Bio 345 Lecture Notes PDF

‫بسم هللا الرحمن الرحيم‬ ‫‪Bio 345 3 rd lecture‬‬ Electron arrangement determines the chemical properties of an atom  Atoms want to fill their outer electron shells –To accomplish this, the atom can share, donate, or receive electrons –This results in attractions bet...

‫بسم هللا الرحمن الرحيم‬ ‫‪Bio 345 3 rd lecture‬‬ Electron arrangement determines the chemical properties of an atom  Atoms want to fill their outer electron shells –To accomplish this, the atom can share, donate, or receive electrons –This results in attractions between atoms called chemical bonds Chemical bonds An ion is an atom or molecule with an electrical charge resulting from gain or loss of electrons - When an electron is lost, a positive charge results; when one is gained, a negative charge results Two ions with opposite charges attract each other - When the attraction holds the ions together, it is called an ionic bond  Formation of an ionic bond, producing sodium chloride ‫ينتج كلوريد الصوديوم من تكوين رابطة أيونية‬ Transfer of electron ‫رحالن اإلليكترون‬ Na Cl Sodium atom Chlorine atom ‫ذرة صوديوم‬ ‫ذرة كلور‬  Formation of an ionic bond, producing sodium chloride ‫ينتج كلوريد الصوديوم من تكوين رابطة أيونية‬ Transfer of electron ‫رحالن اإلليكترون‬ + – Na+ Cl– Na Cl Sodium ion Chloride ion Sodium atom Chlorine atom ‫أيون صوديوم‬ ‫أيون كلور‬ ‫ذرة صوديوم‬ ‫ذرة كلور‬ Sodium chloride (NaCl) ‫كلوريد الصوديوم‬ A crystal of sodium chloride ‫بلورة كلوريد الصوديوم‬ Na+ Cl– Chemical bonds  A covalent bond results when atoms share outer-shell electrons - A molecule is formed when atoms are held together by covalent bonds  ‫طرق مختلفة‬ ‫التوزيع‬ ‫لتمثيل أربع‬ ‫اإلليكتروني‬ ‫جزيئات شائعة‬ ‫الصيغة‬ ‫نموذج‬ ‫الجزيئية‬ ‫فراغي‬ ‫رابطة أحدية‬ ‫الصيغة‬ ‫هيدروجين‬ ‫البنائية‬ ‫رابطة ثنائية‬ ‫أوكسجين‬ ‫ميثان‬ ‫ماء‬  ‫طرق مختلفة‬ ‫لتمثيل أربع‬ ‫جزيئات شائعة‬ ‫الصيغة الجزيئية‬ ‫التوزيع اإلليكتروني‬ ‫الصيغة البنائية‬ ‫نموذج فراغي‬ ‫رابطة أحدية‬ ‫رابطة ثنائية‬  ‫طرق مختلفة‬ ‫لتمثيل أربع‬ ‫جزيئات شائعة‬ ‫الصيغة الجزيئية‬ ‫التوزيع اإلليكتروني‬ ‫الصيغة البنائية‬ ‫نموذج فراغي‬ ‫ميثان‬ ‫ماء‬ Chemical bonds  Unequal electron sharing creates polar molecules  Atoms in a covalently bonded molecule continually compete for shared electrons –The attraction (pull) for shared electrons is called electronegativity –More electronegative atoms pull harder Chemical bonds  In molecules of only one element, the pull toward each atom is equal, because each atom has the same electronegativity The bonds formed are called nonpolar covalent bonds Unequal electron sharing creates polar molecul  Water has atoms with different electronegativities –Oxygen attracts the shared electrons more strongly than hydrogen –So, the shared electrons spend more time near oxygen –The result is a polar covalent bond Unequal electron sharing creates polar molecul  In H2O the oxygen atom has a slight negative charge and the hydrogens have a slight positive charge – Molecules with this unequal distribution of charges are called polar molecules  A water molecule ‫جزيئ ماء‬ (–) (–) O H H (+) (+)  Polarity of Water  In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids ◦ Because oxygen is more electronegative, the region around oxygen has a partial negative charge. ◦ The region near the two hydrogen atoms has a partial positive charge.  A water molecule is a polar molecule with opposite ends of the molecule with opposite charges.   Water has a variety of unusual properties because of attractions between these polar molecules. ◦ The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond. ◦ Each water molecule can form hydrogen bonds with up to four neighbors. Fig. 3.1 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings  HYDROGEN BONDS  Hold water molecules  Extraordinary Properties together that are a result of hydrogen bonds.  Each water molecule can form a maximum ◦ Cohesive behavior of 4 hydrogen bonds ◦ Resists changes in temperature  The hydrogen bonds ◦ High heat of vaporization joining water ◦ Expands when it freezes molecules are weak, ◦ Versatile solvent about 1/20th as strong as covalent bonds.  They form, break, and reform with great frequency  Hydrogen bonds are weak bonds important in the chemistry of life  Some chemical bonds are weaker than covalent bonds  Hydrogen, as part of a polar covalent bond, will share attractions with other electronegative atoms – Examples are oxygen and nitrogen  Water molecules are electrically attracted to oppositely charged regions on neighboring molecules – Because the positively charged region is always a hydrogen atom, the bond is called a hydrogen bond  ‫‪Hydrogen bonds between‬‬ ‫‪water molecules‬‬ ‫الروابط الهيدروجينية بين جزيئات الماء‬ ‫‪Hydrogen bond‬‬ ‫رابطة هيدروجينية‬  Hydrogen bonds hold the substance together, a phenomenon called cohesion  Cohesion is responsible for the transport of the water column in plants  Cohesion among water molecules plays a key role in the transport of water against gravity in plants  Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels.