3/11/09 Chemistry Notes Structure of an Atom Periodicity of Electron Configurations The principal energy level number the number that comes before the sublevel letter designation, is the same as the period number for the s and p sublevels For the d sublevels, the principal energy level number is one less than the period number Valence Electrons for the Main-Group Elements The last filled principal energy level is called the valence level, or valence shell The valence level contains electrons that are highest in energy and occupy orbitals that extend further from the nucleus than those in the lower levels Valence electrons occupy orbitals in the valence level. All the other electrons are called core electrons, or inner electrons. Electron Configuration for Ions In atoms the number of electrons is equal to the protons which is the atomic number In ions the number of electrons does not equal the atomic number. We must ass or subtract electrons, depending on if the ion is an anion or caution. Periodic Properties of Atoms Electron configuration are related to the following properties: ??relative reactivity ??Ionization energy (tendency to lose electrons) ??Atomic radii (atomic size) Chemical Reactivity the alkali metals from +1 ions since they all have 1 valence electron ?oxides of the alkali metals The alkaline earth metals form +2 ions since they all have 2 valence electrons ?oxides of the alkaline earth metals Ionization Energy -Ionization energy (IE) is a measure of the energy required to remove a valence electron from an atom -Increases up a group because the valence electrons become increasingly closer to the nucleus. -increases from the left to right in a period because the valence electrons are increasingly held more tightly by the nucleus, which is increasing in the # of protons.
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