# Chemistry

posted by on .

Hi Dr Bob,

I understand the 2nd method but need to clarify the 1st method.If I only wish to draw 1 Al and 1 O, then I need to include the "plus" sign and draw the 'dot and cross' diagram as
2[Al]+ 3[O] , with the charges of course, right?

Posted by candy on Thursday, April 2, 2009 at 5:10am.

When drawing the 'dot and cross' diagram to show the ionic bonding in aluminium oxide, Al2O3, do we place the nos "2" and "3" in front of the brackets containing the 'dot and cross' diagram, e.g 2[Al]3[O]?
I notice that some books place the "2" and "3" as subscripts while others place the nos. in front.

Responses

Chemistry - DrBob222, Thursday, April 2, 2009 at 11:39am
The link provided by SraJMcGin refers more to the cross product of mathematics than to your question.
What you want to do with ionic compounds is to show
2Al+3 + O-2 with two pairs of electrons on the left, top, right, and bottom of the O. I can't draw them on the board. The other way of doing it is to show one Al ion with another just below it, then to the right show 3 O atoms, one above the other, and put in the dots and crosses on the O. Don't forget to put the -2 charge on each oxygen when you have finished.

• Chemistry - ,

Yes, you will show
2Al^+3 + 3O^-2 and have the dots and crosses around the O. The problem with doing it this way, with a compound such as Al2O3, the crosses and dots don't come out right for just the one O because it takes 3 O atoms to take up electrons from the two Al atoms. I would handle that by placing two crosses on the left side of O, two dots at the top, two dots on the right, and two dots on the bottom of the O atom. Then show a -2 charge on the oxide ion created. That way clearly shows that the O atom had 6 electrons to start and that the two electrons it was short was supplied by Al. You must remember, of course, that an electron is an electron, that all electrons are alike, and that the crosses and dots only represent electrons from a different source.