Core v Valence Electrons

Transcription

1 Bonding

2 Core v Valence Electrons The core electrons (represented by the noble gas from the previous row) are those electrons held within the atom. These electrons are not involved in the bonding, but contribute to the shielding effect The valence electrons are those electrons in the outermost energy level of an atom. These electrons are involved in bonding. Can be determined by location in periodic table.

3 Octet Rule The most stable atoms will have 8 outer electrons (noble gases). The octet rulestates that those elements with 8 outer s and p electrons will be particularly stable. These valence electrons largely determine the chemical properties of an element. Metals have low numbers of valence electrons. Nonmetals have high numbers of valence electrons.

4 Intramolecular Forces vs. Intermolecular Forces Intramolecular forces are forces of attraction between atoms in a compound (ie. ionic bonding, etc.) Intermolecular forces are forces of attraction between compounds in a collection of compounds. (ie. LDFs, dipole-dipole forces, etc.)

5 Bonding Between Atoms Ionic Bonds (cations and anions) Covalent Bonds (sharing) Metallic Bonds (delocalized electron cloud) Network Covalent Bonds (diamond)

6

7 Ionic bonding Electrostatic attraction (in lattice) Metals/nonmetals (widely diff electronegativity) Electrons transferred Polyatomic ion possibly High melting points Do not conduct as solids But conduct when dissolved or melted

8 Covalent Bonding Share electrons Two or more nonmetals Similar electronegativities Low melting points Do not conduct in any phase May or may not dissolve in water (depends upon polarity of compound)

9 Metallic Bonding Metal cations surrounded by their mobile valence electrons (in lattice) Malleable and ductile; shiny Conducts electricity in all phases Melting points vary (generally lowers as you move down a group) Metallic character increases as you move down a group (due to shielding effect)

10

11 Alloys An alloy is a solid solution of two or more metals (could include a nonmetal or a metalloid). Made by melting the elements involved, mixing them together, then re-solidifying. The properties of the alloy are much better than any of the original components. Common alloys Brass (Zn and Cu) Bronze (Sn and Cu)

12 Network Covalent Bonding Atoms held together in lattice of covalent bonds (like one big molecule) Very hard Very high mpand bp Electrons are localized about the atoms In diamond, all of the carbons are bonded in a tetrahedral arrangement

13 Bond Summary

14 Let s Look at Electronegativity

15 Determining Type of Bond Simplest use periodic table metal/non-metal should exhibit ionic bond Non-metal/non-metal should be covalent bond Metals will exhibit metallic bond Find electronegativity difference Large difference (>1.67) generally means ionic Small to medium difference (<1.67) generally means covalent Plot electronegativity computations using Bond Triangle

16 Bond Triangle For years, chemists have determined bond type between atoms by using the idea of electronegativity difference. Large electronegativity difference = ionic Small to medium electronegativity difference = nonpolar or polar covalent The bond triangle also uses electronegativity values, but in a slightly different manner A comparison is made between the average of electronegativities and the difference in the electronegativites of the two elements involved.

17 Bond Triangle, cont. Types elements Ave EN Diff EN Bond type Metal/nonmetal ~ ~ 2.5 Usually large Ionic Metal/metal ~ 1 Very low (~0) metallic Nonmetal/nonmetal Usually > 2.5 Low covalent By plotting Ave EN v Diff EN, you get a triangle that is divided into four areas. Area A represents ionic bonding. Area B represents metallic bonding Area C represents covalent bonding Area SM represents semimetals This bond triangle is not absolute, but it more closely can predict the bond type between atoms than just using electronegativity differences.

18 Ionic Covalent Metallic

19 Covalent bonds and polarity Within the bond, one atom is usually more electronegative than another That atom pulls the electrons in the bond to himself Causes a dipole one side of the bond is more negative and the other side of the bond is more positive Not an ionic bond but a very polar bond

20 Determining Covalent Bonds Lewis dot diagrams can predict type of covalent bond between atoms (single, double, or triple) but it cannot imply the shape of the molecule Structural formulas will replace a bonded pair of electrons with a stick. Then by understanding the VSEPR theory, the shape can be implied on two dimensional paper

21 Valence-shell electron pair repulsion theory The VSEPR theory states that in a small molecule, the pairs of valence electrons are arranged as far apart from each other as possible. This repulsion is different for the following possibilities. Unshared-unshared repulsion is the most. Shared-shared repulsion is the least. Unshared-shared repulsion is midway between the two.

