Physics Topic 7: Atomic and Nuclear Physics

· Nuclear model of atom has positively charged nucleus made up of protons and neutrons. · Negatively charged particles called electrons orbit the nucleus. · Most of the mass of the atom is concentrated in nucleus. · Electrons are much smaller than protons and neutrons. · Neutrons have no charge. · Neutral atoms are where the charges of the protons and electrons balance; same number of protons and electrons.

· Geiger Marsden / Gold Foil Experiment · Alpha particles were shot at thin sheet of gold foil. · Most particles passed through, since atoms are made up mostly of empty space · Some particles deflected at large angles, indicating a solid, positively charged structure within atoms.

Limitations include: the model does not take into consideration isotopes and different atomic weights, and the model does not show how the atom would be stable. Electrons would lose energy with radiation.

· Atomic energy levels are distinct regions around the nucleus where electrons are likely to be found. · As electrons gain energy (become excited), they move up to energy levels farther away nucleus. · When electrons return to their ground state (initial energy level), they emit the energy gained in the form of packets of light called photons. · E=hf (Energy of a photon = Planck’s constant*frequency of light in Hz) · Absorption and Emission Spectra provide evidence for energy levels.

Nuclide: A particular type (species) of nucleus with a certain number of protons and neutrons. Isotope: Nuclei with the same number of protons (Z) but different number of neutrons (N) e.g. C-12 and C-14. Nucleon: A proton or neutron (not necessarily a particles within nucleus).

Nucleon Number: Number of nucleons (Protons + Neutrons) in the nucleus. Also known as mass number. Symbol is A. Proton Number: Number of protons in the nucleus . Also known as atomic number. Symbol is Z. Neutron Number: Number of neutrons in the nucleus. Symbol is N. N=A-Z

Coulomb Force: · The positively charged protons repel one another. · Nucleons are arranged in such a way that like charges repel as far as possible. · Electrostatic force of repulsion between the protons in the nucleus. Strong Nuclear Force: · Strong force of attraction between nucleons that overcomes Coulomb repulsion between protons. · Force is independent in whether particles are neutrons or protons. · At 1.3 femtometers, force is about 100 times stronger than repulsion forces; if the distance between protons is greater than 1.3 fm, force falls to 0, if the distance is smaller than 1.3 fm, strongly repulsive.

· Radioactivity is when elements emit radiation spontaneously. o An unstable nucleus emits a particle (alpha, beta, or gamma). · Radiation is independent of pressure, temperature, and chemical combination. · Radioactive properties caused by nucleus of atoms. · The rate of radioactive decay decreases exponentially with time.

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· When alpha particles pass through air, they take two electrons from the air molecules, and become neutral helium atoms. · Air molecules are left with a charge i.e. the air is ionized. · Beta particles and gamma radiations may also cause ionization; the particles of the radiation remove electrons, creating electron ion-pairs.

· Radioactive radiations can cause severe damage to living organisms. · May affect DNA and genetic coding of future generations. · Probability of genetic damage increases with increasing intensity and exposure to radiation. · Illnesses such as leukemia and cancer can result from exposure; even death

· For elements with Z less than about 20, the protons and neutrons are in equal numbers. · Due to an increase in the electrostatic repulsion forces of protons as the number of protons increases, more neutrons must be found in nucleus to hold atom together. · Each time protons and neutrons are added, they must go into higher energy state, and eventually become unstable. Unstable nuclei emit alpha particles (two protons and two neutrons) in order to reach a more stable state

Radioactive Half-Life (Two Definitions): · The time taken for half the number of radioactive nuclei in a sample to decay. · The time taken for the activity of a sample to decrease to half its initial value.

Artificial Transmutation: When a nucleus is bombarded with a nucleon, an alpha particle or another small nucleus, resulting in a nuclide with a different proton number (a different element). · Can also be used to form radioactive isotopes of elements. Neutrons are effective in inducing artificial transmutation to form artificial radioactive isotopes

Unified Atomic Mass Unit: 1/12^th the mass of a Carbon-12 nucleus. · Units created to compare atomic masses, since individual masses in nuclear reactions are very small. · The mass one proton (or neutron) are approximately 1 u.