static get maxBonds() {
    return {
      'H': 1,
      'C': 4,
      'N': 3,
      'O': 2,
      'P': 3,
      'S': 2,
      'B': 3,
      'F': 1,
      'I': 1,
      'Cl': 1,
      'Br': 1
    };
  }
A map mapping element symbols to their maximum bonds.