No.because atomic number indicates the number of protons,electrons,neutrons in an atomic.we cant count some thing in it is not possible to have an element with atomic number1.5.
1 2 1
" the atomic number of any of the given element would be a whole number "
the examples of the elements with their elements are -
1. Carbon - 6
2. Oxygen - 8 
∴ it is impossible to have a element with a atomic number 1.5 which is needed to be placed in between of hydrogen and helium