3. What is the difference between an emission spectrum and an absorption spectrum? What is the Bohr model of the atom, and how does it explain both emission and absorption spectra?

The Bohr model resembles a planetary system in which the negatively-charged electrons orbit a small and very dense, positively-charged nucleus at the atom's center.

The electrons are held in orbit by the Coulomb (electrical) force between the positively-charged nucleus and the negatively-charged electrons.

The electrons cannot occupy just any orbital radius.

Only orbits with a very specific set of energy values are permitted (which all atoms of a given element have in common and are unique to that element).

The lowest energy (or ground state) corresponds to orbit closest to the nucleus and photons with specific amounts of electromagnetic radiation are absorbed or emitted when an electron moves from one orbit to another (absorbed to move further up the permitted levels and away from the nucleus)

An atomic line spectrum is the whole range of specific photon radiation frequencies that an element can emit or absorb as it's electrons move between the energy levels allowed in those atoms.

The emissions correspond with electrons descending 'down' their energy levels, with the energy differences being carried away by photons with the appropriate frequency. Consequently an emission spectra is a series of specific, single color lines (against a black background) for each of the emitted frequencies.

Photon absorption provides the energy for electrons to 'climb' the set of energy levels for that element. So, putting electrons into higher energy states within an atom.

When the absorbed photons are removed from incident light containing the full spectrum, their absence is seen as a series of fine black lines on an otherwise continuous spectrum background.

The features in absorption and emission spectra coincide exactly for atoms of a given element.