code: def center_of_mass(elements, coordinates):
      
      mass = molecular_weight(elements)
      mass_array = np.array([[atomic_mass[i.upper()]] * 3 for i in elements])
      mass_coordinates = coordinates * mass_array
      return (np.sum(mass_coordinates, axis=0) / np.array([mass, mass, mass]))
docs: Return the centre of mass (COM).
  
      Parameters
      ----------
      elements : numpy.ndarray
          An array of all elements (type: str) in a molecule.
  
      coordinates : numpy.ndarray
          An array containing molecule's coordinates.
  
      Returns
      -------
      numpy.ndarray
          An 1d array with coordinates of the centre of mass including elements'
          masses.