Question

Why was the early 20th century model of the atom flawed

Answer 1

I’ll assume you mean Rutherford’s model: the nuclear model. The nuclear model suggested there was a small tiny positive nucleus in the centre of an atom and delocalised electrons were in a cloud around it. That couldn’t be true because if the electrons were in a cloud around the positive nucleus, they would attract and the atom would collapse in on itself. So in 1913 Niels Bohr suggested that electrons orbit the nucleus in fixed positions, shells.