What is the determinant of "Bulkiness"?

[cwpatter]

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Snip.

 

[inasensegone]

I think the application of the term "bulkiness" is more informal than that and has to do with the branching of a compound. For example, Potassium Tertbutoxide:

Is a more bulky base than Sodium Hydroxide:
O-H

 

[cwpatter]

That makes easy enough sense, as without a doubt three methyl groups are larger than simply one hydrogen.

But say we had three Iodines instead of the three methyl groups, or three Bromines instead of three methyl groups. Which of these would make the bulkiest molecule, and WHY?

There has to be some specific rule to overall bulkiness, because when we're looking at newmann projections of conformational isomers, the two BULKIEST substituents on adjacent carbons have to be in a STAGGERED position to be the most stable.

 

[inasensegone]

First of all, methyl groups will be more bulky than a halogen because a methyl group is a group of 4 atoms: a CH3 group. Halogens are individual atoms.

But, as to comparing Iodine versus Bromine, it works the same way as you would compare the sizes of any atoms. Within a family (column) on the periodic table, atomic size increases as you go down the column (more electronic levels are being added). Within a period (row) on the periodic table, atomic size seems to almost counter-intuitively decrease... This is because you are not adding more electronic levels, but you're adding more electrons. The result is that the force of the nucleus pulling on these electrons increases but the electronic level remains constant (that is, no new orbitals are being added at a further distance from the nucleus within a period).

 

[cwpatter]

So if I understand you correctly.

1) The NUMBER of atoms contributing to a substituent is the most important factor.
(CH3 > NH2)

2) If they are the same number, then you delegate to the ATOMIC RADIUS. The larger the atomic radius, the more bulky the atom.

Atomic radius decreases as you move to the right of the PT due to the increasing amount of protons pulling electrons in, and it increases as you go down the PT as you are adding more non-valence electron orbitals which contribute to shielding.

 

[inasensegone]

So if I understand you correctly.

1) The NUMBER of atoms contributing to a substituent is the most important factor.
(CH3 > NH2)

2) If they are the same number, then you delegate to the ATOMIC RADIUS. The larger the atomic radius, the more bulky the atom.

Atomic radius decreases as you move to the right of the PT due to the increasing amount of protons pulling electrons in, and it increases as you go down the PT as you are adding more non-valence electron orbitals which contribute to shielding.

1) Regarding the 1st thing you listed.... I wouldn't compare the 2... they would be about equivalent because you're talking about 2 similarly sized atoms (C and N) bonded to Hydrogens. The fact that one of them has 2 hydrogens and the other has 3 shouldn't make a huge difference.

I highly doubt that you will have to distinguish this tiny difference on the MCAT and if you do, then it will most likely be able to be answered based off of the passage.

2) Yes

....decreases as you move to the right of the PT (within a PERIOD) That is an important distinction. And yes it decreases in size because now there is a stronger attractive force in the nucleus (which compared to electrons is super dense and all the protons are closer together) pulling on electrons that are at the same level within a period.

And going down a family (column), yes the shielding effect (from the effective nuclear charge) is a good way to describe it.