1 Mr. Shields Regents Chemistry U10 L01

2 Ionic Bonds involve a force of attraction that develops through… ELECTRON TRANSFER In covalent bond formation the bond is formed by ELECTRON SHARING Both lead to the formation of the octet. However, The force of attraction in Covalent compounds involves each atom donating an electron to form a shared pair. What is a covalent bond?

3 Covalent Bonds Many of the compounds we encounter in our daily Lives are covalently bonded compounds - Antibiotics - Food - Plants - Plastics - Clothing (Natural & Synthetic) - Biochemicals - What we breath and what we are - And so many other things its impossible to list them all! ampicillin

4 Molecular Bonds Covalent Bonds fall into several sub-categories: Non-polar Covalent Polar Covalent Coordinate Covalent Network Covalent We’ll start our discussion with non-polar covalent bonds

5 2 hydrogen Atoms Approach each other 1s orbitals overlap Each atom shares its Electron with the Other atom but… Each atom thinks the shared electrons are theirs alone! Formation of a non-polar Covalent Bond  EN < 0.5)

6 This sharing of electrons creates a new bond Between the atoms. Electron density is concentrated in the region between The atoms. When 2 atoms approach the pt. of maximum attraction a SINGLE COVALENT BOND forms Covalent Bond

7 If electron density is equally shared across both atoms One side of the molecule has no more electron density than the other side so no “poles “ develop Non-polar This type of bond is called… Non-polar covalent

8 Electronegativity differences Recall that bonds formed by atoms with electronegativity differences are greater than 1.7 are said to be IONIC 2 atoms with an electronegativity difference that is less than 1.7 will form a COVALENT bond When 2 atoms have an EN difference that is really low, Less than or equal to 0.5 these bonds are said to be… Non-Polar 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

10 Electronegativity differences Between 0.5 and 1.7 there is a large number of Compounds having bonds known as Polar Covalent For example H 2 O  = 1.3 We’ll talk more about Polar compounds later.

11 Multiple atom electron sharing In water, each hydrogen shares 1 electron with oxygen Since oxygen initially has 6 electrons it has 2 pairs Of electrons that remain unshared Unshared electron pairsAll atoms now have an octet

12 Unshared electron pairs are called LONE PAIRS Shared electron pairs are called SHARED PAIRS or BONDING PAIRS

13 Nitrogen has a non-bonding pair of electrons But Methane has no non-bonding pairs

14 Depicting bonds The most common way to depict bonds is not by Lewis dot structures. Instead, a line is drawn for each pair of shared electrons : =

15 HCHHHHCHHHH CH 4 METHANE Carbons S and P orbitals Overlap with Hydrogen’s S orbital to form a Hybrid orbital known as a Sigma bond. Its shape is Unique – the tetrahedron 4 sigma bonds (4 single covalent bonds) How many bonding pairs and Lone pairs are there in CH 4

16 ** PCl Cl Lone pairs are not Connected to the Parent ion with a line. The electrons are indicated using 2 “dots” Phosphorus’ lone pair How would you draw the Water molecule? Lewis dot structure

17 Multiple bonds So far we have looked at atoms sharing one pair of Electrons to form a single covalent bond It is however possible for atoms to share more than a single pair of electrons Oxygen (O 2 ), for example, does this. Let’s see how. Oxygen: 6e -

18 Multiple bonds Oxygen’s oxidation number is This means it has electrons in it’s valence shell To achieve the octet it needs. 1.A pair of Electrons from oxygen #1are shared 2. Oxygen #2 then Shares one of its Pair of electrons to Complete the octet #1 # So how does it do this?

19 The sharing of 2 pairs of electrons is known as a DOUBLE BOND. They can be formed by Molecules having other than just two of the same atoms (like O 2 ) Formaldehyde is an example Propene is another. Do the atoms in these molecules have an Octet? Some compounds may even have several double bonds. :O = C = O:.. H C = O: H C = C H Carbon Dioxide

20 Triple Bonds Nitrogen is an example of a compound that is formed By sharing three pairs of electrons. This is called a TRIPLE BOND How many Electrons does Each atom have?

21 Triple bonds Another example of a Triple Bond is acetylene H - C : : : C - H Electrons from carbon 1 Electrons From carbon 2 H - C C - H Acetylene Oxy-acetylene torch

22 Bond energies The energy necessary to break A bond in a given Compound varies Different kinds of Bonds take different Energies to break them apart. It would take 436 KJ/mol to break A hydrogen bond Bond length Bond Strength

23 Bond Dissociation energies The energy necessary to break a bond is the BOND DISSOCIATION ENERGY The higher the dissociation energy the stronger the Covalent bond (and greater stability) 813 KJ/mol Triple C C 591 KJ/mol Double C = C 331 KJ/mol Single C - C Dissociation Energy Bond type Bonding atoms Covalent Bond strength: Triple > Double > Single

24 Covalent vs. Ionic Properties PROPERTYIONIC COMPOUNDCOVALENT COMPOUND Simplest Component Formula Unit Anion & Cation Single Molecule StructureCrystalline Amorphous solids (not Crystalline) can also be gas/liquid HardnessHard Solids relatively soft Workability Shatters if worked Solids easily break apart Electrical Conductivity Good Conductor ONLY if molten or in solution Non-conductors even if molten Melting Point High (NaCl – 801 deg C) Low Boiling Point High (NaCl – 1413 deg C) Low Ease of Phase Change Difficult – Low VP / High MP & BP’s / High H f Easy – High vapor pressure Solubility in Water Soluble – Covers range from very soluble to very slightly soluble NaCl = 37.5g/100g water Non-polar: insoluable Polar: Soluble to slightly soluble