The Bohr model is applicable only to hydrogen-like species, which contain only one electron (e.g., H, He⁺, Li²⁺, Be³⁺) .

1. Hydrogen atom: Contains 1 electron (Valid).
2. Deuteron: Isotope of hydrogen, contains 1 electron (Valid).
3. Singly ionised helium (He⁺): Atomic number $Z=2$. Electrons = $2-1=1$ (Valid) , .
4. Singly ionised neon (Ne⁺): Atomic number $Z=10$. Electrons = $10-1=9$. Since it is a multi-electron system, the Bohr model is not valid , .