Intermediate bonding and bond polarity

1. Intermediate bonding and bond polarity

2. Electronegativity • Definition: The ability of an atom to attract electron density towards itself and polarise a covalent bond. The table of values of electronegativity: Why are there no values for He/Ne/Ar?

3. δ+ δ- H- F • The electron distribution in this bond is distorted because fluorine is pulling all the electrons towards itself as it is more electronegative. • This leaves the hydrogen being more positive and the fluorine slightly more negative (δ+ means slightly positive).

4. Predicting the Nature of Bonds Link to electronegativity What would the following be ionic or covalent in water HCl , CH4 , CO2 , Li2O , Distorted electron cloud δ+ δ- H- F

5. Very polar covalent compound like Hydrogen Fluoride will dissolve in water (a polar solvent and will not be very soluble in organic solvents). • This is very ionic in nature, since ionic complexes are made up of negative and positive charges. • Usually ionic compounds like Aluminium Chloride do not dissolve in organic solvents as they are charged. • If a cation is very small and has a high positive charge, it will attract the electron back toward itself causing a distortion of the ionic bond. The aluminium takes back a share in the electron, so this is covalent in nature. • Metals that are of this nature are termed amphoteric

6. You have seen examples of ionic substances dissolving in organic solvents and covalent substances dissolving in water • Give some compounds that may also have ionic or covalent character that is not fitting with the rest the definition • Can you draw up a trend or a rule to govern how you can predict a compounds character?

7. Answer 1 • GaI3 • GeBr4 • Al2S3 • ICl4- • CH3COOH • MgI2 • Answer 2 • For metals-The smaller the ion and the more charges it has and the larger the electronegativity the more covalent (as the metal attracts back the donated electrons) • For non-metals –The larger the electronegativity difference the more likely it is to form ions.

8. The result of these phenomenon is that some ionic complexes have covalent properties and covalent molecules that have ionic. This is due to electronegativity and size of a densely charged ion. • Put the labels on the table: • 1.Complete transfer of electrons • .Equal sharing of electrons • .X÷Y • .Large electronegativity difference • .No transfer of electrons • .X+Y- • .No sharing of electron 8.No difference in electronegativity

9. Ionic Radius • Atoms are extremely small. The atomic radius is the distance from the nucleus to the outermost electrons. • In period three the outer most electrons are on the third energy level. How does this vary going across? • Why does this happen? What have the elements on the right got more of which would make the radius reduce?

10. Fill in the table, follow the pattern as sodium and chlorine have been done for you

11. Predict whether a molecule is polar • What do we mean by polar? • What causes the polarity? • Water is polar –why? • Draw it and see if you can see

14. Deciding which bonds are polar • Firstly you have to check their electronegativity value using data table • If there is a difference then the bond is polar • Try this, find out if CH4 is a polar molecule • First check the electronegativity • Answer C=2.4 and H=2.1 • Is the bond polar? • Yes to polar bonds • Is there an overall polarity? • CH4 is not polar, it does not have an overall polarity

15. In your groups work out a rule which can be used for different molecules to determine polarity • You know that H2O is polar and CH4 is not • Work out why then use these reasons for a universal rule

16. Overall polarity • Ask yourself if you put a positive force of attraction near it what would happen? • Does the charge have a vector? Y/N • Or even simpler is the molecule symmetrical? Y/N • If there is no overall vector or it is symmetrical it will not be polar as to be polar means that there are 2 distinct ends of the molecule which have different charges

17. How can we test this theory? • We can use a simple technique you will already be familiar with. Using an electrostatic force to see if there is any attraction or repulsion • Use a polythene wand, rub with a cloth for 2 mins continuously • Turn on the tap and produce a constant stream of water (not full) • Without touching put the wand near the stream –what do you notice. Does this confirm water is polar or non-polar?

18. Use your rule on the following • Cl2 • CH3OH • H2 • C2H5Cl • CCl4 • CCl3F • CO2 • NO2 • O2 Not sure… Find shape Check electronegativity Is it symmetrical? Is there a vector?