Chemical Bonds

CHEMICAL BONDING Cocaine To play the movies and simulations included, view the presentation in Slide Show Mode.

Electron Shells Atomic number = number of Protons in the Nucleus Which also happens to be the number of Electrons Electrons vary in the amount of energy they possess and they occur at certain energy levels or electron shells. Electron shells determine how an atom behaves when it encounters other atoms

Electrons are placed in shells according to rules The 1st shell (orbital) can hold up to 2 electrons and each shell thereafter can hold 8 electrons

Octet Rule = atoms tend to gain, loose or share electrons as to have 8 electrons C would like to Gain 4 electrons N would like to Gain 3 electrons O would like to Gain 2 electrons

Electron Dot Structures Symbols of atoms with dots to represent the valence-shell electrons 1 2 13 14 15 16 17 18 H He:            Li Be  B   C   N   O  : F  :Ne :                    Na Mg  Al  Si  P S :Cl  :Ar :        

Learning Check A. X would be the electron dot formula for  A. X would be the electron dot formula for 1) Na 2) K 3) Al   B.  X  would be the electron dot formula 1) B 2) N 3) P

Forms of Chemical Bonds There are 2 extreme forms of connecting or bonding atoms: Ionic—complete transfer of 1 or more electrons from one atom to another Covalent—some valence electrons shared between atoms Most bonds are somewhere in between.

Formation of Ions from Metals Ionic compounds result when metals react with nonmetals Metals lose electrons to match the number of valence electrons of their nearest noble gas Positive ions form when the number of electrons are less than the number of protons Group 1 metals  ion 1+ Group 2 metals  ion 2+ Group 13 metals  ion 3+

Formation of Sodium Ion Sodium atom Sodium ion Na  – e  Na + 2-8-1 2-8 ( = Ne) 11 p+ 11 p+ 11 e- 10 e- 0 1+

Formation of Magnesium Ion Magnesium atom Magnesium ion  Mg  – 2e  Mg2+ 2-8-2 2-8 (=Ne) 12 p+ 12 p+ 12 e- 10 e- 0 2+

Some Typical Ions with Positive Charges (Cations) Group 1 Group 2 Group 13 H+ Mg2+ Al3+ Li+ Ca2+ Na+ Sr2+ K+ Ba2+

Learning Check A. Number of valence electrons in aluminum 1) 1 e- 2) 2 e- 3) 3 e- B. Change in electrons for octet 1) lose 3e- 2) gain 3 e- 3) gain 5 e- C. Ionic charge of aluminum 1) 3- 2) 5- 3) 3+

Solution A. Number of valence electrons in aluminum 3) 3 e- B. Change in electrons for octet 1) lose 3e- C. Ionic charge of aluminum 3) 3+

Learning Check Give the ionic charge for each of the following: A. 12 p+ and 10 e- 1) 0 2) 2+ 3) 2- B. 50p+ and 46 e- 1) 2+ 2) 4+ 3) 4- C. 15 p+ and 18e- 2) 3+ 2) 3- 3) 5-

Ions from Nonmetal Ions In ionic compounds, nonmetals in 15, 16, and 17 gain electrons from metals Nonmetal add electrons to achieve the octet arrangement Nonmetal ionic charge: 3-, 2-, or 1-

Fluoride Ion unpaired electron octet 9 p+ 9 p+ 9 e- 10 e- 0 1 -     1 - : F  + e : F :     9 p+ 9 p+ 9 e- 10 e- 0 1 - ionic charge

Complete electron transfer from an element (metal) to an element of high affinity for electrons (nonmetal) 2 Na(s) + Cl2(g) ---> 2 Na+ + 2 Cl- Ionic compounds exist primarily between metals at left of periodic table and nonmetals at right Ionic Bonds

Ionic Bonds Between atoms of metals and nonmetals with very different electronegativity Bond formed by transfer of electrons Produce charged ions. Conductors and have high melting point. Examples; NaCl, CaCl2, K2O

Ionic Bonds: One Big Greedy Thief Dog!

1). Ionic bond – electron from Na is transferred to Cl, this causes a charge imbalance in each atom. The Na becomes (Na+) and the Cl becomes (Cl-), charged particles or ions.

Covalent Bonding The bond arises from the mutual attraction of 2 nuclei for the same electrons. Electron sharing results. (Screen 9.5) Bond is a balance of attractive and repulsive forces.

Covalent Bond Between nonmetallic elements of similar electronegativity. Formed by sharing electron pairs Stable non-ionizing particles, they are not conductors at any state Examples; O2, CO2, C2H6, H2O

2. Covalent bonds - Two atoms share one or more pairs of outer-shell electrons. Oxygen Atom Oxygen Atom Oxygen Molecule (O2)

Electron Distribution in Molecules Electron distribution is depicted with Lewis electron dot structures Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. G. N. Lewis 1875 - 1946

Bond and Lone Pairs Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. • •• H Cl shared or bond pair lone pair (LP) This is called a LEWIS ELECTRON DOT structure.

