What is an emission spectrum? Use the Bohr model to explain why the emission spectrum of the hydrogen atom consists of distinct lines at specific wavelengths.

The Bohr model is an essential concept in understanding atomic structure and behavior, particularly for the hydrogen atom. Proposed by Niels Bohr in 1913, this model suggests that electrons orbit the nucleus in specific paths or 'shells'—something akin to planets orbiting a sun but on a microscopic scale. Unlike the planetary model, electrons can only exist in certain allowed orbits with fixed radii, meaning they have quantized energy levels.

This model was a key step away from the classical mechanics that couldn't explain atomic phenomena. Bohr introduced the concept of quantization into atomic structure, which states that electrons orbit at discrete energy levels, with nothing in between. An electron can jump from one energy level to another but cannot exist in a transitional state. These jumps will absorb or emit energy corresponding to the difference in energy levels, which is where the idea of the emission spectrum comes into play.

Electromagnetic radiation is a form of energy that is present all around us and takes various forms, such as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. These different types of electromagnetic radiation make up the electromagnetic spectrum and are characterized by their wavelengths and frequencies. Visible light, the part of the spectrum observable by the human eye, plays a key role in the study of emission spectra.

When discussing the Bohr model, we are particularly interested in the visible spectrum emitted by atoms. Emission occurs when an electron in a higher energy state transitions to a lower state, releasing energy in the form of electromagnetic radiation. The wavelength of this radiation is specific to the energy difference between those two levels, which is why in the emission spectrum of the hydrogen atom, one sees distinct color lines corresponding to these transitions.

Quantized energy levels are a fundamental aspect of the Bohr model and quantum theory at large. The term 'quantized' means that something is restricted to discrete values rather than a continuous range. This is akin to climbing a staircase; you can stand on one step or another, but not between steps.

In the context of an atom, quantized energy levels mean that an electron can only have specific amounts of energy and occupy certain energy levels. The energy difference between these levels can be calculated and is associated with the energy of light emitted or absorbed as an electron moves between these levels. The transition of electrons between these quantized levels is what gives rise to the distinct lines seen in an emission spectrum since each transition results in the emission of light at a very specific energy and therefore a specific wavelength.

The study of the hydrogen atom's spectrum, or hydrogen atom spectroscopy, involves analyzing the light emitted or absorbed by hydrogen to understand its quantum mechanical properties. Because hydrogen is the simplest atom with just one electron, it's a prime subject in early quantum studies and makes it easier to observe the principles of quantum mechanics in action without the complications of larger, multi-electron atoms.

In practice, when a hydrogen atom absorbs energy, its lone electron can become 'excited' to a higher-energy orbit. As it returns to a lower energy state, it emits light of particular wavelengths, which we can record as an emission spectrum. Each line in the spectrum represents a specific electronic transition from a higher to a lower energy level. Advanced techniques such as spectroscopy make it possible to precisely measure these wavelengths, thus providing valuable experimental support for quantum theory and the Bohr model.