Electromagnetic Spectrum
Electromagnetic energy is light energy, which is thought to move in the form of waves.
Visible
light, infrared light, ultraviolet light and X rays all have different frequencies (the number of cycles that
pass a point per second).
Each wave has crests and troughs and the distance which encompasses one crest and one trough is the wavelength (given the symbol λ, measured in nm).
Light has a characteristic speed, 3.0 × 108 m/s
The band of light that the human eye can detect is called the visible spectrum (400 – 700 nm).
Each wavelength of light is associated with a different colour – a rainbow or continuous spectrum.
Atomic Structure
Recall that Neils Bohr was the scientist who proposed the planetary model of the atom.
The nucleus is in the centre of the atom and the circular orbitals radiate out from the centre - the further the orbital is from nucleus, the higher the energy of each electron in that shell.
Electrons can move up or down between the shells, but cannot exist between the orbits.
When the electrons are at the lowest possible energy, the atom is in the ground state.
If an electron jumps from a lower energy orbital to a higher energy orbital, the atom is in the excited state.
Emission Spectra & Fireworks
When an atom in the excited state loses energy and the electron drops back down to the ground state, the excess energy is lost in the form of coloured light.
Each atom has a characteristic colour, indicative of the size of the jump between orbits.
This is how fireworks get their colours.
Emission (or bright line) spectra is observed that has lines of colour separated by black regions.
Line spectra can be observed using a spectroscope – an instrument that separates light into its components colours using a prism.
Flame Tests
Since metal ions produce different colours of light, we can use this knowledge to determine which elements are present in unknown samples by carrying out flame tests.
Flame Test Procedure:
- Soak a wooden splint in water.
- Dip splint into the powdered sample.
- Place sample (on the splint) into a flame.
- Observe colour produced.
➣ Each element produces a unique colour. We can compare the unknown's colour to a list of known colours and determine the identity of the unknown.
This is one example of qualitative analysis.
Homework:
Section 1.4 Questions, p. 18 # 2-4
Watch this video and answer the following questions. Send your answers to me on Edsby. Alert! Alert! This is for marks!!!
- List out the substances (in order of appearance in the video) and the corresponding flame test colour.
- Identify the elements in the middle two pictures above (the picture found at the end of today's lesson).
