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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: dan_c_23 on February 20, 2006, 06:14:56 PM

Title: How do I determine if an element is harder than the other?
Post by: dan_c_23 on February 20, 2006, 06:14:56 PM
How do I determine if an element is harder than the other, for example calcium and magnesium, they're in the same family but how do I know which one is the harder element.
Title: Re:Element Properties
Post by: Mitch on February 20, 2006, 06:58:29 PM
look up their density.
Title: Re: How do I determine if an element is harder than the other?
Post by: dan_c_23 on February 20, 2006, 07:07:13 PM
their mass???
Title: Re: How do I determine if an element is harder than the other?
Post by: Mitch on February 20, 2006, 07:14:01 PM
density. To a first approximation.
Title: Re: How do I determine if an element is harder than the other?
Post by: madscientist on February 20, 2006, 07:17:54 PM
plotting Moh's (minerological hardness) against atomic number gives:

http://www.webelements.com/webelements/properties/text/image-bars/hardness-brinell.html (http://www.webelements.com/webelements/properties/text/image-bars/hardness-brinell.html)

quote "Posted by: Mitch  Posted on: Today at 03:58:29pm  
look up their density. " end quote  

Mitch I thought that at first too but then thought about lead and iron,

Lead has a density of 11.3 g cm3

and density of iron is 7.86 g cm3

in this case lead is more dense but using logic if i took an iron nail to a lump of lead i know that the iron nail would scratch the lead?
Title: Re: How do I determine if an element is harder than the other?
Post by: Mitch on February 20, 2006, 07:21:41 PM
Density is not ideal. But it'll probably do for a highschool homework question.
Title: Re: How do I determine if an element is harder than the other?
Post by: madscientist on February 20, 2006, 07:26:40 PM
Hi dan_c_23,

From what mitch and i have told you, you should be able to answer the question with an explaination (ie lead iron) to the exception of the rule that mitch suggested.

(sorry mitch I forgot this was a high school chem forum )

cheers

madscientist
 :stupid:
Title: Re: How do I determine if an element is harder than the other?
Post by: mike on February 20, 2006, 08:15:56 PM
If you actually have the materials there you can scratch one with the other and see which one is harder.
Title: Re: How do I determine if an element is harder than the other?
Post by: pantone159 on February 21, 2006, 02:06:51 PM
Quote from: mike on February 20, 2006, 08:15:56 PM
If you actually have the materials there you can scratch one with the other and see which one is harder.

That is really the correct method.  Hardness is also very dependent on impurities as well, e.g. pure Fe and Al
are soft but both have 98% pure commercial alloys that are much harder.  It's not as much of a 'pure' property of the elements as other things.

I don't think I buy the denser-is-harder argument.  I'm sure it works in some cases, but there are too many exceptions.  E.g....
Lithium is harder than cesium.  (The metals get softer going down the alkali column.)  Gold, mercury and lead are dense but all are soft.  Iridium, however, is denser than all them and it is in fact very hard.  Copper is harder than silver and gold.

I'd actually guess that Ca is softer than Mg, but my nice Ca sample is sealed in an ampule and I'm not willing to break it out for the correct test.   :)