Chemical bonding Why do atoms form bonds? Reason 1: Lower energy state Energy is always released during bond formation Bond energy: The energy required to break a bond The higher the bond energy, the more stable the bond Reason 2: Atoms are most stable when they have a complete valence shell Noble gases don’t form bonds because they already have a complete valence shell

Octet rule: During bond formation, atoms tend to achieve a complete valence shell (8 e- ) Octet rule can be satisfied in two ways: Transfer of electrons from one atom to another (ionic bonding) Sharing of electrons by two atoms (covalent bonding)

Intramolecular bonds: Bonds between atoms in a compound, ion, or molecule Three major types of intramolecular bonds: Ionic bond: Transfer of e- from one atom to another, leading to formation of + and – ions Result in formation of an ionic compound (salt) Involve a metal cation and a nonmetal anion e- transferred from valence shell of metal to valence shell of nonmetal Metal and nonmetal must have an electronegativity difference (END) of 1.7 or greater continued 

Ionic Bonding: Click For Animation

(+) cation and (–) anion strongly attracted to each other Ionic bonds very strong (high MP and BP) Ionic compounds usually water soluble Conduct electricity in aqueous or molten form Ionic compounds form ionic crystals; they do not form molecules

2) Covalent bond: Atoms share one or more pair of e- to complete octet e- are strongly attracted to the nuclei of each atom in the bond Involve atoms of two nonmetals END < 1.7 Weaker bond than ionic (lower MP and BP) Can involve sharing of one, two, or 3 pairs of e- (single, double, or triple bond) Covalent bond results in the formation of a molecule Resulting compound called covalent or molecular compound

Polar covalent bond: A pair of e- is shared unevenly between two different nonmetals Element with higher EN attracts e- more strongly Small partial charge denoted by + or -

B) Non-polar covalent bond: A pair of e- is shared equally by two of the same nonmetal END is usually zero, but can be 0.4 or less C) Coordinate covalent bond: Identical to a polar covalent bond, but both e- are donated by one of the atoms in the bond Polyatomic ions frequently involve coordinate covalent bonds Look for an unshared pair of e-

Ionic vs. Covalent Bonding: Click For Animation

Molecular shapes Valence shell electron pair repulsion (VSEPR): Electron pairs repel each other to get as far away from each other as possible

Different molecular shapes: Linear Any molecule with only two atoms must be linear CO2

2) Bent: Unshared e- pairs force the atoms out of a straight line Requires at least three atoms Note: even when drawn like this, one needs to know that the unshared e- pairs cause bend

3) Pyrimidal: Unshared e- pair forces atoms into a tented shape Requires at least four atoms

4) Tetrahedral: Requires at least five atoms When four atoms are all bonded to one central atom, the shape they assume is called a tetrahedron

Molecular Shapes: Click For Animation

Predicting the polarity of covalent molecules: Bond polarity vs. molecular polarity: Can have a non-polar molecule with polar bonds or a polar molecule with non-polar bonds Molecular polarity is determined by: Shape Charge symmetry

SNAP How to determine a molecule’s polarity: *DOES NOT PERTAIN TO IONIC COMPOUNDS Step 1) Draw Lewis dot diagram Step 2) Use VSEPR to determine shape Step 3) Is charge distributed symmetrically in all directions? If so, molecule is non-polar (doesn’t have + and - ends). If charge is distributed asymmetrically, the molecule is polar In a two-atom molecule, if the bond is polar then the molecule is polar SNAP ymmetrical onpolar symmetrical olar Click For Animation

Significance of polar vs. non-polar molecules: Will dissolve ionic and other polar substances Have relatively high MP and BP Non-polar molecules: Will dissolve other non-polar substances (like dissolves like) Have relatively low MP and BP

Polar vs. Nonpolar Molecules: Click For Animation

D) Network covalent bond Atoms covalently bound to each other in a giant repeating pattern Form huge molecules Strongest of all bonds Form very hard substances

3) Metallic bond: Positive metal ions in a “sea of mobile (delocalized) electrons Since metals have low EN, valence e- are loosely held and free to move from one atom to another Sea of mobile e- holds all the atoms together tightly, accounting for metal’s higher MP and BP, as well as malleability Also accounts for metal’s luster and conductivity

Metallic Bonding: Click For Animation

 1.7 < 1.7 NA High High Low Yes (usually) No Yes No No (usually) TYPE OF BOND IONIC COVALENT METALLIC Type of Elements Involved Electronegativity Difference Role of Electrons in the Bond Melting Point / Boiling Point Polar Non-Polar Solubility in H2O? Conductivity of Electricity General Nature of the Substance Metal and nonmetal (or polyatomics) Nonmetals only Metals only  1.7 < 1.7 NA Shared pair between atoms Shared among all the atoms Transferred form (+) to (-) ion High Very low High Low Yes (usually) No Yes No In aqueous or molten form only No (usually) Yes Hard crystal Gas, liquid, or soft solid Hard solid

Intermolecular forces of attraction: Attractive forces between two molecules of the same substance or between two different substances Dipole-dipole attraction: A polar molecule, because it has + and - poles, is called a dipole. The + end of one molecule is attracted to the - end of another molecule, like opposite poles of a magnet. continued Click For Animation

Only involves polar covalent molecules Due to dipole-dipole attraction, polar compounds have higher MP and BP than do non-polar compounds “Like dissolves like.” Polar substances will dissolve in other polar substances, not in non- polar ones

Hydrogen bond: a special type of dipole- dipole attraction H of one molecule is strongly attracted to the F, O, or N of another molecule Strongest intermolecular force When H is bonded to a very small atom with high EN (F, O, or N), one gets a very polar molecule Molecules with H-bonding have higher BP and MP than other polar substances continued

Hydrogen atom is essentially bound to two atoms at the same time Responsible for surface tension, meniscus of H2O Ex: H2O, NH3, HF Holds together the double-helix of DNA

Hydrogen Bonding: Click For Animation

Molecule-ion attraction: The charged ends of a polar molecule are attracted to the charged ions of a salt The reason most ionic solids dissolve in H2O Ions become surrounded by H2O (hydrated) and thus dissolve in H2O

Molecule-Ion Attraction: Click For Animation

Van der Waals forces: Even in non-polar molecules, random fluctuations in e- distribution can make them temporarily slightly polar The weakest of all intermolecular forces Reason for low MP and BP of nonpolar substances The more e- a molecule has, the greater the attraction Ex: CH4 vs. C3H8