How do we know if this following molecule is a superimposable mirror image or non-superimposable mirror image ? [See Image attached]

How do we know if this following molecule is Super Imposable mirror image or Non Super Imposable mirror image ? [See Image attached]


[![Molecule](https://i.stack.imgur.com/zefvY.png)](https://i.stack.imgur.com/zefvY.png)




* Take 2 round, soft objects like potatoes or apples.
* Pin into each 4 pins of 3 different kinds ( 1 + 2 + 1 ),  

pointing to the most different directions.
* Call the 1st pin **bromine**, the 2nd one **hydrogen**, the 3rd one **ethyl**.
* Do the same for the other potato to be the **mirror image** of the first one.
* Try to rotate one of them to look like the other.
* If you succeed, they are superimposable.
* Now, replace one of the same pins on both objects by another -
4th kind - of pins in such a way that they are still mutual mirror images.
* Try to match them the same way you tried previously.
* If you succeed, they are superimposable.


**The summary:**


* While there are 2 hydrogen atoms on a carbon atoms - or more generally, there are only 3 kinds of groups on a C atom - the groups/atoms of the same kind are interchangeble and you can rotate the molecule to match the mirror image.
* If there are 4 different groups on C atoms, you cannot make such a match.
* Said by other way, the former case has a plane of symmetry, the latter case does not.


 2