module StringMap = Map.Make(String)

let periodic_table =
  StringMap.empty
  |> StringMap.add "H" 1.008  (* Hydrogen *)
  |> StringMap.add "He" 4.0026 (* Helium *)
  |> StringMap.add "Li" 6.941  (* Lithium *)
  |> StringMap.add "Be" 9.0122 (* Beryllium *)
  |> StringMap.add "B" 10.811 (* Boron *)
  |> StringMap.add "C" 12.011 (* Carbon *)
  |> StringMap.add "N" 14.007 (* Nitrogen *)
  |> StringMap.add "O" 15.999 (* Oxygen *)
  |> StringMap.add "F" 18.998 (* Fluorine *)
  |> StringMap.add "Ne" 20.180 (* Neon *)
  |> StringMap.add "Na" 22.990 (* Sodium *)
  |> StringMap.add "Mg" 24.305 (* Magnesium *)
  |> StringMap.add "Al" 26.982 (* Aluminum *)
  |> StringMap.add "Si" 28.085 (* Silicon *)
  |> StringMap.add "P" 30.974 (* Phosphorus *)
  |> StringMap.add "S" 32.06  (* Sulfur *)
  |> StringMap.add "Cl" 35.45 (* Chlorine *)
  |> StringMap.add "K" 39.098 (* Potassium *)
  |> StringMap.add "Ar" 39.948 (* Argon *)
  |> StringMap.add "Ca" 40.078 (* Calcium *)
  |> StringMap.add "Sc" 44.956 (* Scandium *)
  |> StringMap.add "Ti" 47.867 (* Titanium *)
  |> StringMap.add "V" 50.942 (* Vanadium *)
  |> StringMap.add "Cr" 51.996 (* Chromium *)
  |> StringMap.add "Mn" 54.938 (* Manganese *)
  |> StringMap.add "Fe" 55.845 (* Iron *)
  |> StringMap.add "Ni" 58.693 (* Nickel *)
  |> StringMap.add "Co" 58.933 (* Cobalt *)
  |> StringMap.add "Cu" 63.546 (* Copper *)
  |> StringMap.add "Zn" 65.38  (* Zinc *)
  |> StringMap.add "Ga" 69.723 (* Gallium *)
  |> StringMap.add "Ge" 72.63  (* Germanium *)
  |> StringMap.add "As" 74.922 (* Arsenic *)
  |> StringMap.add "Se" 78.971 (* Selenium *)
  |> StringMap.add "Br" 79.904 (* Bromine *)
  |> StringMap.add "Kr" 83.798 (* Krypton *)
  |> StringMap.add "Rb" 85.468 (* Rubidium *)
  |> StringMap.add "Sr" 87.62  (* Strontium *)
  |> StringMap.add "Y" 88.906 (* Yttrium *)
  |> StringMap.add "Zr" 91.224 (* Zirconium *)
  |> StringMap.add "Nb" 92.906 (* Niobium *)
  |> StringMap.add "Mo" 95.95  (* Molybdenum *)
  |> StringMap.add "Tc" 98.0   (* Technetium *)
  |> StringMap.add "Ru" 101.07 (* Ruthenium *)
  |> StringMap.add "Rh" 102.91 (* Rhodium *)
  |> StringMap.add "Pd" 106.42 (* Palladium *)
  |> StringMap.add "Ag" 107.87 (* Silver *)
  |> StringMap.add "Cd" 112.41 (* Cadmium *)
  |> StringMap.add "In" 114.82 (* Indium *)
  |> StringMap.add "Sn" 118.71 (* Tin *)
  |> StringMap.add "Sb" 121.76 (* Antimony *)
  |> StringMap.add "I" 126.90 (* Iodine *)
  |> StringMap.add "Te" 127.60 (* Tellurium *)
  |> StringMap.add "Xe" 131.29 (* Xenon *)
  |> StringMap.add "Cs" 132.91 (* Cesium *)
  |> StringMap.add "Ba" 137.33 (* Barium *)
  |> StringMap.add "La" 138.91 (* Lanthanum *)
  |> StringMap.add "Ce" 140.12 (* Cerium *)
  |> StringMap.add "Pr" 140.91 (* Praseodymium *)
  |> StringMap.add "Nd" 144.24 (* Neodymium *)
  |> StringMap.add "Pm" 145.0  (* Promethium *)
