thumb|Bohr model in 1921 after Sommerfeld expansion of 1913 model showing maximum electrons per shell with shells labeled in X-ray notation Until the second decade of the 20th century, atomic models were generally speculative. Even the concept of atoms, let alone atoms with internal structure, faced opposition from some scientists.
thumb|Bohr model in 1921 after Sommerfeld expansion of 1913 model showing maximum electrons per shell with shells labeled in X-ray notation Until the second decade of the 20th century atomic models were generally speculative. Even the concept of atoms, let alone atoms with internal structure, faced opposition from some scientists.