Reinstate neutron as element Z=0, but exclude it from the iterator

periodictable/core.py

@@ -239,7 +239,10 @@ def __iter__(self):
         """
         Process the elements in Z order
         """
-        for _, el in sorted(self._element.items()):
+        # CRUFT: Since 3.7 dictionaries use insertion order, so no need to sort
+        elements = sorted(self._element.items())
+        # Skipping the first entry (neutron) in the iterator
+        for _, el in elements[1:]:
             yield el
 
     def symbol(self, input):
@@ -604,7 +607,7 @@ def _make_isotope_ion(table, Z, n, c):
 element_base = {
     # number: name symbol common_ions uncommon_ions
     # ion info comes from Wikipedia: list of oxidation states of the elements.
-    # 0: ['Neutron',     'n',  [],         []],
+    0: ['neutron',     'n',  [],         []],
     1: ['Hydrogen',    'H',  [-1, 1],    []],
     2: ['Helium',      'He', [],         [1, 2]],  # +1,+2  http://periodic.lanl.gov/2.shtml
     3: ['Lithium',     'Li', [1],        []],
@@ -764,7 +767,7 @@ def define_elements(table, namespace):
     for el in table:
         names[el.symbol] = el
         names[el.name] = el
-    for el in [table.D, table.T]:
+    for el in [table.D, table.T, table.n]:
         names[el.symbol] = el
         names[el.name] = el
 

periodictable/density.py

@@ -170,7 +170,7 @@ def init(table, reload=False):
             el.density_caveat = ""
 
 element_densities = dict(
-    # n=None,  # No longer using element 0 for a bare neutron
+    n=None,  # No longer using element 0 for a bare neutron
     H=(0.0708, "T=-252.87"),
     He=(0.122, "T=-268.93"),
     Li=0.534,

periodictable/mass.py

@@ -120,12 +120,12 @@ def init(table, reload=False):
         iso._abundance, iso._abundance_unc = 0, 0
 
     # # A single neutron is an isotope of element 0
-    # from .constants import neutron_mass, neutron_mass_unc
-    # el = table[0]
-    # el._mass, el._mass_unc = neutron_mass, neutron_mass_unc
-    # iso = el.add_isotope(1)
-    # iso._mass, iso._mass_unc = neutron_mass, neutron_mass_unc
-    # iso._abundance, iso._abundance_unc = 100, 0
+    from .constants import neutron_mass, neutron_mass_unc
+    el = table.n
+    el._mass, el._mass_unc = neutron_mass, neutron_mass_unc
+    iso = el.add_isotope(1)
+    iso._mass, iso._mass_unc = neutron_mass, neutron_mass_unc
+    iso._abundance, iso._abundance_unc = 100, 0
 
     # Parse element mass table where each line looks like:
     #    z  El  element mass(unc)|[low,high]|- note note ...

periodictable/nsf.py

@@ -567,9 +567,6 @@ def init(table, reload=False):
         symbol = parts[1]
         isotope_number = int(parts[2]) if len(parts) == 3 else 0
 
-        if Z == 0: # Skip row 0-n-1
-            continue
-
         # Fetch element from the table and check that the symbol matches
         element = table[Z]
         assert element.symbol == symbol, \
@@ -580,7 +577,7 @@ def init(table, reload=False):
         nsf._number_density = element.number_density # N/cm^3 = N/cm^3
 
         if isotope_number == 0:
-            # Bulk values using laboratory abundances of isotopes
+            # Value for element using laboratory abundances of isotopes
             element.neutron = nsf
         else:
             # Values for the individual isotope

test/test_nsf.py

@@ -5,7 +5,7 @@
 import periodictable
 from periodictable import elements, formula, nsf
 from periodictable.nsf import neutron_scattering, neutron_sld
-from periodictable.constants import avogadro_number as N_A
+from periodictable.constants import avogadro_number as N_A, neutron_mass
 
 def test():
     H,He,D,O = elements.H,elements.He,elements.D,elements.O
@@ -135,6 +135,19 @@ def test():
     assert all(abs(v-w)<1e-14 for v,w in zip(xs,xs2))
     assert abs(depth-depth2)<1e-14
 
+def test_bare_neutron():
+    n = elements.n
+    assert n == elements[0]
+    assert n == periodictable.neutron
+    n_iso = elements[0][1]
+    assert n.mass == neutron_mass
+    assert n_iso.mass == neutron_mass
+    assert n.neutron.b_c == -37.0
+    assert n.density is None
+    assert n.number_density is None
+    assert n.neutron.scattering()[0] is None
+
+
 def test_formula():
     density = 2.52
     M = formula('B4C', density=density)