Polar, Coordinate, & Network Covalent Bonds

1. Polar, Coordinate, & Network Covalent Bonds Mr. Shields Regents Chemistry U10 L02

2. C H EN = 0.5 Bond vs. Electronegativity Differences We know that if the EN difference between 2 atoms in a bond is less than 1.7 then the bond is covalent covalent The COVALENT range between 0 & 1.7 can also be Further Segmented: If the difference is ≤0.5 the bond is NON-POLAR If the difference is 0.6 – 1.7 the bond is said to be POLAR

3. 3.2 2.1 Ionic Least Ionic - Most covalent Most Ionic – Least covalent Molecules with differences 0.5 or less are said to be Non-polar Covalent 0.5 Polar Covalent Region

4. Electron cloud Non-polar covalent A hydrogen molecule forms a covalent bond in Which the electrons are equally shared by both atoms This is the basis of a non-polar bond

5. Polar Bond Distorted Electron cloud When one atom in a bond is much more electronegative Than the other it pulls electron density towards it

6. Br 2.1 - 3.0 = 0.9 A Polar Covalent Compound The atom that pulls electron density towards it more strongly becomes partially negative while the other atom becomes partially positive

7. Polarity of Water The electronegativity of O = 3.5 The electronegativity of H = 2.1 The difference is 1.4; the O-H bond is Polar covalent

8. Water Water is polar because electron density is pulled more Strongly towards the Oxygen atom than the Hydrogen atom. This makes Oxygen Partially negative And the less Electronegative H Partially positive so… There are 2 POLAR Covalent Bonds formed

9. Negative end Positive end Bond Polarity vs. Molecular polarity If two atoms form a polar bond you might assume the molecule is polar. However this is not always the case! Let’s see why… Recall that a polar bond Must have 2 ends with Opposite charge. Let’s look at the water Molecule first…

10. Negative end Positive end Polarity vs. Symmetry Polar bonds Polar molecule • Water is polar due to 2 effects: • It has polar covalent O-H bonds • 2) The molecule has a • BENT geometric shape. • The bent shape gives it an • Asymmetry that allows • two opposite polar ends • To develop.

11. Carbon Dioxide Now let’s look at Carbon Dioxide. The 2 C = O bonds in CO2 are in fact polar covalent, Just like water (what’s the electronegativity difference?) Because the electronegativity difference is >0.5 we expect the CO2 molecule to be polar … But it isn’t. How Come ????

12. VECTORS CANCEL . . . . :O = C = O: - + - CO2 is actually a linear molecule. As a linear molecule it Has a symmetry through Carbon. That means that the ends Of the molecule are BOTH Negative and there is NO Defined Positive END It’s this symmetry that makes The molecule NON-POLAR Instead of POLAR.

13. Other examples of symmetry related non-Polar molecules What is the electronegativity difference between C and Cl? Would it form a polar or non-polar Bond? Based on this is CCl4a polar or non-polar Molecule (it has 4 C-Cl bonds)? CCl4 is in fact a non-polar molecule. Let’s see why

14. Carbontetrachloride (w/partial charges) Methane H C H H H Cl- C+ Cl- Cl- Cl- CCl4 has the shape of a Tetrahedron (Like many C-H Containing Compounds such as methane) CCl4 CH4 Notice the symmetry of these molecules. When each Is rotated is looks the same from any angle. Due to this symmetry there is no negative end, no Positive end and the molecule is NON-POLAR.

15. Summary • Molecules containing only non-polar bonds are • always Non-polar • examples: O2, N2, I2, CH4 • 2. Molecules containing polar bonds with symmetrical • charge distributions are Non-polar • examples: CO2, CCl4 • 3. Molecules containing polar bonds with asymmetrical • charge distributions are Polar • examples: HF, H2O, NH3, CH3Cl

16. Coordinate Covalent Bonds Recall… A covalent bond is formed between two atoms, EACH donating 1 electron to form a shared pair. When one of the two atoms donates BOTH electrons To form the shared pair the bond is called COORDINATE COVALENT. This type of bond is found in many POLYATOMIC IONS (but not exclusively)

17. + H O: H H Coordinate Covalent Bonds Let’s look at some example of molecules with coordinate Covalent bonds and how they’re formed? Nitrogen contributes electron pair Oxygen contributes electron pair Carbon Monoxide Ammonium ion Oxygen contributes electron pair Hydronium ion

18. Formation of a Coordinate Covalent bond Std Polar covalent bonds In the water molecule A polyatomic: The hydronium ion Electron pair Donated solely By Oxygen The added Proton to the Water molecule leads to The positive charge H20 + H+ H3O+

19. Problem: 1. Ammonium Chloride is an ionic compound. Why? NH4Cl 2. Draw the Lewis dot structure for this compound 3. What kind of covalent bonds are present?

20. Network Solids Some atoms are able to form multiple repeating covalent bonds with themselves. - for example: carbon Certain other atoms can also form multiple repeating covalent bonds with another atom - for example: Si and O or B and N, The result is a three dimensional structure known as a NETWORK SOLID Let’s look at some examples.

21. Diamond is an example of a network solid A highly interconnected carbon Covalent Structure consisting of Hexagonal rings 1 carbon Bonded to 4 others

22. Allotropes - Carbon can covalently bond with itself in several different geometric structures • Different combinations or geometric structures of the • same element or molecule are known as ALLOTROPES • - Examples of allotropes: • - Diamond, Graphite, Coal, buckyballs • - O2, O3  These are are allotopes but • not network solids • - Allotropes have different physical and chemical • properties.

23. Graphite is an allotrope of diamond and like diamond it consists of connected hexangonal rings. It is also a network solid Unlike diamond however, these rings form layers that are not connected and can slide across one another. It’s this structure that gives graphite its lubricating quality & Electrical Conductivity

24. 4 Other Examples of Network Solids SiO2 BN Quartz MP 1650 C A Lubricant like Graphite MP 2967 C Si3N4 MP 1900 C SiC Extremely Hard MP 2700 C

25. Network solids • PROPERTIES: • Extraordinarily high melting points • Very hard • Not soluble in polar or non-polar liquids • Non conductors of electricity in either solid or liquid state • Notable Exception: Graphite does conduct electricity!