Valence Electrons and Chemical Bonding

Valence Electrons and Chemical Bonding • Review valence electrons • Principles of “Bonds Away” • Ionic Bonds • Metallic Bonds • Covalent Bonds • Intermolecular Forces • Common Chemical Reactions

Take Home Message • When atoms combine to produce molecules and compounds, expect the chemical properties of the molecules/compounds to be far different than that of the constituent atoms (hierarchy theory) • Atoms bind together by re-arranging and sharing their electrons • Ionic bonds • Metallic bonds • Covalent Bonds • Intermolecular forces (e.g., hydrogen bond) • Chemical interactions make and break bonds between atoms and in so doing effect a change in energy (potential and kinetic) • Weak chemical bonds (e.g., covalent bonds) play a very important role in the chemistry of life

Chapter Deletions • Pp 262 (Covalent compounds and formulas) – 265 (Coordinate covalent bonds) • Pp 267 (Ionic compound names) – 270 (Covalent compound names) • Pp 279 (Percent Composition of Compounds) – 295 (Quantitative use of equations)

Atoms in Proximity: Bonds Away • Hypothesis: when two atoms are brought together, electrons will tend to re-arrange themselves to the lowest energy state where the valence electrons are most stable • Product: electrons are re-arranged into bonds • Give away electrons • Accept electrons • Share electrons

Ionic Bonding • Atoms give away electrons while other atoms receive electrons • Example of lithium chloride 36Li + 1735.5Cl = LiCl

Ionic Bonding • Lithium (Li) Li gives up 1 electron and is left with 2 electrons (-) and 3 protons (+); net positive (+) charge • Chlorine (Cl) Cl has 1 unpaired electron in valence shell, so Cl tends to accept an electron and is left with 18 electrons (-) and 17 protons; net negative (-) charge

Ionic Bonding • Atoms give away electrons while other atoms receive electrons • Example of lithium chloride Li + Cl = LiCl • Bonding via electrical attraction between Li+ and Cl- • Li+ + Cl - = Li+Cl- • Consequence: ionic bonds are underpinned by charged ions and tend to form crystals of very specific and repeating geometry (very rigid) • Example: NaCl is based on ionic bonds and is salt

Metallic Bonds • Elements that do not give or take electrons (ionic bonds) BUT share electrons • Valence electrons tend to move freely between both atoms (contrast with ionic bonds) • Significance of sharing electrons: compounds tend to show two features • Malleability (easily worked or pounded) • Conductive of electricity (good conductors) • Examples • Gold jewelry • Copper wire

Covalent Bonds • Extremes of behavior in bonding • Accept or give away electrons (ionic bonds) • No tendency to share (noble gases) • Intermediate between these two extremes but • Do not form ionic bonds • Do not form metallic bonds • Yet share 1,2, 3 and 4 electrons in unique arrangement called covalent bonds • Key: orbits of valence electrons are shared so that electrons are shared (and move) between valence shells of adjacent atoms

Covalent Bonds • Example of hydrogen fluoride (HF) • 11H and 919F • Note: Valence shell for both atoms are full • Single bond shared • Double bond

Next Week’s Lab: Evaporation and Chemical Structure • Vaporization and chemical properties of molecules • Liquid to gas state change • State change has energy cost: endothermic (temperature decrease) • Temperature change is a function of chemical structure of molecule • Bonding and polarity

Evaporation and Chemical Structure • Organic compounds • Carbon based or hydrocarbons bond with other elements via covalent bonds) • Alkanes: C and H only • Pentane (C5H12) • Alcohols: C, H and OH (hydroxyl group) • Ethanol (C2H5OH) • Structural formula • Hydrogen bonding: H bonded to N, O or F (tight bond) • Process: as chemical vaporizes, temperature change is chemical specific and is a “window” onto the chemical structure

Evaporation and Chemical Structure • Hypothesis: temperature changes with vaporization in a manner that is predictable, based on the bonding among atoms involving C, H and OH • Method • Measure temperature change electronically • Record for 5 hydrocarbons • Analyze data (graphically) based on understanding of the bonds for each molecule

Valence Electrons and Chemical Bonding