2 原子结构 Atomic Structure课件.pptVIP

Atomic Structure (P11-14) 1. Nucleus This contains protons and neutrons, called collectively nucleons. 2. The mass number gives the number of nucleons, that is the number of protons + neutrons. 3. The atomic number gives the number of protons in the nucleus. It also gives the number of electrons in the neutral atom and the position of the element in the periodic table. 4. Isotopes are atoms with the same atomic number but different mass numbers. 5. The masses of atoms of particular isotopes, called the relative isotopic mass, are expressed on a relative scale on which the mass of an atom of the isotope carbon-12 has 12 units exactly. 6. Relative atomic mass All elements exist in several isotopic forms and it is useful to have an average value for the atoms of each element. This is called the relative atomic mass and is defined as the weighed mean of the masses of the naturally occurring isotopes of the element expressed on the carbon-12 scale. (An example is shown below. P11) 7. Radioactivity Some isotopes are stable, but others, often with uneven numbers of protons and/or neutrons are unstable. This instability increases with atomic number, resulting from the growing repulsion between increasing numbers of protons. When an unstable isotope decays it gives off radiation known as radioactivity. This can be in one of three forms as this table shows. (Seeing in the following table. P11) 8. Uses of isotopes Isotopes are often used as tracers. Very small traces of an isotope can be detected and hence followed through a progress. biology e.g. 32P to study nutrient uptake in plants medicine e.g. 131I to study thyroid function industry e.g. 57Fe to study wear and lubrication in engines generating power e.g. 235U in fission reactors; 3H in fusion geography e.g. 57Fe to study river flow archaeology e.g. 14C in carbon dating 9. Electronic energy levels, orbitals, and shells: how electrons are arranged in atoms. energy levels Atomic emission and absorption spectra tell u