Chemical Bonding. Chapter 6

Transcription

1 Chemical Bonding Chapter 6

2 Introduction to Bonding Chemical bond mutual attraction between the nuclei and valence electrons of different atoms that binds the atoms together Q: Why do atoms bond? A: By bonding with each other, the system decreases in potential energy, thereby creating more stable arrangements of matter

3 Types of Chemical Bonding Ionic chemical bonding that results from the electrical attraction between large numbers of cations and anions Covalent results from the sharing of electron pairs between two atoms

4 Types of Chemical Bonding Ionic or covalent? Determined by the difference in electronegativities of the two atoms

5 Types of Chemical Bonding

6 Types of Chemical Bonding Ionic or covalent?

7 Types of Chemical Bonding Atoms S and H Ca and Cl C and F K and O N and N Difference in Electronegativity Bond Type

8 Formation of a Covalent Bond Nature favors chemical bonding because most atoms are at a lower potential energy when bonded to other atoms than they are as independent particles

9 Formation of a Covalent Bond Bond length distance between two bonded atoms at their minimum potential energy

10 Formation of a Covalent Bond Bond length distance between two bonded atoms at their minimum potential energy Bond Bond Length (pm) H-H 74 Cl-Cl 199 Br-Br 228 I-I 267 C-C 154 C=C 134

11 Formation of a Covalent Bond Bond energy the energy required to break a chemical bond and form neutral isolated atoms

12 Formation of a Covalent Bond Bond energy the energy required to break a chemical bond and form neutral isolated atoms Bond Bond Length (pm) Bond Energy (kj/mol) H-H Cl-Cl Br-Br I-I C-C C=C

13 Covalent Bond Formation Molecule neutral group of atoms that are held together by covalent bonds CO 2 H 2 O C 6 H 12O 6 Molecular formula shows the number and types of atoms combined in a single molecule

14 Molecular Geometry The properties of molecules depend not only the bonding of the atoms but also on the molecular geometry The polarity of the bonds as well as the arrangement of the atoms in space determines the molecular polarity

15 Molecular Geometry VSEPR Theory Valence Shell Electron Pair Repulsion VSEPR Theory Repulsion between the sets of valence electrons around an atom causes these sets to be oriented as far apart from each other as possible

16 Molecular Geometry 180

17 Molecular Geometry Two areas of electron density Linear 180

18 Molecular Geometry Three areas of electron density Trigonal planar 120

19 Molecular Geometry

20 Molecular Geometry Four areas of electron density Tetrahedron 109.5

21 Molecular Geometry

22 Molecular Polarity Bonds Non-polar covalent bond two atoms involved share electrons equally Polar covalent bond electrons are shared unequally Molecules Non-polar molecules even distribution of electrons (electron density) Polar molecules uneven distribution of electrons (electron density)

23 Molecular Polarity Molecular polarity depends on both the polarity of the individual bonds and their orientation in threedimensional space. I 2 CO 2 HF H 2 O

24 Molecular Polarity Draw the Lewis structures for the following molecules. For each molecule, predict its geometry and polarity. 1. IBr 2. H 2 S 3. SiCl 4 4. PF 3

25 Monoatomic Ions Ion atom that has lost or gained electrons We must adjust the number of electrons to account for the charge Na is a metal and will lose its valence electron. Na Na (atom) (ion) Cl is a non-metal and will gain an electron to fill its valence shell. Cl Cl (atom) (ion)

26 Polyatomic Ions Polyatomic ion group of covalently bonded atoms that has collectively lost or gained electrons We must adjust the number of electrons to account for the charge NH 4 + SO 4 2-

27 Polyatomic Ions Draw the Lewis structure of the following ions OH - CO 3 2-

28 Ionic Compounds Composed of positive and negative ions Usually involves metals and non-metals Combine in a ratio where charges cancel out Li Li + and F F - so they combine in a 1:1 ratio

29 Ionic Compounds Draw Lewis structures of the following compounds. KF MgO CaI 2

30 Molecular vs Ionic The force that holds ionic compounds together is a very strong attraction between + and ions In a molecule, the forces that hold the atoms together within a molecule are very strong (covalent bonds) but the forces that hold one molecule to another are much weaker

31 Molecular vs Ionic Melting point, boiling point, and hardness of a compound depend on how strongly its particles are attracted to one another Decreasing forces of attraction

32 Molecular vs Ionic Melting point, boiling point, and hardness of a compound depend on how strongly its particles are attracted to one another Because the forces of attraction between individual molecules are not very strong, many molecular compounds melt at low temperatures In contrast, many ionic compounds are held together by strong forces (ionic bonds) and have higher melting and boiling points than molecular compounds

33 Forces of Attraction As a liquid is heated, the kinetic energy of its particles increases At some temperature, the energy is sufficient to overcome the forces of attraction between the molecules and the liquid boils

34 Forces of Attraction Boiling point is therefore a good measure of the strength of the force of attraction between the particles of a liquid The higher the boiling point, the stronger the forces

35 Forces of Attraction The forces of attraction between molecules are called intermolecular forces The strongest intermolecular forces exist between polar molecules Polar molecules act as tiny dipoles because of their uneven charge distribution Dipole-dipole attraction

36 Forces of Attraction Some hydrogen containing molecules have unusually high boiling points This is explained by the presence of a particularly strong IM force Hydrogen bonding IM force that results when a molecule contains a H atom bonded to a F, O, or N atom

37 Forces of Attraction Even noble gases and molecules that are non-polar experience forces of a attraction Electrons in molecules are in constant motion and create instantaneous dipoles These temporary differences in charge distribution cause forces of attraction known as London dispersion forces The more electrons a molecule has, the stronger its London dispersion interactions

38 Forces of Attraction The boiling points of alkanes (a type or organic compound) increases with increasing molar mass As the molecules get bigger, so does the size of their electron cloud This increases the London dispersion forces and thus more energy, or heat is required to pull them apart Boiling Point ( C) Methane -164 Ethane Propane Butane pentane 36.1

39 Forces of Attraction If we compare the alkane to its corresponding alcohol we can see another trend Boiling Point ( C) Methane -164 Methanol 64.7 Ethane Ethanol 78.3 Propane Propanol 97.2 Butane Butanol 117

40 Metallic Bonding Chemical bonding in metals is different from that in ionic or molecular compounds This difference is reflected in their unique properties Results from the attraction between metal ions and the surrounding sea of mobile valence electrons