  |> StringMap.add "Sm" 150.36 (* Samarium *)
  |> StringMap.add "Eu" 151.96 (* Europium *)
  |> StringMap.add "Gd" 157.25 (* Gadolinium *)
  |> StringMap.add "Tb" 158.93 (* Terbium *)
  |> StringMap.add "Dy" 162.50 (* Dysprosium *)
  |> StringMap.add "Ho" 164.93 (* Holmium *)
  |> StringMap.add "Er" 167.26 (* Erbium *)
  |> StringMap.add "Tm" 168.93 (* Thulium *)
  |> StringMap.add "Yb" 173.05 (* Ytterbium *)
  |> StringMap.add "Lu" 174.97 (* Lutetium *)
  |> StringMap.add "Hf" 178.49 (* Hafnium *)
  |> StringMap.add "Ta" 180.95 (* Tantalum *)
  |> StringMap.add "W" 183.84 (* Tungsten *)
  |> StringMap.add "Re" 186.21 (* Rhenium *)
  |> StringMap.add "Os" 190.23 (* Osmium *)
  |> StringMap.add "Ir" 192.22 (* Iridium *)
  |> StringMap.add "Pt" 195.08 (* Platinum *)
  |> StringMap.add "Au" 196.97 (* Gold *)
  |> StringMap.add "Hg" 200.59 (* Mercury *)
  |> StringMap.add "Tl" 204.38 (* Thallium *)
  |> StringMap.add "Pb" 207.2  (* Lead *)
  |> StringMap.add "Bi" 208.98 (* Bismuth *)
  |> StringMap.add "Po" 209.0  (* Polonium *)
  |> StringMap.add "At" 210.0  (* Astatine *)
  |> StringMap.add "Rn" 222.0  (* Radon *)
  |> StringMap.add "Fr" 223.0  (* Francium *)
  |> StringMap.add "Ra" 226.0  (* Radium *)
  |> StringMap.add "Ac" 227.0  (* Actinium *)
  |> StringMap.add "Pa" 231.04 (* Protactinium *)
  |> StringMap.add "Th" 232.04 (* Thorium *)
  |> StringMap.add "Np" 237.0  (* Neptunium *)
  |> StringMap.add "U" 238.03 (* Uranium *)
  |> StringMap.add "Am" 243.0  (* Americium *)
  |> StringMap.add "Pu" 244.0  (* Plutonium *)
  |> StringMap.add "Cm" 247.0  (* Curium *)
  |> StringMap.add "Bk" 247.0  (* Berkelium *)
  |> StringMap.add "Cf" 251.0  (* Californium *)
  |> StringMap.add "Es" 252.0  (* Einsteinium *)
  |> StringMap.add "Fm" 257.0  (* Fermium *)
  |> StringMap.add "Md" 258.0  (* Mendelevium *)
  |> StringMap.add "No" 259.0  (* Nobelium *)
  |> StringMap.add "Lr" 262.0  (* Lawrencium *)
  |> StringMap.add "Rf" 267.0  (* Rutherfordium *)
  |> StringMap.add "Db" 268.0  (* Dubnium *)
  |> StringMap.add "Sg" 269.0  (* Seaborgium *)
  |> StringMap.add "Bh" 270.0  (* Bohrium *)
  |> StringMap.add "Hs" 269.0  (* Hassium *)
  |> StringMap.add "Mt" 278.0  (* Meitnerium *)
  |> StringMap.add "Ds" 281.0  (* Darmstadtium *)
  |> StringMap.add "Rg" 282.0  (* Roentgenium *)
  |> StringMap.add "Cn" 285.0  (* Copernicium *)
  |> StringMap.add "Nh" 286.0  (* Nihonium *)
  |> StringMap.add "Fl" 289.0  (* Flerovium *)
  |> StringMap.add "Mc" 289.0  (* Moscovium *)
  |> StringMap.add "Lv" 293.0  (* Livermorium *)
  |> StringMap.add "Ts" 294.0  (* Tennessine *)
  |> StringMap.add "Og" 294.0  (* Oganesson *)

let element_find_closest s : string list = 
  StringMap.filter (fun k _ -> String.get k 0 = String.get s 0) periodic_table
  |> StringMap.to_list
  |> List.map (fun x -> fst x)