化学专业外语.ppt

Lesoon 1 Size of Atoms and Ions 氢 Hydrogen 氦 Helium 锂 Lithium铍 Beryllium硼 Boron 硫 Sulfur 氯 Chlorine 氩 Argon 钾 Potassium 钙 Calcium Lesson 2 Metallic Character Metals are electropositive and have a tendency to loss electrons, if supplied with energy: M→M+ + e. Thus in Group Ⅳ, carbon is a nonmetal, germanium shows some metallic properties, and tin and lead are metals. The most electropositive elements are found in the lower left of the periodic table and the most nonmetallic in the top right. Electropositivity is really the converse of electronegativity, but it is convenient to retain the concept of electropositivity when describing metals. Strongly electropositive elements give ionic compounds. Metallic oxides and hydroxides are basic since they ionize, and given hydroxyl ions: NaOH →Na+ + OH- CaO + H2O →Ca2+ + 2OH- Oxides, which are insoluble in water, are regarded as basic if they react with acids to form salts. Thus in the main groups of the periodic table, basic properties increase on descending a group because the elements become more electropositive and more ionic. The degree of electropositivity is shown in a variety of ways. Strongly electropositive elements react with water and acids. The degree of electropositivity is shown in a variety of ways. Strongly electropositive elements react with water and acids. Weakly electropositive elements are unaffected by water and are much less readily attacked by acids. The electropositive nature of a metal is also shown in the degree of hydration of the ions. In the change: the positive charge becomes spread over the whole complex ion. Since the charge is no longer localized on the metal, this is almost the same as the change M+→M.