A 2 Elements Oxygen, Silicon, Sulfur, Chlorine, and Bromine

2 A -I- 2 elements oxygen, silicon, sulfur, chlorine, and bromine 

A second isotope makes an especially prominent appearance in the spectrum if it is more than one unit higher in mass than the most abundant isotopic species. Bromine and chlorine, and to a lesser extent silicon and sulfur, are striking common examples. The presence of these elements in an ion is often easily recognized from the isotopic clusters produced in the spectrum. Thus elements of Unknowns 2.2 and 2.3, like Unknown 2,1, can be recognized from the 

The characteristic patterns resulting from combinations of the chlorine, bromine, sulfur, and silicon isotopes are illustrated by Table 2.3 (inside front cover), arranged in increasing order of the relative abundances of (A -I- 2) and (A + 4). Numerical values for combinations of A -4- 2 elements are given in Table A.2. These were calculated from the binomial expansion (a + bf = a + na h n(n — )a b /2 n n — 1)( — 2)a b / 3 ) H-. Thus for the peaks con- 

Absence of A + 2 elements. The value of negative information should not be overlooked. Another reason for checking first for the presence of A + 2 elements is that one often finds them to be absent. Consider any peak A (frag- 

---