Nuclear Chemistry - Chem Honors

Nuclear stability

• changes occur in atom’s nucleus in nuclear reactions

• strong nuclear force holds protons and neutrons together in nucleus

  • some mass of nucleons converted to energy (E = mc²) - mass defect

• in larger atoms, nucleus spontaneously breaks apart releasing energy and particles - called radioactivity

• nuclear symbol

  • lower # = atomic number; upper # = mass number

  • Ex. ²³⁸U or uranium-238 or U-238

Ration of protons to neutrons

• anything over Bi is unstable

Why are some atoms stable?

• every element has one or more isotopes (isotope is element with dif atomic mass but same atomic number - difference in neutrons)

  • most isotopes are stable and not naturally radioactive

  • some are unstable

• smaller atoms (elements under element 15) have ration of 1:1 neutrons to protons - stable

• larger atoms have slightly higher ratio of neutrons to protons of 1.5:1

• radioactive decay occurs when nucleus is unstable and starts to break apart releasing energy

Radioactive decay

• spontaneous disintegration (decomposing/breaking down) of nucleus into slightly lighter and more stable nucleus

  • nuclear particles and/or electromagnetic radiation caused from reaction

• transmutation - change in identity of an element as a result of a change in number of protons

  • Ex. ²³⁸₉₀U → ²³⁴₉₀U + ⁴₂He - shows conservation of mass and charge

• no stable isotopes of elements with atomic number above Pb-82

  • bismuth and larger are all unstable radioactive isotopes

• decay of these naturally unstable elements is called natural radioactivity

Alpha decay α

• emission of an alpha particle by the nucleus

• least penetrating power

• equivalent to a helium nucleus - ⁴₂He

• Ex. ²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He

Beta decay β^-

• emission of an electron by the nucleus

• medium penetrating power

• equivalent to an electron ⁰_-1e

• Ex. ¹⁴₆C → ¹⁴₇N + ⁰_-1e

  • 7 and -1 = 6 so conservation of charge

  • neutron decays into proton and electron which is why you gain a proton from C to N

Gamma emission γ

• emission of high energy electromagnetic waves

• high penetration power

• no mass and no charge ⁰₀γ

Positron emission β⁺

• emission of a positively charged electron

• lower penetrating power

• positively charged electron ⁰₊₁e

• Ex. ³⁸₁₉K → ³⁸₁₈Ar + ⁰₊₁e

  • opposite of neutron decaying into proton and electron

Electron capture

• capture of inner orbital electron by nucleus

• electron combines with proton to form neutron

• Ex. ¹⁰⁶₄₇Ag + ⁰_-1e → ¹⁰⁶₄₆Pd

Rays experiment (Rutherford)

• alpha particles will be attracted to negative plate

• beta particles will be attracted to positive plate

• gamma rays will not be affected by the charged field

Radioactive decay series

• radioactive decay of naturally occurring radioisotopes of heavy elements often initiate chain of successive decay reactions

• decay series ultimately leads to formation of a stable isotope

Half life

• time it takes for 1/2 of atoms in a sample of radioactive material to decay (transmute) into another element

  • shorter the half life, more unstable the element

• half life helps determine how long a sample of a useful isotope will be available

  • Ex. medical scans

• determine how long a sample of an undesirable isotope must be stored before it decays to a low radiation level

• Ex. Co-60

  • start with 10g of Co-60

  • after 5.27 years there will be 5 g

  • after 10.54 years there will be 2.5g

  • after 15.81 years there will be 1.25g

Example of a 10g substance with a 6 year 1/2 life

Examples

• problem: half life of I-131 is 8 days. what mass remains unchanged from a 400g sample after 32 days?

• solution:

• problem: half life of ¹³¹I is 8 days and 4g remain unchanged after 48 days. what was the mass of original sample and how many half lives have passed?

• solution:

• problem: after 36 minutes, 5g remains from original sample of 40g. calculate half life of nuclide

• solution:

• problem: what fraction of K-42 remains after 24.8 hours

• solution: