Valence Electrons, Bonds and Chemical Reactions 05 October 2006

Transcription

1 Valence Electrons, Bonds and Chemical Reactions 05 October 2006 Principles of Valence Electrons and Bonds Ionic Bonds Metallic Bonds Covalent Bonds Intermolecular Forces Common Chemical Reactions

2 Atoms in Proximity Observation: when two atoms are brought together, electrons re-arrange to the lowest energy state (i.e., valence electrons are most stable) Consequence: distribution of electrons among atoms are re-arranged into bonds Give away electrons Accept electrons Share electrons

3 Atoms in Proximity

4 Ionic Bonding Atoms give away electrons, whereas other atoms receive electrons Example of lithium (Li) chloride (Cl) 3 6 Li + 17 Cl = LiCl Cl =

5 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

6 Ionic Bonding Some atoms tend to give away electrons, whereas other atoms tend to 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

7 Ionic Bonding: Salt

8 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

9 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 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

10 Covalent Bonds Example of hydrogen fluoride (HF) 1 1H H and 9 19 F Note: Valence shell for both atoms are full Single bond shared Double bond

11 Covalent Bonds: Carbon 6 12 C is a special case (important in biology & chemistry) Valence electrons for C are 4 (1 in each orbit) and intermediate between giving and accepting C - C single covalent bond (1 orbit) C C - C two covalent bonds involving 2 orbits Unique behavior of C C C-C-C C

12 Behavior of Valence Electrons: Five Options No action (e.g., inert gases) Give away one or more electrons in valence state (positive ion leading to ionic bond) Accept one or more electrons to valence state (negative ion leading to ionic bond) Share an electron with many other atoms without respect to an orbit (metallic bond) Share one or more electrons plus their orbits with another atom (covalent bond)

13 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

14 Intermolecular Forces: Polarization & Hydrogen Bonding Example of water (H 2 O) + H H + O - When one molecule s distribution of atoms results in one side of the molecule having either a + or charge Resulting distribution of charges causes adjoining H 2 O molecule to align with + and charges to be most stable Called polarity of molecule (e.g., magnet)

15 Intermolecular Forces: Van der Waal Forces In polarity, specific and rigid + and fields on each molecule that does not change over time When molecules converge, inevitable that electrons shift and re-distribute (e.g., planar compound) In re-distribution, small net attraction between molecules arise and two molecules for weak bond Graphite pencil lead Stack of paper

16 Valence Electrons and Chemical Bonding Review valence electrons Principles of Bonds Away Ionic Bonds Metallic Bonds Covalent Bonds Intermolecular Forces Common Chemical Reactions (ph)

17 Acid Base Reaction: Measurement ph scale Any increase in H+ results in more acid solution from 7 to 0 Any increase in OH- results in more basic solution from 7 to 14 Examples Rainwater of 5.6 means what? Cell ph value of means what? Importance to biological systems and buffering

18 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 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

19 Chapter Deletions (No. 9) Pp 202 (A Closer look) Pp 204 (A Closer Look) Pp 203 (Composition of Compounds) 211 (Ion Exchange Reactions)