Additional
1) particles of a pure substance that cannot be decomposed by ordinary chemical means are called what ?

Particles of a pure substance that cannot be decomposed by ordinary chemical means are called Elements .

Explanation:

Elements are the purest form of the substances that cannot be decomposed by any of the ordinary chemical means such as heating, electrolysis and reaction. Only those substances that has been formed through combination of the elements can be decomposed by the ordinary chemical means due to the presence of elements like sodium, hydrogen, carbon dioxide as well as oxygen because these things upon heating decomposes.  

that is wrong, its not thymus.