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Lecture 1 Dr: Sahar Mousa
Electrons and nuclei The Familiar Planetary Model of the atom was proposed by Rutherford in 1912 • All the mass of an atom was concentrated in the positively charged (+) Nucleus. Electrons have Negatively charged (-). • electrostatic force: it’s a force required to attracted the electrons to the nucleus.
Nuclear structure • Atomic number (Z) ,is the no. of protons and also the no. of electrons in a neutral atom. • Mass number,is the no.of protons + the no. of neutrons
The lightest atomic nucleus (hydrogen) is 1830 times more massive than an electron. • The size of a nucleus is around 10−15 m (1 fm)
Nuclear structure • Nuclei contain positively charged Protons and uncharged Neutrons • Protons and neutrons are held together by an attraction force (strong interaction). • An electrostatic ruplsion between protons occurs inside the nucleus • The balance of the two forces controls some important features of nuclear stability
Nuclear structure • The lighter nuclei are generally stable with approximately equal no. of protons and neutrons as O16 andC12. • The heavier ones have higher proportion of neutrons as pb 208. • As Z increases the electro static repulsion comes to be dominate. • There is a limit to the number of stable nuclei. • All elements beyond Bi (z=83)being radioactive.
Magic number The nuclei with even no. of either protons or neutrons ( or both) are generally more stable than ones with odd no. O16and Pb208 are example of nuclei with magic no. -certain magic numbers of protons or neutrons, which give extra stability
Isotopes • are atoms of the same element that have a different number of neutrons. Therefore, isotopes have the following characteristics: • Isotopes have the same atomic number (same number of protons), but a different atomic mass number (a different number of neutrons). • Isotopes behave the same chemically, because they are the same element. The only difference is that one is heavier than the other, because of the additional neutrons. • For example, carbon-12 and carbon-14 are both isotopes of carbon. Carbon-12 has 6 neutrons; carbon-14 has 8 neutrons.
Isotopes Some elements have: • only one stable isotope (e.g. 19F, 27Al, 31P). • others may have several (e.g. 1H and 2H, the latter also being called deuterium, 12C and 13C). • Molar mass is also known as Relative atomic mass (RAM) is determine by the Proportions (Mixture of the isotopes of an element).
Isotopes • NMR (nuclear magnetic resonance ) technique depend on NMR of H1 and C13. • All elements have unstable radioactive isotopes, some of these occur Naturally and the other can be mad e artificially. • Importance and uses of isotopes (students)
Radioactivity Radioactive decay: • It is a process whereby unstable nuclei change into more stable ones by emitting particles of different kinds as Alpha, beta and gamma (α, β and γ) radiation
Radioactivity • Gamma radiation, is high energy electro magnetic radiation accompanies alpha and beta decays. • Half-life is the time taken for half of a sample to decay. It can vary from a fraction of a second to billions of years. • All elements beyond Bi (z=83) are radioactive and non beyond U(z=92) occur naturally on Earth.
Radioactivity Spontaneous fission • It occurs for heavy elements ,where the nucleus splits into two fragments of similar mass.