Answer:
He developed the concept of concentric electron energy levels
Explanation:
Before Bohr's model, Rutherford's model was proposed. This model explains most of the properties of the atom but failed to explain the stability of the atom.
As per Rutherford's model, electrons revolve around the nucleus in the orbit.
But revolving electron in their orbit around nucleus would give up energy and so gradually move towards the nucleus and therefore, eventually collapse.
Bohr's proposed that the electrons around the nucleus move orbit of fixed energy called "stationary states". Electrons in these stationary states do not radiate energy.
Therefore, proposal of concentric electron energy levels refine the atomic models.
Before we describe the phases of the Moon, let's describe what they're not. Some people mistakenly believe the phases come from Earth's shadow cast on the Moon. Others think that the Moon changes shape due to clouds. These are common misconceptions, but they're not true. Instead, the Moon's phase depends only on its position relative to Earth and the Sun.
The Moon doesn't make its own light, it just reflects the Sun's light as all the planets do. The Sun always illuminates one half of the Moon. Since the Moon is tidally locked, we always see the same side from Earth, but there's no permanent "dark side of the Moon." The Sun lights up different sides of the Moon as it orbits around Earth – it's the fraction of the Moon from which we see reflected sunlight that determines the lunar phase.
