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Different Bonds

I am working on different lessons-- Ionic bond, non polar covalent, polar covalent and now polyatomic bonds.

If I am give a molecule, how can I look at it and know what type of bond it is right away??

I mean in each lesson I am working on they use only molecules with that TYPE of bond. so outside of these lessons I am working on in my textbook I am confused as to how to determine the bonds easily!!!

any help? hope that makes sense.

kingchemist Fri, 01/15/2010 - 11:37

Ionic compounds do not exist a molecules, only groups of ions. Ionic compounds usually have a metal joined to one or more non-metals eg NaCl

A molecule is a small group of similar atoms, normally all non-metal atoms, joined together by covalent bonds (electrons are shared) eg. H2, N2, O2, F2, Cl2, Br2, I2, CO2, CH4, SO2, H2O, C60

Now covalent bonds can either involve equal sharing of electrons or unequal sharing of electrons. In diatomic molecules such as H2, the two atoms are identical and have equal pulling power for electrons and the electrons are shared equally. Sometimes this is called a pure (or non-polar) covalent bond.
In H2O, O is better at attracting electrons than H, so the shared electrons are not shared equally and will spend more of their time closer to O than H. This makes O slightly negatively charged and H slightly positively charged. So the molecule is polar and the bond is a polar covalent bond. The further to the right an element is in the periodic table, the better it is at attracting shared electron but there are also patterns up and down the table eg F has few protons than Cl in group 7 but is better at attracting electrons than Cl.

If you know about electronegativity values, these can be used to help predict the type of bond. Metals have low values and do not readily attract electrons. Some courses suggest looking at the differences in the electronegativity values of the two elements.
If the difference is 1.7, the bond tends to be ionic.
These rules are only a guide and there are exceptions.

Electronegativity definition is a measure of the attraction for a pair of shared electrons.

Hope this helps

Some examples
CH4 has C = 2.55 and H = 2.1 so it suggest this bond is non-polar covalent
H2O has H = 2.1 and  O = 3.44 so this bond is polar covalent
PH3 has H= 2.1 and P = 2.19, so this bonds is almost pure (non-polar) covalent
HF has H = 2.1 and F= 4.0 and this would suggest this will be ionic. However, in the gas state, this is highly polar covalent but in solution it is ionic
NaCl has Na = 0.93 and Cl = 3.16 and this is definitely ionic