According to the standard atomic model¹, an atom consists of a nucleus surrounded by orbiting electrons.

A chemical element has an atomic nucleus consisting of a fixed number of protons and a variable number of neutrons. The number of protons determines the atomic number of the element and its chemical properties. The number of neutrons determines, among other things, the stability of the nucleus and — together with the number of protons — the atomic mass. Each element has a unique name consisting of an uppercase letter followed by one or more lowercase letters (e.g. Tungsten, Lead, Mercury, Helium) and a unique symbol consisting of an uppercase letter followed by zero or more lowercase letters (e.g. W, Pb, Hg, He).

Isotopes are atoms of the same chemical element — so they have the same number of protons but a different number of neutrons in their atomic nucleus. An isotope is denoted by its mass number (the number of protons and neutrons in the nucleus) in superscript to the left of the element's chemical symbol (e.g. $${}^{3}\mathrm{He}$$, $${}^{4}\mathrm{He}$$, $${}^{12}\mathrm{C}$$, $${}^{14}\mathrm{C}$$, $${}^{235}\mathrm{U}$$, $${}^{239}\mathrm{U}$$).

Which famous chemist is hidden in this sequence of isotopes? \[ {}^{281}\mathrm{Mt}\,{}^{203}\mathrm{Tl}\,{}^{202}\mathrm{Hg}\,{}^{249}\mathrm{Es}\,{}^{291}\mathrm{Fl}\,{}^{247}\mathrm{Cm}\,{}^{167}\mathrm{Er}\,{}^{93}\mathrm{Zr}\,{}^{251}\mathrm{Cf}\,{}^{146}\mathrm{Nd} \]

Assignment

We represent a sequence of isotopes that hides the name of a famous chemist as a string (str) consisting of the alternating sequence of mass numbers (digit sequences) and symbols (letter sequences) from the isotope notations. For example, the sequence of isotopes  \[ {}^{281}\mathrm{Mt}\,{}^{203}\mathrm{Tl}\,{}^{202}\mathrm{Hg}\,{}^{249}\mathrm{Es}\,{}^{291}\mathrm{Fl}\,{}^{247}\mathrm{Cm}\,{}^{167}\mathrm{Er}\,{}^{93}\mathrm{Zr}\,{}^{251}\mathrm{Cf}\,{}^{146}\mathrm{Nd} \] that hides the name Marie Curie, is represented as the string

281Mt203Tl202Hg249Es291Fl247Cm167Er93Zr251Cf146Nd

To find the chemist hidden in the sequence of isotopes, we need the names of the chemical elements whose symbol appears in the sequence of isotopes. For this purpose, we use a text file in which each line describes some properties of a single chemical element, with information fields separated by a character that otherwise does not appear elsewhere in the information fields. The file contains at least information about elements whose symbol appears in the sequence of isotopes, and contains at least information fields with the name and the symbol of the elements. The order of the information fields is not fixed. The first line of the file may consist of a header with descriptive names for the information fields.

For example, this is part of a file (elements.txt²) containing information about all known chemical elements. The file has a header, and each line consists of 15 tab-separated information fields. The second field contains the symbol for the elements, and the third field contains their name.

Atomic number	Symbol	Name	Group	Period	Block	Atomic weight (Da)	Density (g/cm3)	Melting point (K)	Boiling point (K)	Specific heat capacity (J/g · K)	Electro-negativity	Abundance in Earth's crust (mg/kg)	Origin	Phase
…
40	Zr	Zirconium	4	5	d-block	91.224	6.52	2128	4682	0.278	1.33	165	primordial	solid
…
60	Nd	Neodymium	f-block groups	6	f-block	144.24	7.01	1297	3347	0.19	1.14	41.5	primordial	solid
…
68	Er	Erbium	f-block groups	6	f-block	167.26	9.066	1802	3141	0.168	1.24	3.5	primordial	solid
…
80	Hg	Mercury	12	6	d-block	200.59	13.534	234.43	629.88	0.14	2.00	0.085	primordial	liquid
81	Tl	Thallium	13	6	p-block	204.38	11.85	577	1746	0.129	1.62	0.85	primordial	solid
…
96	Cm	Curium	f-block groups	7	f-block	[247]	13.51	1613	3383	–	1.28	–	synthetic	solid
97	Bk	Berkelium	f-block groups	7	f-block	[247]	14.78	1259	2900	–	1.3	–	synthetic	solid
98	Cf	Californium	f-block groups	7	f-block	[251]	15.1	1173	(1743)	–	1.3	–	synthetic	solid
99	Es	Einsteinium	f-block groups	7	f-block	[252]	8.84	1133	(1269)	–	1.3	–	synthetic	solid
…
109	Mt	Meitnerium	9	7	d-block	[278]	(27–28)	–	–	–	–	–	synthetic	unknown phase
…
114	Fl	Flerovium	14	7	p-block	[289]	(11.4±0.3)	(284±50)	–	–	–	–	synthetic	unknown phase
…

