Partial charges occur when partial charges occur when (a) a covalent bond links

both (a) a covalent bond links atoms of two kinds, and (b) atoms share electrons unequally.

Explanation:

1) Atoms bond each other throgh covalent bonds by sharing electrons.

2) By sharing electrons the atoms can fill their valence electron shell, reaching a favorable configuration like that of a noble gas (8 valence electrons, or 2 in the case of helium).

3) Nevertheless, different atoms exert different attraction on the valence electrons (due to the number of protons and the kind of valence orbital). The electronegativity is the property that aacounts for relative trend to attract electrons.

4) Then, altough different elements can share electrons, one of the elements will pull the shared electrons closer to its nucleus. The atoms that pulls the electrons more effectivley, will end with a partial negative charge, and the other atoms will end with a partial positive charge.

So, it is the fact (a) a covalent bond links atoms of two kinds, and (b) atoms share electrons unequally, what result in the apparition of partial charges (the negatively charged electrons will be closer to the nucleus of the more electronegative atom).

What nuclear reaction product has the least mass?  

C.beta particle