材料科學(xué)基礎(chǔ)英文版課件_(8)-資料下載頁
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【正文】 aals bonds – Hydrogen bonds Secondary bonds (2) 1) Van der Waals bonds Describe a dipolar attraction between uncharged atoms with stable electron structures or between molecules in molecular structures . inert gases helium (He), neon (Ne), argon (Ar), and H2, N2, F2。 when they are in the liquid state, their atoms or molecules are bonded by Van der Waals bonds Reason: Electronic charge on an atom is always in motion。 on the average – spherically symmetric relative to the nucleus, but at any instant – unsymmetric – dipole moment Two dipoles on two adjacent atoms attract each other Bond energy: Secondary bonds (3) Van der Waals bonding – the atoms are held together by the dipole charge distribution Typical example: liquid nitrogen at –198oC – covalently bonded N2 molecules are glued together by Van der Waals forces Secondary bonds (4) 2) Hydrogen bonds Keep water liquid at room temperature, and bind polymer chains together to form solid polymers In water (H2O): each hydrogen atom shares its charge with the nearest oxygen atoms Hydrogen (H) – losing part of its share – positively charged Oxygen (O) – getting more share – negatively charged Positively charged H atom – bridging between two neighbouring O atoms Charge redistribution – a dipole moment for each H2O molecule Secondary bonds (5) Arrangements of H2O molecules in the form of ice, showing the hydrogen bonds Condensed states of matter Primary and secondary bonds – matter can condense from the gaseous state to liquids and solids Interatomic forces F = dU/dr Hooke’s Law ?? = E ?
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