1. Biology I Covalent Bonds
3.00_Chemistry of Biology
Biology I Covalent Bonds
Slides

2. Covalent Bonding

3. Covalent Bonding
A covalent bond results when electrons are shared by nuclei

4. Formation of a Covalent Bond
Nature favors COVALENT bonding because most atoms are at lower potential energy when bonded to other atoms that they are at as independent particles.
The electron-proton attraction is stronger than the electron-electron and proton-proton repulsions,
Therefore the atoms are drawn to each other and their potential energy is lowered.

5. Covalent Bonding In covalent bonds atoms share electrons.
There are several electrostatic interactions in these bonds:
Attractions between electrons and nuclei
Repulsions between electrons
Repulsions between nuclei

6. Covalent Bonding
The electrons in each atom are attracted to the nucleus of the other.
The electrons repel each other.
The nuclei repel each other.
They reach a distance with the lowest possible energy.
The distance between is the bond length.

7. H2 Bond Formation
When hydrogen atoms are brought close together, there are two unfavorable potential energy terms:
Proton-proton repulsion
Electron-electron repulsion
One favorable term:
Proton-electron attraction

8. H2 Bond Formation
Under what conditions will the hydrogen molecule by favored over the separated atoms.
Whichever orientation gives the lowest possible energy.
The system will act to minimize the sum of the positive (repulsive) energy terms and the negative (attractive) energy terms.
The distance where the energy is minimum is called the bond length.

10. Characteristics of the Covalent Bond
Bond Length - the distance between two bonded atoms at their minimum potential energy
Average distance between 2 bonded atoms.
Bond Energy - the energy required to break a chemical bond and form neutral isolated atoms.
Measured in Kilojoules/mole (KJ/mol)
Bond lengths and bond energies vary with the types of atoms that have combined

11. Electrons & Bonds Single bond one pair of electrons is shared.
Double bond two pair of electrons are shared.
Triple bond three pair of electrons are shared.
More bonds, shorter bond length.

12. Covalent Bond Strength
Most simply, the strength of a bond is measured by determining how much energy is required to break the bond.
This is the bond enthalpy.
The bond enthalpy for a Cl-Cl bond, D(Cl-Cl), is measured to be 242 kJ/mol.

13. Average Bond Energy
This table lists the average bond enthalpies for many different types of bonds.
Average bond enthalpies are positive, because bond breaking is an endothermic process.

14. Average Bond Energy
NOTE: These are average bond enthalpies, not absolute bond enthalpies; the C-H bonds in methane, CH4, will be a bit different than the C-H bond in chloroform, CHCl3.

15. Non-Polar Covalent Bonding
A non-polar covalent bond results when electrons are not shared unequally by nuclei 15

16. Why Non-Polar Covalent Bonds Are Not Common
Electronegativity of an atom determines how it will share the electrons in a covalent bond. If an atom has a high electronegativity and it is bonded to an atom with a low one, the shared electrons will stay closer to the one with higher electronegativity. This molecule will be polar.
If two atoms are bonded with similar electronegativities, the electrons will be shared evenly and it will be nonpolar. Nonpolar bonds are uncommon because there are many atoms with different electonegativities, so the chance that they are very close is low.
In order for a bond to be completely nonpolar, the electronegativities of the two atoms must be equal. Otherwise, there is unequal sharing of the electrons. 16

17. Polar Covalent Bonding
Sometimes the electrons in a covalent bond aren’t shared equally, because one atom attracts electrons more strongly than the other. When this happens, the electrons spend more time with one atom, and that atom becomes slightly negatively charged. The other atom becomes slightly positively charged. This is a polar covalent bond, because the atoms form positive and negative poles.
Water is a polar compound, because the oxygen is slightly negative and the hydrogens slightly positive.
Oxygen attracts electrons more than hydrogen
Note that the total charge on the molecule is balanced, same number of electrons as protons, but within the molecule the charges are slightly separated.
(Bonds where the electrons are shared equally are called non-polar.)
Polar molecules attract each other: the opposite charges attract. 17

18. when electrons are shared but shared unequally
POLAR COVALENT BONDS
when electrons are shared but shared unequally
H2O

19. Polar Covalent Bonds: Unevenly matched, but willing to share.

20. Covalent Bond
Or a polar covalent bond when it is formed between two different atoms of different electronegativity, thus one atom has a stronger pull on the pair of electrons resulting in a shift of electron density toward the more electronegative atom.
Such a covalent bond is polar, and will have a dipole (one end is positive and the other end negative). The degree of polarity and the magnitude of the bond dipole will be proportional to the difference in electronegativity of the bonded atoms.

21. Covalent Bond
The dipolar nature of these bonds if often indicated by a partial charge notation (δ+/-) or by an arrow pointing to the negative end of the bond.
Although there is a small electronegativity difference between carbon and hydrogen, the C-H bond is regarded as weakly polar at best and hydrocarbons in general are considered the be non-polar compounds.

22. - water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

23. Examples of Covalent Bonding

24. Examples of Covalent Bonding

25. Nonpolar/Polar Covalent Bonds

26. Polar and Nonpolar Covalent Bond - Clear & Simple