Bonding: General Concepts Part 2 Dr. Deborah Walker

Announcements From building atoms to building molecules! By the end of today, you should be able to: – Identify a bond as covalent, polar covalent, or ionic. – Explain bonding (what, when, why, how). – Predict which bond is more strongly polar (has strongest dipole). – Identify polar bonds and identify the partial charges. – Determine which ionic compound has the stronger lattice energy. MON TUES Deb 12-1:30pm WEL WED Deb 12-1:30pm WEL THURS Deb 12-1:30pm WEL FRI Seyma 11-12noon

Review Ionic bonds involve the exchange of electrons from one atom to another, or Ionic bonds involve a cation bonding to an anion. Covalent bonds involve the sharing of electrons between two atoms. In all cases, bonds form because they form a lower energy state.

Knowledge Check Which of the following involves a covalent bond? – A. Cl 2 – B. BeF 2 – C. Fe 2 O 3 – D. None of these.

Puzzler Which of the following bonds exhibits the most covalent character? – A. BF 3 – B. CH 4 – C. Al 2 O 3 – D. HCl – E. I don’t know.

The Catch… Continuum So, electronegativity is key to identifying bond type. Nonpolar covalent ΔEN=0 Polar covalent Ionic ΔEN=1.5

Electronegativity What is electronegativity?

Electronegativity

Knowledge Check For the H-F bond, which element has the greater electronegativity? – A. H – B. F – C. They have the same electronegativity because H could be in column 1A or in column 7A. – D. I have no clue.

Possibilities 1. X and Y have identical electronegativities. (ex. X and Y are the same element). Electrons are equally shared. This is a nonpolar covalent bond. 2. X and Y have dissimilar electronegativities. (ex. Y is more electronegative) Y has greater control of the electrons. The bond is polar covalent. 3. X and Y have very different electronegativities. (ex. Y is much more electronegative) Y has full control of the electrons. Y is negative; X is positive The bond is ionic. X Y ++ -- X + Y - (Note: Metallic bond not included in above discussion as it is not on the continuum.)

Continuum Bond Types X Y ++ -- X + Y -

The Catch… Continuum So, electronegativity is key to identifying bond type. Ex. H 2 Ex. F 2 Ex. HF Ex. NaF Nonpolar covalent ΔEN=0 Polar covalent Ionic ΔEN=1.5

Example H-F Electrons in bond not equally shared. How do we know? – Test: magnetic field OffOn

Generic Example Consider two atoms, X and Y, bound together by two electrons. What are the possible bond types? – (Note: we’re ignoring metallic bonds for now)

Practice (see packet) Identify the dEN for the following bonds and classify each as having mostly covalent, mostly polar covalent, or mostly ionic character. Identify partial charges when applicable. – H-H – Cl-Cl – H-Cl – C-H

Your Turn (see packet)

Now more about ionic bonds… How do we tell which ionic bond is stronger? – (Note: A stronger bond indicates a more stable bond)

Ionic Compounds Stabilize by forming crystalline solids Alternating cations and anions in a repeating pattern

Lattice Energy Lattice energy is the change in energy that occurs when the separated gaseous ions are packed together to form an ionic solid. Question: Which is more stable, M + & X - or MX? We can determine lattice energy for an ionic compound via a series of steps.

Determining Lattice Energy Step 1: Metal to gas Step 2: Ionize metal Step 3: Break diatomic gas Step 4: Ionize Step 5: Combine ions

Lattice Energy

Lattice Energy = k Q 1 Q 2 r r = average distance between cations and anions k = a constant for this crystal structure Q 1, Q 2 = charges on the ions Lattice Energy

Figure 13.10: NOTE: Making F - releases energy but making O 2- requires energy! The driving force for making MgO is the bigger release of energy (big negative LE value)

Knowledge Check Lattice Energy will be large for: A. small r (close spacing - permitted by small ions) B. large r (large spacing required by larger ions)

Knowledge Check Lattice Energy will be large for: A. large Q (highly charged ions (+2,-3 for example) B. small Q (low charged ions (+1, -1 for example)

Knowledge Check Which would have the larger lattice energy? A. NaCl B. KCl

Knowledge Check Which would have the larger lattice energy? A. Calcium phosphate B. Calcium chloride

Summary Lattice Energy The difference in energy between ions packed together in a solid and ions widely separated in a gas. High lattice energy indicates the ions are held together tightly in the solid. Lattice energy is large when charge is large and distance between ions is small. High lattice energy leads to high melt point for ionic compounds, ex. LiF & Ca 3 (PO 4 ) 2.

Summary So Far Multiple bond types exist. Bonds form to lower energy for the atoms involved. Bond type is estimated by calculating the difference in electronegativity. Partial charges are assigned to atoms in polar covalent bonds. Full charges are assigned to ions in ionic bonds.

(below if time or for next time)

Knowledge Check 1. Which of the following bonds would you expect to be classified as polar covalent? KFCOO 2 A. KF, with the F end having a partial negative charge. B. KF, with the K end having a partial negative charge. C. CO, with the O end having a partial negative charge. D. CO, with the C end having a partial negative charge. E. O 2, with neither end having a partial negative charge.

Knowledge Check Which bond would you expect to be MOST polar? (i.e., Which would have the largest partial positive and partial negative charges?) O-FN-FC-F A. O-F B. N-F C. C-F

Ionic Character