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 O 16 andC 12.  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. O 16 and Pb 208 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. 19 F, 27 Al, 31 P).  others may have several (e.g. 1 H and 2 H, the latter also being called deuterium, 12 C and 13 C).  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 H 1 and C 13.  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.