Add isoelectronic sequence property and helper function

fiasco/__init__.py

@@ -3,7 +3,12 @@
 """
 from fiasco.collections import IonCollection
 from fiasco.elements import Element
-from fiasco.fiasco import list_elements, list_ions, proton_electron_ratio
+from fiasco.fiasco import (
+    get_isoelectronic_sequence,
+    list_elements,
+    list_ions,
+    proton_electron_ratio,
+)
 from fiasco.gaunt import GauntFactor
 from fiasco.ions import Ion
 from fiasco.levels import Level, Transitions

fiasco/base.py

@@ -71,6 +71,10 @@ def ionization_stage(self):
     def charge_state(self):
         return self.ionization_stage - 1
 
+    @property
+    def isoelectronic_sequence(self):
+        return plasmapy.particles.atomic_symbol(self.atomic_number - self.charge_state)
+
     @property
     def _ion_name(self):
         # Old CHIANTI format, only preserved for internal data access

fiasco/fiasco.py

@@ -11,8 +11,9 @@
 import fiasco
 
 from fiasco.io import DataIndexer
+from fiasco.util import parse_ion_name
 
-__all__ = ['list_elements', 'list_ions', 'proton_electron_ratio']
+__all__ = ['list_elements', 'list_ions', 'proton_electron_ratio', 'get_isoelectronic_sequence']
 
 
 def list_elements(hdf5_dbase_root=None, sort=True):
@@ -74,7 +75,29 @@ def list_ions(hdf5_dbase_root=None, sort=True):
     # NOTE: when grabbing straight from the index and not sorting, the result will be
     # a numpy array. Cast to a list to make sure the return type is consistent for
     # all possible inputs
-    return ions.tolist() if type(ions) == np.ndarray else ions
+    return ions.tolist() if isinstance(ions, np.ndarray) else ions
+
+
+def get_isoelectronic_sequence(element, hdf5_dbase_root=None):
+    """
+    Return a list of ions in the isoelectronic sequence of ``element``.
+
+    Parameters
+    ----------
+    element: `str`, `int`
+        Name of sequence. Can be either the full name (e.g. "hydrogren"),
+        the atomic symbol (e.g. "H") or the atomic number (e.g. 1)
+    hdf5_dbase_root: path-like, optional
+        If not specified, will default to that specified in ``fiasco.defaults``.
+    """
+    Z_iso = plasmapy.particles.atomic_number(element)
+    all_ions = list_ions(hdf5_dbase_root=hdf5_dbase_root)
+
+    def _is_in_sequence(ion):
+        Z, z = parse_ion_name(ion)
+        return Z_iso == (Z - z + 1)
+
+    return [ion for ion in all_ions if _is_in_sequence(ion)]
 
 
 @u.quantity_input

fiasco/ions.py

@@ -88,6 +88,7 @@ def __repr__(self):
 Name: {self.ion_name}
 Element: {self.element_name} ({self.atomic_number})
 Charge: +{self.charge_state}
+Isoelectronic Sequence: {self.isoelectronic_sequence}
 Number of Levels: {n_levels}
 Number of Transitions: {n_transitions}
 
@@ -262,7 +263,7 @@ def hydrogenic(self):
         -----
         This is `True` if :math:`Z - z = 1`.
         """
-        return (self.atomic_number - self.charge_state == 1)
+        return self.isoelectronic_sequence == 'H'
 
     @property
     def helium_like(self):
@@ -273,7 +274,7 @@ def helium_like(self):
         -----
         This is `True` if :math:`Z - z = 2`.
         """
-        return (self.atomic_number - self.charge_state == 2)
+        return self.isoelectronic_sequence == 'He'
 
     @property
     @u.quantity_input

fiasco/tests/test_base.py

@@ -25,6 +25,7 @@ def test_create_ion_input_formats(hdf5_dbase_root, ion_name):
     assert ion.atomic_number == 26
     assert ion.ionization_stage == 21
     assert ion.charge_state == 20
+    assert ion.isoelectronic_sequence == 'C'
     assert ion._ion_name == 'fe_21'
     assert ion.ion_name_roman == 'Fe XXI'
     assert ion.ionization_stage_roman == 'XXI'

fiasco/tests/test_fiasco.py

@@ -19,6 +19,16 @@ def test_list_ions(hdf5_dbase_root):
     assert isinstance(ions, list)
 
 
+def test_get_isoelectronic_sequence(hdf5_dbase_root):
+    iso_seq = fiasco.get_isoelectronic_sequence('iron',
+                                                hdf5_dbase_root=hdf5_dbase_root)
+    assert iso_seq == ['Fe 1',
+                       'Co 2',
+                       'Ni 3',
+                       'Cu 4',
+                       'Zn 5',]
+
+
 def test_proton_electron_ratio(hdf5_dbase_root):
     t = np.logspace(4, 9, 100) * u.K
     # NOTE: this number will not be accurate as we are using only a subset of