4 Basic Forces of Nature strong force = very strong, but very short-ranged. It acts only over ranges of order centimeters and is responsible for holding the nuclei of atoms together. It is basically attractive. electromagnetic force = causes electric and magnetic effects such as the attraction and repulsion between electrical charges or magnetic poles. It is long-ranged, but much weaker than the strong force. weak force = responsible for radioactive decay and neutrino interactions. It has a very short range and, as its name indicates, it is very weak. gravitational force = weak, but very long ranged. Furthermore, it is always attractive, and acts between any two pieces of matter in the Universe since mass is its source.

Nuclear terms Nucleons = p + and n o in the nucleus of an atom Atomic # = Z = # p + in the nucleus Mass # = A = # p + and n o in the nucleus Nuclear symbol A X 12 C 235 U z 6 92 Isobars = same A diff Z Atoms of diff elements with the same mass

Nuclear Radioactivity History Henri Becquerel 1897 discovered radioactivity in a sample of a uranium salt Radioactivity-emission of energy by a substance without absorption of energy by that substance Marie and Pierre Curie 1898 isolated two new elements which were radioactive from pitchblende ore The elements were Po and Ra. Both of these elements are more radioactive than uranium Wilhelm Roentgen 1895 discovered X-rays Emitted from uranium ore X-rays are high energy forms of electromagnetic radiation.

Ernest Rutherford’s discovery Alpha particles (  ), (+) charged, massive, 2 4 He Beta particle (ß), (-) charged, low mass, ß or -e (high energy electron) Gamma radiation- discovered by P. Villard, high energy form of light, no mass and no charge  Radium produced two types of particles upon decay

Stability of Atomic Nuclei Except for H and He, the mass number of an isotope (A) is at least twice as large as the atomic number (Z). This is at least 1 neutron/ 1 proton. Neutrons moderate the repulsive forces between the protons Protons in the nucleus repel each other due to electromagnetism but held together in the nucleus by the strong force Strong force is stronger than electromagnetism over small distances

Nuclear Stability When the nucleus gets too large (Z>83) the protons are so far apart the strong force cannot hold them together and electromagnetic repulsion becomes stronger the nucleus is unstable (radioactive) and gives off radiation (radioactivity) dropping to a lower, more stable energy position Radioactivity can result in the change of an isotope of one element into an isotope of a diff element (transmutation)

Types of Nuclear Radiation The mass and charge of the reactants and products must balance Gamma rays = high energy EMR (no mass, only radiation) alpha particle = 4 He or 4  (these are helium nuclei) 2 2 beta particle = 0  or 0 e (high energy electrons) neutron = 1 n 0 proton = 1 p 1 positron = 0  or 0 e (high energy positrons) (antimatter, same particle as electron with opposite charge)

Four natural processes that occur in the nucleus K e- capture the nucleus gains an electron belonging to the atom’s inner energy (K) level. the e- joins a proton and forms a neutron Positron emission a proton ejects an e+ from the nucleus and changes to a neutron Alpha decay emission of an alpha particle from the nucleus Beta decay a neutron ejects an e- from the nucleus, and turns into a proton

Belt of stability elements with stable nuclei and Z < 25 exhibit approx. 1:1 n:p ratio elements with stable nuclei and Z > 25 exhibit approx. 1.5:1 n:p ratio Nuclei below the belt of stability undergo  + decay or k capture the #of neutrons and the # protons Nuclei with Z > 83 undergo  decay # of p + and n o by 2’s

Nuclear decay Atoms are radioactive for different reasons. Radioisotopes decay by giving off a specific type of radiation to change their n o /p + ratio and become more stable. Elements with Z > 83 tend to lose alpha particles to become less massive Elements with too many neutrons (N >> Z) tend to lose beta particles Lighter elements that have few neutrons will often emit a positron, increasing their number of neutron

Magic numbers The nucleus is inhabited by protons and neutrons - the nucleons Any combination of nucleons which adds up to these numbers 2, 8, 20, 28, 50, 82, 126 provides a more stable isotope Isotopes containing both a magic number of protons and neutrons are especially stable Elements with an atomic number beyond Bi (83) are all unstable isotopes

Balancing Nuclear Equations Atomic and mass numbers are conserved Radium-226 decays by  decay. What is the product? 226 Ra ____ + 4 He Ra 222 Rn + 4 He Phosporous - 30 decays by beta decay. Write the reaction. 30 P ____ + 0 e Ra 30 S + 0 e

Radioactive Decay Series Some radioactive atoms undergo a series of reactions leading from a radioactive isotope to a stable (nonradioactive) isotope. Such a series is called a radioactive decay series

Artificial Transmutations Nuclear bombardment accelerate particles in a cyclotron or linear accelerator and collide them to fuse them together to form a diff element Bullet and target Created transuranic elements by nuclear fusion 238 U + 1 n 239 Np + 0 e Pu + 4 He 242 Cm + 1 n

Binding Energy the energy released when a nucleus is formed from its constituent parts Greater binding energy = Greater nuclear stability binding energy is given by the equation:  E = (  m)c 2 c = 9 x kJ/g  m is the missing mass in grams Called mass deficit

Calc the binding energy of C-14 in kJ per mol Calc  m for C-14 (mass deficit)  m = mass of nucleons C-14 – atomic mass C-14  m = 6 mol p+ + 8 mol no – g = 6( g) + 8( g) – g = g per Then calc  E from E =  mc 2  E = 9.00 X kJ x g = 1.02 x kJ/mol C-14 mol mol C-14