Your task:

• Write a function select_letter that takes two arguments: i) a number $$n \in \mathbb{N}_0$$ (int) and ii) a string $$s$$ (str) that only contains letters (uppercase and lowercase letters). The function must return the $$n$$-th letter (str) of string $$s$$, if we start counting from 1 at the first letter of string $$s$$ and continue counting from the start of string $$s$$ after reaching the end.

• Write a function isotopes that takes the string representation (str) of a sequence of isotopes. The function must return a list containing a tuple for each isotope in the sequence, with the mass number (int) and the symbol (str) of the isotope.

• Write a function read_elements that takes three arguments: i) the location (str) of a text file containing properties of chemical elements, ii) the index (int) of the information field containing the symbols for the elements and iii) the index (int) of the information field containing the names of the elements. Information fields are numbered from left to right, starting at 1. The function also has an optional parameter separator that may take the field separator (str). If no separator is passed explicitly, the information fields are tab-separated. The function also has an optional parameter header that takes a Boolean value (bool; default value: True), indicating whether the first line of the file is a header. The function must return a dictionary (dict) that maps the symbol (str) for each element described in the file onto the name (str) of the element.

• Write a function chemist that takes two arguments: i) the string representation (str) of a sequence of isotopes that hides the name of a famous chemist and ii) a dictionary (dict) mapping symbols (str) of chemical elements onto their names (str). The function may assume that the symbol of each isotope in the sequence is a key in the dictionary, without the need to check this explicitly. The function must return the chemist (str) hidden in the given sequence of isotopes.

Example

In this interactive session, we assume the current directory contains the text file elements.txt³.

>>> select_letter(281, 'Meitnerium')
'M'
>>> select_letter(203, 'Thallium')
'a'
>>> select_letter(202, 'Mercury')
'r'

>>> isotopes('281Mt203Tl202Hg249Es291Fl247Cm167Er93Zr251Cf146Nd')
[(281, 'Mt'), (203, 'Tl'), (202, 'Hg'), (249, 'Es'), (291, 'Fl'), (247, 'Cm'), (167, 'Er'), (93, 'Zr'), (251, 'Cf'), (146, 'Nd')]
>>> isotopes('121Sb232Ac107Ag139Nd137Pr163Ho225Ac261Md258No290Og20Mg115Cd238Am36Cl')
[(121, 'Sb'), (232, 'Ac'), (107, 'Ag'), (139, 'Nd'), (137, 'Pr'), (163, 'Ho'), (225, 'Ac'), (261, 'Md'), (258, 'No'), (290, 'Og'), (20, 'Mg'), (115, 'Cd'), (238, 'Am'), (36, 'Cl')]

>>> symbol2name = read_elements('elements.txt4', 2, 3)
>>> symbol2name['Mt']
'Meitnerium'
>>> symbol2name['Tl']
'Thallium'
>>> symbol2name['Hg']
'Mercury'

>>> chemist('281Mt203Tl202Hg249Es291Fl247Cm167Er93Zr251Cf146Nd', symbol2name)
'MarieCurie'
>>> chemist('121Sb232Ac107Ag139Nd137Pr163Ho225Ac261Md258No290Og20Mg115Cd238Am36Cl', symbol2name)
'AmedeoAvogadro'