22 5 Basic Shapes This theory does explain a wide variety of molecular shapes. Example Name of Shape Bond Angle BeF 2 Linear 180º BF 3 Trigonal planar 120º CH 4 tetrahedral 109.5º NH 3 Trigonal pyramid (pyramidal) 107º H 2 O Bent 104.5º

23 More complex shapes These shapes arise because certain atoms can expand their octets (which means they can have more than 8 electrons around them). The only atoms that can expand their octet are those who have a d sublevel available to them. Name of Shape Bond Angle Trigonal bipyramidal 90º & 120º See-saw 90º & <120º T-shaped 90º octahedral 90º Square pyramidal 90º Square planar 90º

24 Lewis structures show the arrangement of valence electrons in covalently bonded molecules and ions. We can use Lewis structures to predict the properties of the bonds in molecules and ions, such as the properties of bond length and bond energy, and use these bond properties to predict physical and chemical properties of molecules and ions.

25 Lewis Dot Diagram steps 1. Count valence electrons 2. If charged, add or subtract the charge (ie. changes the number of electrons) 3. Arrange symbols, make no trains 4. Put in shared pairs 5. Determine remaining electrons 6. Determine who wants octet Groups 1, 2, 3 do not want eight Groups 4-7 want eight (know who can expand octet) 7. Put in unshared electrons 8. Rearrange bonds as needed

26 Polar vs. Non-polar If a molecule contains only nonpolar bonds, it will be a nonpolar molecule. (seven diatomics, ozone, and diamond) If a molecule contains polar bonds it is not necessarily a polar molecule. The shape of a molecule and the polarity of its bonds together determine whether the molecule is polar or nonpolar. If the shape of the molecule is symmetrical (which means that there are no unshared electrons on the central atom), then the molecule will be nonpolar. If the shape of the molecule is asymmetrical (which means that there is at least one unshared pair of electrons on the central atom), then the molecule will be polar.

27 Dipole moment The dipole moment measures the polarity of a molecule (based on shape of molecule) The larger the dipole moment, the more polar the molecule The greater the charge at the ends of the dipole and the greater the distance between the charges, the greater the value of the dipole moment

28 Bond Lengths In general, single bonds are longer than double bonds, which are longer than triple bonds. How do they determine bond length? X-ray diffraction Neutron diffraction X-ray crystallography Microwave spectroscopy

29 FYI Computational Chemistry Computational chemistry is simply the application of chemical, mathematical and computing skills to the solution of interesting chemical problems. It uses computers to generate information such as properties of molecules or simulated experimental results. Some common computer software used for computational chemistry includes: Gaussian xx (Gaussian 94 currently) GAMESS MOPAC Spartan Sybyl

30 Computational chemistry has become a useful way to investigate materials that are too difficult to find or too expensive to purchase. It also helps chemists make predictions before running the actual experiments so that they can be better prepared for making observations. For instance, you can calculate: electronic structure determinations geometry optimizations frequency calculations transition structures protein calculations, i.e. docking electron and charge distributions potential energy surfaces rate constants for chemical reactions (kinetics) thermodynamic calculations-heat of reactions, energy of activation

31 Hybridization One of the theories used to explain molecular geometry (AP expects you to know bold ) sp linear (2 identical lobes) sp 2 trigonal planar, bent (3 identical lobes) sp 3 tetrahedral, trigonal pyramid, bent (4 lobes) sp 3 d trigonal bipyramidal, seesaw, T-shaped, linear (5 identical lobes) sp 3 d 2 octahedral, square pyramidal, square planar (6 identical lobes)

32

33 σ versesπ Sigma bonds overlap end to end Pi bonds overlap above and below

34 What can σ and π tell you? A sigma bond results in a single bond A sigma bond and a pi bond results in a double bond A sigma bond and 2 pi bonds results in a triple bond Therefore the picture from the previous slide represents a double bond

35 Molecular Orbital Theory In the molecular orbital theory, as two atomic orbitals approach each other they create two molecular orbitals. Bonding orbital of lower energy Antibonding orbital of higher energy Type of covalent bond (single, double, or triple) can be predicted using the formula bond order = # bonding electrons # antibondingelectrons 2

36 Molecular Orbital Diagram

37 Resonance Some Lewis dot diagrams can have a variety of arrangements Ex. Carbonate ion CO 3 2- Resonance explains the disparity in bond lengths

38 Van der Waals forces (Intermolecular Forces) Ion-dipole forces attraction between an ion and a polar molecule Dipole-dipole forces attraction between two polar molecules

39 Dipole-induced dipole forces attraction between a polar and nonpolar molecule Dispersion forces attraction between two nonpolar species

40 Very Important van der Waals force Hydrogen bonds (FON) -- attraction between molecules containing fluorine, oxygen, or nitrogen and hydrogen the hydrogen is electron starved holds hands with neighboring F, O, or N

41 Three famous molecules H 2 O NH 3 : : HF H:F:

42 Intermolecular Forces of Attraction A substance will normally exist in the solid, liquid, or gas phase depending upon the force of attraction between the particles A solid substance will have strong intermolecular forces of attraction (lots of snuggling) A gaseous substance will have weak intermolecular forces of attraction between the particles. A liquid substance has intermediate forces of attraction between its particles

43 Three States of Matter