Note that each atom has a single, unpaired electron. Bond Formation A bond can result from a “head-to-head” overlap of atomic orbitals on neighboring atoms. Cl H •• • + Note that each atom has a single, unpaired electron.

Valence Electrons Electrons are divided between core and valence electrons

Rules of the Game # of valence electrons of a main group atom = Group number

Building a Dot Structure Ammonia, NH3 1. Decide on the central atom (the atom with lowest electron affinity); never H. Hydrogen atoms are always terminal. Therefore, N is central - Generally Symmetrical 2. Count valence electrons H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons; 4 pairs

Building a Dot Structure 3. Form a single bond between the central atom and each surrounding atom H N 4. Remaining electrons form LONE PAIRS to complete octet as needed. H •• N 3 BOND PAIRS and 1 LONE PAIR. Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair.

Sulfite ion, SO32- Step 1. Central atom = S Step 2. Count valence electrons S = 6 3 x O = 3 x 6 = 18 Negative charge = 2 TOTAL = 26 e- or 13 pairs Step 3. Form bonds 10 pairs of electrons are now left.

Sulfite ion, SO32- Remaining pairs become lone pairs, first on outside atoms and then on central atom. • •• O S •• Each atom is surrounded by an octet of electrons.

Carbon Dioxide, CO2 1. Central atom = _______ 2. Valence electrons = __ or __ pairs 3. Form bonds. This leaves 6 pairs. 4. Place lone pairs on outer atoms.

Carbon Dioxide, CO2 4. Place lone pairs on outer atoms. 5. So that C has an octet, we shall form DOUBLE BONDS between C and O.

Double and even triple bonds are commonly observed for C, N, P, O, and S H2CO SO3 C2F4

Urea, (NH2)2CO

Urea, (NH2)2CO 1. Number of valence electrons = 24 e- 2. Draw single bonds.

Urea, (NH2)2CO 3. Place remaining electron pairs in the molecule.

Urea, (NH2)2CO 4. Complete C atom octet with double bond.

# of bonds between a pair of atoms Bond Order # of bonds between a pair of atoms Double bond Single bond Acrylonitrile Triple bond

Polar Covalent Bonds: Unevenly matched, but willing to share.

- water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

Bond Polarity HCl is POLAR because it has a positive end and a negative end. Cl has a greater share in bonding electrons than does H. Cl has slight negative charge (-d) and H has slight positive charge (+ d)

Electronegativity,   is a measure of the ability of an atom in a molecule to attract electrons to itself. Concept proposed by Linus Pauling 1901-1994

Linus Pauling, 1901-1994 The only person to receive two unshared Nobel prizes (for Peace and Chemistry). Chemistry areas: bonding, electronegativity, protein structure

Electronegativity Figure 9.9

Molecular Polarity All above are NOT polar Molecules will be polar if a) bonds are polar AND b) the molecule is NOT “symmetric” All above are NOT polar

Consequences of H2O Polarity Microwave oven Consequences of H2O Polarity

B—F bonds in BF3 are polar. But molecule is symmetrical and NOT polar Molecular Polarity, BF3 B atom is positive and F atoms are negative. B—F bonds in BF3 are polar. But molecule is symmetrical and NOT polar

Molecular Polarity, HBF2 B atom is positive but H & F atoms are negative. B—F and B—H bonds in HBF2 are polar. But molecule is NOT symmetrical and is polar.

Is Methane, CH4, Polar? Methane is symmetrical and is NOT polar.

Is CH3F Polar? C—F bond is very polar. Molecule is not symmetrical and so is polar.

Substituted Ethylene C—F bonds are MUCH more polar than C—H bonds. Because both C—F bonds are on same side of molecule, molecule is POLAR.

Substituted Ethylene C—F bonds are MUCH more polar than C—H bonds. Because both C—F bonds are on opposing ends of molecule, molecule is NOT POLAR.

Metallic Bond - Bonds found in metals, which hold metal atoms together very strongly.

Metallic Bond Formed between atoms of metallic elements Electron cloud around atoms Good conductors at all states, lustrous, very high melting points Examples; Na, Fe, Al, Au, Co

Metallic Bonds: Mellow dogs with plenty of bones to go around.

Ionic Bond, A Sea of Electrons

Metals Form Alloys Metals do not combine with metals. They form Alloys which is a solution of a metal in a metal. Examples are steel, brass, bronze and pewter.