1 Mr. ShieldsRegents Chemistry U10 L02

2 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 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 C H EN = 0.5 covalent

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

4 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 Non-polar covalent Electron cloud

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

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

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 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… Negative end Positive end

10 Polarity vs. Symmetry Negative end Positive end Water is polar due to 2 effects: 1) 1)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. Polar bonds Polar molecule

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

12 CO 2 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. :O = C = O: VECTORS CANCEL

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 CCl 4 a polar or non-polar Molecule (it has 4 C-Cl bonds)? CCl 4 is in fact a non-polar molecule. Let’s see why

14 CCl 4 has the shape of a Tetrahedron (Like many C-H Containing Compounds such as methane) 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. HCHHHHCHHHH Methane Cl- C+Cl- Cl- Carbontetrachloride (w/partial charges) CH 4 CCl 4

15Summary 1.Molecules containing only non-polar bonds are always Non-polar always Non-polar examples:O 2, N 2, I 2, CH 4 2. Molecules containing polar bonds with symmetrical charge distributions are Non-polar charge distributions are Non-polar examples: CO 2, CCl 4 3. Molecules containing polar bonds with asymmetrical charge distributions are Polar charge distributions are Polar examples: HF, H 2 O, NH 3, CH 3 Cl

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

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

19 2. Draw the Lewis dot structure for this compound 3. What kind of covalent bonds are present? Problem: 1. Ammonium Chloride is an ionic compound. Why? NH 4 Cl

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 - Different combinations or geometric structures of the same element or molecule are known as ALLOTROPES same element or molecule are known as ALLOTROPES - Examples of allotropes: - Diamond, Graphite, Coal, buckyballs - Diamond, Graphite, Coal, buckyballs - O 2, O 3  These are are allotopes but - O 2, O 3  These are are allotopes but not network solids - Allotropes have different physical and chemical properties. properties. Allotropes - Carbon can covalently bond with itself in several different geometric structures different geometric structures

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 Electrical Conductivity Conductivity

24 4 Other Examples of Network Solids SiC BN SiO 2 Si 3 N 4 Quartz MP 1650 C A Lubricant like Graphite MP 2967 C ExtremelyHard MP 2700 C MP 1900 C

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