In order to use the number of electrons in an atom to identify it, we need to know that there is a relationship between number of electrons, protons and atomic number (assuming that this is a neutral atom). In a neutral atom, all three values are equal z = p = e, and in this element we have 55 electrons because Krypton represents 36 electrons and we 19 more in the shells, therefore 55 electrons, and we will also have 55 of atomic number, and this element will be Cs, Caesium or Cesium.