Lecture 150130 Electronegativity a measure of the ability of an atom to attract electrons from an atom it is bonded to determined based on 1 Number of protons 2 Distance between nucleus and the valence shell 3 O and N are more electronegative than C and H Atoms with a high electronegativity will hold the electrons more tightly than that with a lower electronegativity and have a partial negative charge whereas the other atom in the molecule will have a partial positive charge The upper left corner has the highest electronegativity fluorine is the most likely to bond with anything but oxygen bonds a lot too bottom left corner of periodic table has lowest electronegativity Chemical bonds differences in electronegativity dictate how electrons are distributed in covalent bonds 1 Nonpolar covalent bond electrons are evenly shared between two atoms and the bond is symmetrical 2 Polar covalent bond electrons are asymmetrically shared water r O H H O is a covalent bond r H is a radical bond could be anything or nothing the electron in the OH bond will stay with the O so the H will be positive a little bit the partial charges are why water has the bend in the molecule electrons hate each other and want to be as far away as possible and there s 3 pairs on the outer electron shell and the angle where there s maximum distance between all electrons is 120 the and can make very weak bonds called hydrogen bonds oxygen gas molecules are non polar because the electronegativity of oxygen is in fact the same as the electronegativity of oxygen so the atoms have even pull and the electrons are shared evenly some pictures of bonding from the book Ionic bonds an atom or molecule that carries a charge is called an ion 1 Cation loses electron gains positive charge 2 Anion gains electron becomes negatively charged They form bonds like the way magnets do negative attracts positive Some pictures of a salt crystal Electron sharing continuum the degree to which electrons are shared in chemical bonds forms a continuum from equal sharing in nonpolar covalent bonds to unequal sharing in polar covalent bonds to the transfer of electrons in ionic bonds Picture from the book Properties of water called the universal solvent life is based on water because water is a great solvent the covalent bonds in water are polar Ions and polar molecules dissolve in water because of the partial charges they are hydrophilic water won t dissolve lipids or non polar stuff because they don t interact with water LIKES DISSOLVE LIKES which is why you need soap because it is both polar and non polar and can dissolve anything including your skin if given enough time Structure and properties Water is unique due to its small size bent shape highly polar covalent bonds and overall polarity 1 2 3 4 Cohesive it sticks to itself why water makes spherical droplets Adhesive it sticks to other stuff Denser as a liquid than a solid ice floats due to its open crystal structure Able to absorb large amounts of energy it takes a 4 calories to raise a gram of water one degree it has a high specific heat Carbon carbon is the most versatile atom on earth Because it has a valence of four carbon can form many covalent bonds Carbon containing molecules can form an almost limitless array of molecular shapes with different combinations of single and double bonds The formation of carbon carbon bonds was an important event in chemical evolution because we are carbon based life forms Things usually get named after their number of carbons octane C8H18 hexane C6H14 Functional groups the carbon atoms in an organic molecule furnish the skeleton that gives the molecule its overall shape Amino groups negative charge attract protons protium Carbonyl link molecules into larger compounds Carboxyl releases a proton negative charge Hydroxyl groups polar group and act as weak acids Phosphate groups have two negative charges Sulfhydryl groups link together via disulfide bonds NH3 C O COOH COOOH PO4 SH QUESTION why do most polar covalent bonds involve nitrogen or oxygen Because they are the most electronegative so any bond with them will probably not have equal sharing of electrons QUESTION predict what part of water molecules would interact with amino carboxyl and hydroxyl groups amino the oxygen part is negative NH3 is positive carboxyl the H parts are positive and COOH is kinda negative hydroxyl negative charge will like the H parts that are positive