Atomic Structure - Electrons
Rutherford’s Model:
Atoms have negative charges surrounding their positive nucleus
What do we know about how positive and negative charges interact? They attract each other.
So, what does that tell us about electrons and the nucleus? They should be pulled towards each other.
Rutherford could not explain why electrons did not spiral in to the nucleus. His model suggested an unstable structure that would eventually collapse.
Bohr’s Solar System Model:
In 1913 Danish physicist Neils Bohr proposed an atomic model with the nucleus in the center and electrons rotating around the nucleus in orbits, similar to how the planets rotate around the Sun.
These electrons are located in specific energy levels or shells.
· The lower the energy level, the closer the electrons are to the nucleus.
· The lowest level is called the ground state, and is where the electrons are normally located.
· If energy is added to an electron, the electron will move to a new orbit. This orbit will be farther from the nucleus, and is a position of higher energy content. This new position is known as an excited state.
Energy levels are like rungs of a ladder – an electron cannot exist between energy levels. It jumps from one level to another by absorbing specific amounts of energy called quanta. (Maybe you’ve heard the phrase “quantum leap”?)
The excited state is unstable, so the electron will fall back and return to the lower energy level. When it does this it releases this same quanta of energy in the form of light, also called a photon.
Let’s talk about light:
Light travels in waves. All waves can be described in terms of amplitude, wavelength, frequency and speed.
Amplitude is the height of the wave. The brightness or intensity of light depends on the amplitude of the light wave.
Wavelength is the distance between successive crests of a wave. Different colors of light have different wavelengths.
Frequency tells how fast the wave oscillates up and down. It is measured as the number of times a light wave completes a cycle of upward and downward motion.
How are frequency and wavelength related? Inversely – as one goes up, the other goes down.
The electromagnetic spectrum consists of all of the different types of electromagnetic radiation, some of which we can see, and some which that can’t.
We already mentioned that different colors of light have different wavelengths. The visible range of light goes from about 400 to 750nm, with violet having the shortest wavelength, and red the longest.
A spectrum that contains only certain wavelengths, or colors, is called a line spectrum. Samples of all elements emit light when they are vaporized. Each element emits light of only certain wavelengths, and therefore has a unique line spectrum. This can be used as a kind of fingerprint to identify elements, and is also known as atomic emission.
Remember: Energy is released when an electron changes from a higher to lower Principal Energy Level or Sublevel.
When an atom releases energy a photon of light appears.
Line Spectra are the result of electrons falling from higher energy levels to lower ones (EXCITED to GROUND STATE).
Common atomic emission spectra:
Hydrogen (H)Principal lines (wavelengths in nm)
· 434
· 486
· 656 /
Helium (He)
Principal lines (wavelengths in nm)
· 447
· 471
· 492
· 502
· 588
· 668 /
An example question: