1. CHEMISTRY January 7, 2015 CHEMICAL BONDS

2. SCIENCE STARTER Log onto www.coursesites.com 5 MINUTES

3. ANNOUNCEMENT LAB ON MONDAY FINAL EXAM ON JANUARY 16 AND 20 NO HOMEWORK ACCEPTED AFTER JANUARY 22, 2015

4. REVIEW ELECTRONEGATIVITY IS THE ATTRACTION FOR ELECTRONS ELECTRONEGATIVITY INCREASES ACROSS A PERIOD ELECTRONEGATIVITY DECREASES DOWN A GROUP ELECTRONEGATIVITY DOES NOT EXIST FOR NOBLE GASES

5. ELECTRONEGATIVITY DIFFERENCE Electronegativity differences determine the type of bonding that may occur.

6. COVALENT BONDING Usually, if the electronegativity between 2 atoms is less than 1.7 than the bond is a covalent bond. For example: In CO 2 : – C has an electronegativity value of 2.6 and O has an electronegativity of 3.4. – The electronegativity difference is 3.4 - 2.6 which is 0.8. – Since 0.8 is less than 1.7, the bond is a covalent bond.

7. IONIC BONDING Generally, if the electronegativity difference is greater than 2.0, then the bond is an ionic bond. For example: NaCl – Electronegativity value of Na = 0.9 – Electronegativity value of Cl = 3.2 – Electronegativity difference (high number - low number) = 3.2 - 0.9 = 2.3 – Since 2.3 is greater than 2.0, the bond is therefore an ionic bond

8. IONIC OR COVALENT If the electronegativity difference is between 1.7 and 2.0, then – if one of the atoms is classified as a metal, then an ionic bond exist – if both atoms are nonmetals, then a covalent bond exists.

9. POLARITY Definition - a separation of charge Occurs in covalent bonds

10. POLARITY DETERMINATION Polarity is determined based on the electronegativity of the atom. For example: HCl – Electronegativity value of H = 2.2 – Electronegativity value of Cl = 3.2 – Even though the pair of electrons are shared between H and Cl, the electrons are positioned closer to Cl than H because Cl has the higher electronegativity value.

11. NONPOLAR Nonpolar occurs when the electronegativity differences between the two atoms in a compound is less than 0.5. Example : F 2 - each F atom has an electronegativity value of 4.0. So, the electronegativity difference is 0. As a result, F 2 is nonpolar. Hence, F 2 has a nonpolar covalent bond CH 4 – Electronegativity value of C = 2.6 – Electronegativity value of H = 2.2 – Electronegativity difference = 2.6 - 2.2 = 0.4. – Thus, CH 4 is nonpolar and has a nonpolar covalent bond.

12. DIPOLE Definition of Dipole: difference in charge between the two atoms caused by a shift in electron density in the bond.

13. DIPOLE Bond polarity is measured by its dipole moment. In other words, the electron density around the higher electronegative atom results in giving the atom a partial negative charge (designated as d-). The less electronegative atom has some of its electron density taken away giving it a partial positive charge (d+). Example: HCl – Cl has the higher electronegativity value so Cl will have a greater electron density. As a result, Cl will have a partial negative charge while H will have a partial positive charge.

14. DIPOLE DIAGRAM Dipole moment for a polar covalent bond can be shown as in the example below. For HCl, Cl has higher electron density. Thus, more of the electrons are positioned closer to the Cl. To illustrate this, draw the dipole arro w as shown below.

15. REMEMBER The greater the electronegativity difference, the more polar the bond. Example: – HCl vs. HF – HF has a greater electronegativity difference. Thus, HF will be more polar.