Electrons and atomic spectra

Lithium

Crimson red

Sodium

Yellow

Potassium

Lilac

Calcium

Brick red

Barium

Apple green

Copper

Blue-green

How to do a flame test?

• Clean nichrome wire with hydrochloric acid

• Dip nichrome wire into ionic compound

• Place the wire into a blue Bunsen burner flame

What does an atomic emission spectrum look like?

Black background with coloured lines representing different frequencies being emitted by the sample.

What causes the lines in an atomic emission spectrum?

• When the sample is heated, the electrons gain energy

• They get excited up to higher energy levels

• The quickly drop down to lower energy levels and emit energy

• Energy is generally emitted as light that can be seen as a line at a particular frequency in the emission spectrum

Why is there a limited number of lines on an emission spectrum?

As there is only a limited number of quantised energy levels, only certain drops to lower levels are possible.

• Energy released is particular to each possible drop

• Frequency= energy/Planck’s constant so each lot of energy released corresponds to a particular frequency of light

• Only particular frequencies can be emitted and as each frequency corresponds to a line on the spectrum, the can only be limited lines

Why did different elements have lines at different frequencies on their emission spectrum?

• Different elements have have differently spaced electronic energy levels

• Different amounts of energy are released as electrons drop to lower energy levels in different elements

• Since f=E/h, the different amounts of energy corresponds to different frequencies which correspond to different lines in the spectrum

Why do the lines in an emission spectrum occur in groups?

• Each group is caused by electrons dropping down to a specific energy level

• Another group is caused by electrons dropping down to a different energy level

Why do the lines in an emission spectrum get closer together at higher frequencies?

The electron energy levels in an atom get closer at higher energies.

Since f=E/h, the drops from the higher levels with have similar energies which will the cause similar frequencies. Therefore the lines will be close on the spectrum.

What is atomic absorption spectroscopy?

Electrons being excited up to higher energy levels in the sample absorbs photons of light

What does an atomic absorption spectrum look like?

Coloured background with black lines representing light of a different frequency

What causes the lines in an atomic absorption spectrum?

When visible light photons are passed through a gaseous sample of a substance, some of the frequencies correspond to the energy required to excite an electron. These frequencies are absorbed by the sample whilst others pass through. The frequencies that were absorbed cause black lines on the spectrum.

Why do the lines on the emission spectrum and absorption spectrum for an element occur at the same time?

The energy required to jump up energy levels is the same emitted when they drop back down. Since E=hf, the same frequency will be involved in both processes so the lines with be the same.

What is h in E=hf?

Planck’s constant

What does Planck’s constant equal?

6.63 ×10^-34

Does red have a long or short wavelength?

Long

Does red have a high or low frequency?

Low

Does red require more or less energy than blue?

Less

What is the equation linking energy to frequency of light emitted?

Change in energy= Planck’s constant x frequency

What equation links wavelength and frequency?

Speed of light = frequency x wavelength

What is the speed of light in a vacuum?

3 × 10^8