What is the next number: 3, 11, 19, 37, 55, XX?

87 -- Francium

Metal Li Na K Rb Cs Fr
Configuration of outer electron layer of atom 2S1 3S1 4S1 5S1 6S1 7S1
Atomic number 3 11 19 37 55 87
Relative Atomic mass 6.939 2.9898 39.098 85.47 132.905 *
Atomic radius (nanometre) 0.155 0.189 0.236 0.248 0.268 0.280
Density (g/cm3) 0.534 0.97 0.86 1.53 1.88 2.4
Melting point (0c) 174.54 97.8 63.7 38.89 28.4 27.1
Boiling point (0c) 1347 882-889 774 688 678 677


Bannockburn

Also, the numbers listed above are every fourth number where all digits are odd. So the next number would be 73, followed by 91 and 99.