1. Chapter 6: Chemical Bonds Section 2: Covalent Bonds, Polar and Non Polar Molecules

2. Covalent - The other type of bond Many elements do not easily donate or receive electrons. Instead they share them with another atom. Covalent bonds are formed when elements share electrons between the elements (this is generally formed between non metals). Covalently formed compounds are called molecules.

3. Covalent Bonds It is easy to distinguish a covalent bond from an ionic bond as in a covalent bond all of the elements bonded are non metals (right of stair step plus hydrogen). Water is a covalently bonded compound. Two hydrogen atoms and one oxygen atom share their electrons to form a stable configuration for both atoms (8 for Oxygen and 2 for Hydrogen) Binary covalent bonds never include metals.

4. Covalent Bonds are in a Tug-of-War Since the atoms in a covalent bond are not truly giving up their electrons, there is a constant pull on the shared electron between the atoms. The larger atom has a greater pull and pulls the shared atom closer. This results in “partial charges” with the larger mass elements taking on a partial negative charge and the smaller mass elements taking on a positive charge.

5. Polar Molecules When atoms have a shape that gives them a slightly positive end and a slightly negative end, it is called a polar molecule. Water (H 2 O) and ammonia (NH 3 )are polar molecules. Positive end Negative end The bent shape of the molecule gives them positive and negative poles δ+δ+ δ -

6. Non Polar Molecules Some other covalent bonds have a symmetrical shape. In these situations, electrons are shared equally and a nonpolar molecule is formed as there are no poles. Oxygen molecule No pos. and neg. poles Carbon dioxide molecule

7. Diatomic molecules Many non metals exist naturally in a covalent bond that forms a diatomic molecule. In this situation, two atoms share electrons to become stable. O 2 is a common example of a diatomic molecule. This diagram shows how two chlorine atoms share electrons to form the diatomic molecule Cl 2 Here, two hydrogen atoms become a stable H 2 molecule by sharing electrons

8. Naming Covalent Compounds Writing the chemical formulas of these compounds is pretty straight forward, but the naming of them can get complicated. For example N 2 O, NO, NO 2 and N 2 O 5 would all be read as nitrogen oxide under the naming rules for ionic compounds. However, each of these covalent compounds is distinctly different. Covalent compound naming has one twist.

9. Greek prefixes Add the following Greek prefixes to your periodic table (English meaning in parenthesis):  mono (1)hexa (6)  di (2)septa (7)  tri (3)octa (8)  tetra (4)nona (9)  penta (5)deca (10)

10. Naming covalent compounds includes Greek prefixes for numbers To solve the naming problem, we turn to Greek prefixes for numbers (now on your periodic table). In covalent compounds, you have to include the numbers in the subscripts as part of the name. N 2 O then becomes dinitrogen oxide NO then becomes nitrogen monoxide NO 2 then becomes nitrogen dioxide N 2 O 5 then becomes dinitrogen pentoxide