4 Basic Forces of Nature

strong force = very strong, but very short-ranged. It acts only over ranges of order 10-13 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 no in the nucleus of an atom

Atomic # = Z = # p+ in the nucleus Mass # = A = # p+ and no in the nucleus Nuclear symbol

AX 12C 235U 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

4He 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 Radium produced two types of particles upon decaydecay

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 = 4He or 4 (these are helium nuclei)

2 2

beta particle = 0 or 0e (high energy electrons) 1- 1-

neutron = 1n 0

proton = 1p 1

positron = 0 or 0e (high energy positrons) 1+ 1+

(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 no by 2’s

Nuclear decay Atoms are radioactive for different

reasons. Radioisotopes decay by giving off a specific type of radiation to change their no/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? 226Ra ____ + 4He 88 2

226Ra 222Rn + 4He 88 86 2

Phosporous - 30 decays by beta decay. Write the reaction.

30P ____ + 0e 15 1-

30Ra 30S + 0e 15 16 1-

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 238U + 1n 239Np + 0e 92 0 93 -1

239Pu + 4He 242Cm + 1n 94 2 96 0

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)c2

c = 9 x 1010 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 – 13.99995 g

= 6(1.00728 g) + 8(1.00867 g) – 13.99995 g = 0.1131 g per

Then calc E from E = mc2

E = 9.00 X 1010 kJ x 0.1131 g = 1.02 x 1010 kJ/mol C-14 mol mol C-14