive force & nuclear strong force. Certain #’s of protons & neutrons make a stable nucleus. Other #’s of protons & neutrons are unstable. So the atom decays. Certain #’s of protons & neutrons make a stable nucleus. Other #’s of protons & neutrons are unstable. So the atom decays.

Beyond Element 83 No amount of neutrons can hold a nucleus together once it has more than 83 protons. No amount of neutrons can hold a nucleus together once it has more than 83 protons. Elements 84 & above are radioactive. Elements 84 & above are radioactive.

Stability and the n/p ratio For atoms below atomic number = 20, best neutron/proton ratio  1. For atoms below atomic number = 20, best neutron/proton ratio  1. As atomic number , atoms need more neutrons to be stable. As atomic number , atoms need more neutrons to be stable. So n/p ratio for stable atoms increases to 1.5 for big atoms. So n/p ratio for stable atoms increases to 1.5 for big atoms.

Stability Line

Type of radiation emitted depends on position relative to stable nuclei. Blue: too many neutrons. Yellow: not enough neutrons for the protons. Red/Orange: too many protons and neutrons

Natural Radioactivity – Unstable Nuclei Emit Radiation Spontaneous nuclear change to attain good n/p ratio (high stability, low energy state). Spontaneous nuclear change to attain good n/p ratio (high stability, low energy state). Form a new kind of atom. Form a new kind of atom. Each isotope or nuclide decays in a certain manner to get a better n/p ratio. The decay mode is named for the particle emitted. See Table N. Each isotope or nuclide decays in a certain manner to get a better n/p ratio. The decay mode is named for the particle emitted. See Table N.

Nuclear Equations Describe the decay process. Describe the decay process. reactant or starting side (left)  product or ending side (right). reactant or starting side (left)  product or ending side (right).  separates two sides  separates two sides

Nuclear vs. Chemical How is a nuclear change different from a chemical change? How is a nuclear change different from a chemical change?

Involve a change in an atom’s nucleus. Involve a change in an atom’s nucleus. Radioactive atoms spontaneously emit radiation and change into other kinds of atoms. Radioactive atoms spontaneously emit radiation and change into other kinds of atoms. Nuclear reactions involve 1,000,000 X more energy than ordinary chemical rxns. Nuclear reactions involve 1,000,000 X more energy than ordinary chemical rxns. Involve changes in the outermost electrons. Involve changes in the outermost electrons. 1 or more substances changed into new substances. 1 or more substances changed into new substances. Atoms are rearranged, but their identities do not change. Atoms are rearranged, but their identities do not change. NUCLEARCHEMICAL

Mass  Energy In nuclear reactions, mass is converted into energy. In nuclear reactions, mass is converted into energy. –Mass Defect is the difference between the mass of a nucleus and the sum of the masses of its constituent particles. E = mc 2

Binding Energy Energy released when a nucleus is formed from its constituent particles. Energy released when a nucleus is formed from its constituent particles. It is a measure of the stability of an atom formed: It is a measure of the stability of an atom formed: –The higher the binding energy the more stable the nucleus. –The lightest and heaviest elements are the most unstable (low BE) –Intermediate elements are the most stable (highest BE).

Ni-62 Fe-56 Binding Energy