Trends in the Periodic Table and Bonding Polar Covalent Bonds

Covalent Bonding A covalent bond is a shared pair of electrons electrostatically attracted to the positive nuclei of two atoms. Both nuclei try to pull the electrons towards themselves - + - + This is like a tug-of-war where both sides are pulling on the same object. It creates a strong bond between the two atoms. The atoms achieve a stable outer electron arrangement (a noble gas arrangement) by sharing electrons.

Covalent Bonding Picture a tug-of-war: If both teams pull with the same force the mid-point of the rope will not move.

Pure Covalent Bond H e H This even sharing of the rope can be compared to a pure covalent bond, where the bonding pair of electrons are held at the mid-point between the nuclei of the bonding atoms.

Covalent Bonding What if it was an uneven tug-of-war? The team on the right are far stronger, so will pull the rope harder and the mid-point of the rope will move to the right.

Polar Covalent Bond A polar covalent bond is a bond formed when the shared pair of electrons in a covalent bond are not shared equally. This is due to different elements having different electronegativities.

Polar Covalent Bond I δ- δ+ H e.g. Hydrogen Iodide e If hydrogen iodide contained a pure covalent bond, the electrons would be shared equally as shown above. This makes iodine slightly negative and hydrogen slightly positive. This is known as a dipole. However, iodine has a higher electronegativity and pulls the bonding electrons towards itself (winning the tug-of-war)

Polar Covalent Bond C Cl δ+ δ- In general, the electrons in a covalent bond are not equally shared. δ- δ+ e.g. C Cl 2.5 3.0 Electronegativities δ- indicates where the bonding electrons are most likely to be found.

Polar Covalent Bond Consider the polarities of the following bonds: Electronegativities Difference C Cl 2.5 3.0 0.5 P H 2.2 2.2 O H 3.5 2.1 1.4 C Cl δ- δ+ O H δ- δ+ P H Increasing Polarity Complete a similar table for C-N, C-O and P-F bonds.