Chapter 23 Chemical Bonding 23.1 – An Atomic Model is Needed to Understand How Atoms Bond • Review shell model of atom (18.4) • Valence electrons – • Electron Dot Diagrams – – Tells valence electrons – How many (ve) are paired • Nonbonding pairs - Electron Shells K= L= M= N= O= P= Q= 23.2 – Atoms Can Lose or Gain Valence Electrons to Become Ions • Atoms are electrically neutral – Why? • Ions are not electrically neutral – Why? • Ion – – Can be negative or positive • Atoms tend to lose or gain electrons so they end up with an outermost occupied shell that is filled to capacity • The Periodic table can be used to determine the type of ion that an atom tends to form Positive Ion (cation) Negative Ion (anion) 23.3 – Ionic Bonds Result from an Transfer of Valence Electrons • Result of transfer of electrons forms a positive ion and a negative ion • Ionic Bond – • Ionic Compounds – • Characteristics of Ionic Bonds - Ionic Bonds: One big greedy thief dog! Ionic bonding can be best imagined as one big greedy dog steeling the other dog's bone. If the bone represents the electron that is up for grabs, then when the big dog gains an electron he becomes negatively charged and the little dog who lost the electron becomes positively charged. The two ions (that's where the name ionic comes from) are attracted very strongly to each other as a result of the opposite charges. 23.4 – Covalent Bonds Result from a Sharing of Valence Electrons • Covalent bond – • Covalent Compound – • Molecule – • Characteristics of Covalent Bonds – • Can be single, double, or triple Covalent Bonds: Dogs of equal strength. Covalent bonds can be thought of as two or more dogs with equal attraction to the bones. Since the dogs (atoms) are identical, then the dogs share the pairs of available bones evenly. Since one dog does not have more of the bone than the other dog, the charge is evenly distributed among both dogs. The molecule is not "polar" meaning one side does not have more charge than the other. 23.5 – Polar Covalent Bonds Result from an Uneven Sharing or Electrons • Dipole – • Electronegativity – – Difference in electronegativity • Nonpolar Bond – • Polar Bond - Polar Covalent Bonds: Unevenly matched but willing to share. These bonds can be thought of as two or more dogs that have different desire for bones. The bigger dog has more strength to possess a larger portion of the bones. Sharing still takes place but is an uneven sharing. In the case of the atoms, the electrons spend more time on the end of the molecule near the atom with the greater electronegativity (desire for the electron) making it seem more negative and the other end of the molecule seem more positive. 23.6 – Molecular Polarity Results from an Uneven Distribution of Electrons • Polar and nonpolar are easy when 2 atoms, more complex when more than 2 atoms are bonded • End result may be a even distribution or an uneven distribution • Can make the dipole stronger • Explains why water is “sticky” • May explain other macroscopic properties This molecular polarity causes water to be a powerful solvent and is responsible for its strong surface tension. The molecular arrangement taken by ice (the solid form of the water molecule) leads to an increase in volume and a decrease in density. Expansion of the water molecule at freezing allows ice to float on top of liquid water.
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