Emission Spectra Bohr Model of the Atom

Emission When current is passed through a gas at low pressure the atoms of the gas change from being in a ground energy state to an excited energy state. The atoms can go back to a ground energy state by releasing some of the excess energy in the form of light. Different elements produce different colors of light.

Wavelengths When current is passed through hydrogen gas it gives off a characteristic pinkish light. If this light is passed through a prism it can be separated into the different frequencies of light that make it up. The resulting bands of light are known as hydrogen’s line emission spectrum.

Emission Spectra Demonstrations Element Wavelengths of Spectra Lines Hydrogen Helium

Hydrogen Red- 656.3nm Blue- 486.1nm Indigo- 434nm Violet- 410.0nm

Helium Red: 667.8nm Yellow: 587.5nm Blue: 501.5nm, 492.1nm, 471.3 nm Indigo: 447.1nm Violet: 402.8 nm

Neon and Argon Neon and Argon each have many different emission lines. Therefore, each produces many different wavelengths of light.

How do the wavelengths of light get produced? When an atom becomes “excited” the electrons in the atom move to a higher energy level. When the atom returns to its ground state the electron falls back down to a lower energy level. In order to reach the lower energy level the electron releases energy in the form of light.

How is light produced cont’d…. When the electrons return to a lower energy level they release photons of light. Scientists expected elements to release photons of all colors, instead the elements only released photons of certain colors. These photons carry a specific amount of energy. (EPhoton=hv) So each element is releasing only specific amounts of energy!

Why are only certain amounts of energy released? The amount of energy released is equal to the energy difference between the two energy levels. This means that the energy differences between the atoms’ energy states are fixed. Electrons of each element can only exist at very specific energy states.

Hydrogen Emission → Bohr Model Because only certain amounts of energy are being released, (certain frequencies of light) Neils Bohr hypothesized that electrons in the atoms could only exist at specific distances or energy levels from the nucleus. These levels are quantized- electrons must absorb or release a specific amount of energy to move between them. Electrons can’t have any amount of energy they want. There are only certain amounts of energy they can have.

Bohr Model Also known as the “planetary model.” Positively charged nucleus as Rutherford discovered. Electron orbiting the nucleus at specific energy levels. Energy levels furthest away from the nucleus have the most energy. Each energy level can only hold a specific number of electrons. Bohr model of Hydrogen fit the line emission spectra of hydrogen perfectly but it didn’t fit the other elements very well.