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Nuclear Chemistry. Nuclear, i.e. pertaining to the nucleus. Nucleus. Most nuclei contain p + and n 0 When packed closely together, there are strong attractive forces (nuclear forces) btw.. protons-protons p rotons-neutrons. Atomic Size. The radius of an atom is 40 – 270 pm
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Nuclear Chemistry Nuclear, i.e. pertaining to the nucleus
Nucleus • Most nuclei contain p+ and n0 • When packed closely together, there are strong attractive forces (nuclear forces) btw.. • protons-protons • protons-neutrons
Atomic Size • The radius of an atom is 40 – 270 pm • The nucleus has a radius of only 0.001 pm, with an incredible density of 2 x 108 metric tons/cm3 Remember, most of atom’s volume is taken by e- cloud
The Nucleus (aka Nuclide) • Nucleus is made of p+ and n0, which are collectively called nucleons (p+ and n0 are types of nucleons) • In nuclear chemistry, an atom is referred to as a nuclide and is identified by the number of p+ and n0 in its nucleus 228 Ra or Radium-228 88 Nuclides can be written 2 ways.
Atomic Mass • An atom is made of p+, n0, and e- • So you could expect its mass to = • Ex. Helium 2 p+ (2 x 1.007276 amu) = 2.014552 amu 2 n0 (2 x 1.008665 amu) = 2.017330 amu 2 e- (2 x 0.0005486 amu) = 0.001097 amu total mass = 4.032979 amu
Mass Defect • BUT, the atomic mass of the He atom has been measured at 4.002602, NOT our calculated mass of 4.032979 • Mass of helium atom is 0.030377 amu less than we expected • The difference btw the mass of an atom and the sum of its “parts” is called the mass defect
Mass can be converted to energy • What causes the “loss” in mass? • According to E = mc2, mass can be converted to energy and energy converted to mass • The mass defect is caused by mass being converted to energy upon formation of the nucleus
E = mc2 • Using Einstein’s equation and the mass defect, we can calculate the energy released when a nucleus is formed (nuclear binding energy) • It is also the energy required to break apart the nucleus
Nuclear Stability • Binding energy per nucleon = the binding energy of the nucleus divided by the number of nucleons it contains • The higher binding energy per nucleon, the more tightly they are held together and the more stable the nucleus Elements with intermediate atomic masses are the most stable.
Trends • Among atoms with low atomic numbers, the most stable nuclides are those with a neutron:proton ratio of 1:1 (ex. He has 2:2) • Increasing atomic numbers push the stable ratio closer to 1.5:1 (ex. Pb has 124:82) • Stable nuclei tend to have even numbers of nucleons (p+ and n0)
Opposing Forces in the Nucleus • p+ repel p+ (electrostatic repulsion), except the ones very close to them (nuclear forces) • As #p+ increases, electrostatic repulsion overcomes nuclear forces, therefore more neutrons are required to increase the nuclear force and stabilize the nucleus
Vocabulary Recap • Nucleon • Nuclide • Mass defect • Nuclear stability • Nuclear binding energy • Electrostatic repulsion v. nuclear forces
Nuclear reaction = a reaction that affects the nucleus of the atom • The number of protons and neutrons change! • Large amounts of energy are given off • Stability is increased p+ repel p+ (electrostatic repulsion), except the ones very close to them (nuclear forces) 9Be + 4He → 12C + 1n 4 2 6 0 • If the atomic number of the atom changes,……. • Transmutation = a change in the identity of a nucleus A Be nucleus and a He nucleus fuse to form a C nucleus and a neutron
Electromagnetic radiation, e.g. gamma rays, UV, visible light, x-rays, infrared waves, etc. Radioactive Decay The spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by the emission of particles, electromagnetic radiation, or both Disintegration = breaking apart Emission = sending out
Nuclear Radiation = particles or electromagnetic waves emitted during radioactive decay ex. Uranium is a radioactive nuclide, and unstable nucleus that undergoes radioactive decay • This week think of a neutron as a particle made up of a proton and an electron fused together • mass similar to p+ because mass of e- is negligible • no charge because p+ and e- cancelled each other out
5 Types of Radioactive Decay 1. Alpha Emission • Restricted to heavy nuclei (large # of p+ and n0) • Emits Alpha particles = He nuclei (2 p+ and 2 n0 bound together) to increase the stability of nucleus 210Po → 206Pb + 4He 84 82 2 A polonium nucleus is too large and therefore unstable. In radioactive decay, it emits an alpha particle to become lead, a smaller and more stable nucleus
1n → 1p + 0β 0 1 -1 5 Types of Radioactive Decay • Beta Emission • occurs in nuclides with a high neutron:proton ratio • To decrease the # of n0, a n0 can be converted into a p+ and an e- and the e- is sent from the nucleus as a beta particle • 14C → 14N + 0β 67 -1 Atomic number increases by 1, but mass number stays the same. n0:p+ decreases
5 Types of Radioactive Decay • Positron Emission • Occurs in nuclides whose n0:p+ ratio is too small • Too decrease the number of p+, a p+ can be converted into a n0 by emitting a positron 1p → 1n + 0β 1 0 +1 a particle that has the same mass as an electron, but has a positive charge ex. 38K → 38Ar + 0β 19 18 +1
5 Types of Radioactive Decay • Electron Capture • Occurs in nuclides with a n0:p+ ratio that is too small • An inner e- is captured by its own nucleus and combines with a p+ to form a n0 0e + 1 p → 1n -1 +1 0 ex. 106Ag + 0e → 106Pd 47 -1 46
5 Types of Radioactive Decay • Gamma Emission • Usually occurs following other types of decay, when particles (e.g. alphas, betas, positrons) leave the nucleus in an excited state • Gamma rays (γ) are high energy electromagnetic waves emitted from a nucleus as it changes from an excited energy state to a ground energy state Ground state = the lowest energy state
Half-Life • No 2 radioactive nuclides decay at the same rate • More stable nuclides decay slowly and have long half-lives • Less stable nuclides decay quickly and have shorter half-lives • Half-life (t1/2) = the time required for half the atoms in a radioactive nuclide to decay
Example Problem • P-32 has a half-life of 14.3 days. How many mg of P-32 will remain after 57.2 days if you start with 4.0 mg? • Find number of half-lives that have passed. • # of half-lives = (t1/2/t) • Reduce the original mass by half for every half-life that has passed. • Final mass = initial mass x 0.5 for each half life that has passed t = time passed t1/2 = half-life for the nuclide
Decay Series • One nuclear reaction is often not enough to produce a stable nuclide • A decay series = a series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached • All naturally occurring nuclides with atomic # greater than 83 are radioactive and belong to one of 3 natural decay series (U-235, U-238, Th-232)
