[helenee]

This is an AP Chem prep mcq:

Which pair of elements is expected to have the most similar properties?
A) Potassium and lithium
B) Sulfur and phosphorus
C) Silicon and carbon
D) Lithium and magnesium

I had thought it'd be C or even A (they have the same # of valence electrons) but apparently the answer is D. Could someone help me understand why?

[wildfyr]

I just plain don't agree with the book answer. I think A or C both have merits.

Perhaps they should define "properties" better. Elements have many disparate properties! Melting point, ionization energy, # of core shell electrons etc etc.

Crappy question.

[Flatbutterfly]

The chemistry of lithium and Magnesium is remarkably similar.  It is called a Diagonal Relationship
https://en.wikipedia.org/wiki/Diagonal_relationship

[wildfyr]

What chemistry? Conductivity? Reduction Potential? Bonding (unlikely, magnesium is 2+ and is better at coordination chemistry than tiny 1+ lithium).

The properties listed in that wiki article seem very cherry picked. I mean, form regular oxides rather than superoxides? Their chloride salts are deliquiescent? If this is what the textbooks say, I guess it's a test answer but it seem extremely arbitrary.

[Enthalpy]

The chemistry of lithium and Magnesium is remarkably similar. [...]

One reacts with cold water, the other not. I wouldn't call that "similar".

[DrCMS]

The chemistry of lithium and Magnesium is remarkably similar. [...]

One reacts with cold water, the other not. I wouldn't call that "similar".

Both react with hot water I would call that similar.

Really this is a very badly worded question.  Depending on what properties you want to pick pretty much every pair has some similarities but also some differences.

[Flatbutterfly]

I did not read the Wikipedia article on diagonal relations ships, but I have done so now.  It is poorly written.  The concept goes back to the early years (1920/30s) when chemists noted the similarity of the properties such as solubility of lithium and magnesium salts. For example, they are the only two metals in Groups 1/2 that form nitrides (Li3N, Mg3N2). In the ~60s chemists realized that both lithium and magnesium have an extensive organometallic chemistry.  Consider the common methylating agents of methyl lithium ([LiMe]4) and methyl Grignard (MeMgX).  No other element in Group 1 or 2 has such an extensive organometallic chemistry.  The theory of why this happens is not fully understood but it is related to the polarizing power, (M^n+)/(r^n+) of each metal; this OK for ionic cmpds but covalent cmpds?

[helenee]

Well, here's what the solution says:

Li and Mg are diagonally related in size and many physical properties stemming from atomic and ionic size. Although Si and C are next to each other in the fourth group, Si is a metalloid and semiconductor, and C is not. There is a larger difference between Si and C than between Mg and Li.

That last sentence confuses me a lot—how am I to know which pair has a "larger difference" and how do you determine that? Should I just accept this is perhaps not the best question and move on? I'm worried that there might be a similar problem on the AP exam, however.

[wildfyr]

I would just remember the phrasing of this question and recognize it for the garbage that it is on the test. After this much discussion, I'm sure you'll notice it right away and can apply this BS diagonal rule.

[Flatbutterfly]

This is a poor answer to a poor question.  How old is the question: late 50s?  There is no mention of the organometallic chemistry of Li and Mg which most chemists believe is the best example of a diagonal relationship.  And to compare the chemistries of C and Si is ludicrous as there are no bigger differences in chemistry for two elements in the same chemical group than for C and Si (they both form tetrachlorides but that’s about it!).

[Enthalpy]

Si is a metalloid and semiconductor, and C is not.

Diamond is a semiconductor. Presently it serves to make components. Like ZnO, GaN and other "wide-gap semiconductors" that were called insulators few decades ago.