Bulkiness of Br vs CH3

[PocketJacks]

Ran into this in TBR Orgo (Chapter 4 - Passage 9 - Question 16)

The solution basically says that on a carbon with CH3 and Br as subs the Br is less bulky. Just wanted to get confirmation on this as I always thought the Br would be more bulky (mainly due to its mass) but I guess a polyatomic group is more bulky than a monoatomic one.

Let me know if that logic is correct.

Thank You!

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[FeinMS]

Yup CH3 is bigger than Br.

 

[idkididk]

I just ran into the same problem. Can someone else confirm that bulkiness depends on polyatomic-ness more than mass?

 

[Lunasly]

As far as I know, it is never based on mass, but rather the size of the compound. The periodic trend for atomic size is from right to left and from top to bottom on the periodic table. Bromine is a relatively small element according to this trend. The CH3 group has a carbon and three hydrogen atoms sticking out from it. It is worth noting that CH3 is larger than a bromine.

 

[DrknoSDN]

Confirmed, a Methyl group is much bigger than Bromine. Bromine has a 40% larger radius than carbon weighing in at 94pm radius.
Methane has a 108 pm bond length between carbon and hydrogen plus the 53 pm size of the hydrogen cloud, so probably double to triple the size of bromine.

Table of calculated atomic sizes:
http://www.ptable.com/#Property/Radius/Calculated