function atomic_weight, n, iso

;+
; NAME:
;  atomic_weight
; PURPOSE: (one line)
;  Return the atomic weight
; DESCRIPTION:
;  Atomic weights from CRC 63rd edition, pp B-255
;  If only the atomic number is given, then this returns
;  the weight assuming natural abundances.
;  If both the atomic number and isotope are given,
;  then this returns the atomic weight from the
;  CRC, if it was tabulated there, otherwise it
;  simply returns the inputed
;  isotopic number.
; CATEGORY:
;  General
; CALLING SEQUENCE:
;  w = atomic_weight(n, iso) or w = atomic_weight(n)
; INPUTS:
;  n - atomic number (e.g., 1 for H)
;  iso - isotope (e.g., 12 for Carbon 12)
; OPTIONAL INPUT PARAMETERS:
;  none
; KEYWORD INPUT PARAMETERS:
;  none
; KEYWORD OUTPUT PARAMETERS:
;  none
; OUTPUTS:
;  atomic weight (g/mole, e.g., 12. for atomic_weight(6,12) )
; COMMON BLOCKS:
;  None
; SIDE EFFECTS:
; RESTRICTIONS:
;  For n > 18, the only element w/ isotopes is Br.
; PROCEDURE:
; MODIFICATION HISTORY:
;  Written 2000 October, by Leslie Young, SwRI
;-

; array of natural atomic weights.  Because of IDL limitations, 
; make this as the concatination of 2 arrays.
atwt = $  
;   H                                                                                                                      He
[ 1.008,                                                                                                                 4.003, $
; Li      Be                                                                            B      C     N      O      F      Ne
 6.941, 9.012,                                                                       10.81, 12.01, 14.01, 16.00, 19.00, 20.18, $
; Na     Mg                                                                             Al    Si     P      S      Cl     Ar
 22.99, 24.31,                                                                       26.98, 28.09, 30.97, 32.06, 35.45, 39.95, $
;  K      Ca     Sc      Ti    V      Cr     Mn     Fe     Co     Ni    Cu     Zn      Ga     Ge     As     Se     Br     Kr
 39.10, 40.08, 44.96, 47.90, 50.94, 52.00, 54.94, 55.85, 58.93, 58.70, 63.55, 65.38, 69.72, 72.59, 74.92, 78.96, 79.90, 83.80, $
; Rb      Sr     Y      Zr     Nb     Mo     Tc     Ru     Rh     Pd    Ag      Cd     In     Sn     Sb     Te     I      Xe
 85.47, 87.62, 88.91, 91.22, 92.91, 95.94, 98.  , 101.1, 102.9, 106.4, 107.9, 112.4, 114.8, 118.7, 121.8, 127.6, 126.9, 131.3]

atwt = [atwt, $
;  Cs     Ba     La
[132.9, 137.3, 138.9, $ 
;                              Ce      Pr     Nd    Pm     Sm      Eu    Gd     Tb     Dy      Ho    Er     Tm     Yb     Lu
                              140.1, 140.9, 144.2, 145.,  150.4, 152.0, 157.3, 158.9, 162.5, 164.9, 167.3, 168.9, 173.0, 175.0, $
;                       Hf     Ta      W      Re    Os     Ir      Pt    Au     Hg     Tl      Pb    Bi     Po     At     Rn
                       178.5, 180.9, 183.9, 186.2, 190.2, 192.2, 195.1, 197.0, 200.6, 204.4, 207.2, 209.0, 209., 210.,  222.,  $
;  Fr      Ra    Ac
 223.,  226.,  227. , $
;                              Th     Pa      U     Np     Pu      Am    Cm     Bk    Cf       Es   Fm      Md     No     Lr
                              232.0, 231.,  238.0, 244.,  242.,  243.,  247.,  247.,  251.,  252.,  257.,  258.,  259.,  260. ] ]
 
 
; User passed just atomic number.
if n_params() eq 1 then return, atwt[n-1]

; User passed atomic number and isotope
if n_params() ne 2 then return, 0.
case n of
  1: case iso of           ; H
        1: return,  1.007825
        2: return,  2.0140
        3: return,  3.01605
     else: return, float(iso)
     endcase
  2: case iso of           ; He
        3: return,  3.01603
        4: return,  4.00260
        5: return,  5.0123
        6: return,  6.01888
        8: return,  8.0375
     else: return, float(iso)
     endcase
  3: case iso of           ; Li
        5: return,  5.0125
        6: return,  6.01512
        7: return,  7.01600
     else: return, float(iso)
     endcase
  4: case iso of           ; Be
        6: return,  6.0197
        7: return,  7.0169
        8: return,  8.0053
        9: return,  9.01218
       10: return, 10.0135
     else: return, float(iso)
     endcase
  5: case iso of           ; B
        8: return,  8.0246
        9: return,  9.0133
       10: return, 10.0129
       11: return, 11.00931
       12: return, 12.0143
       13: return, 13.0178
     else: return, float(iso)
     endcase
  6: case iso of           ; C
       12: return, 12.0000  ; By definition!
       13: return, 13.00335
     else: return, float(iso)
     endcase
  7: case iso of           ; N
       14: return, 14.00307
       15: return, 15.00011
     else: return, float(iso)
     endcase
  8: case iso of           ; O
       16: return, 15.99491
     else: return, float(iso)
     endcase
  9: case iso of           ; F
       19: return, 18.99840
     else: return, float(iso)
     endcase
 10: case iso of           ; Ne
       20: return, 19.99244
       21: return, 20.99138
       22: return, 21.99138
     else: return, float(iso)
     endcase
 11: case iso of           ; Na
       23: return, 22.9898
     else: return, float(iso)
     endcase
 12: case iso of           ; Mg
       24: return, 23.98504
       25: return, 24.98584
       26: return, 25.98259
     else: return, float(iso)
     endcase
 13: case iso of           ; Al
       27: return, 26.98153
     else: return, float(iso)
     endcase
 14: case iso of           ; Si
       28: return, 27.97693
       29: return, 28.97649
       30: return, 29.97376
     else: return, float(iso)
     endcase
 15: case iso of           ; P
       31: return, 30.97376
     else: return, float(iso)
     endcase
 16: case iso of           ; S
       32: return, 31.97207
       33: return, 32.97146
       34: return, 33.96786
       36: return, 35.96709
     else: return, float(iso)
     endcase
 17: case iso of           ; Cl
       35: return, 34.96885
       37: return, 36.96590
     else: return, float(iso)
     endcase
 18: case iso of           ; Ar
       36: return, 35.96755
       38: return, 37.96272
     else: return, float(iso)
     endcase
; Sparce from here on down
 35: case iso of           ; Br
       79: return, 78.9183
       81: return, 80.9163
     else: return, float(iso)
     endcase
 53: case iso of           ; I
      127: return, 126.9044
     else: return, float(iso)
     endcase
else: return,float(iso)
endcase

end