Title: Lesson 2 The EM Spectrum and Emission Line Spectra

Learning Objectives:

• Understand how we can use the EM spectrum to study the behaviour of electrons within atoms

The Electromagnetic Spectrum

• Waves that travel at the speed of light in a vacuum (3.0 x 108 m/s)

• These ‘waves’ display properties of a particle so we can refer to them as photons.

• Visible light – White light that can be split up into the colours of the spectrum.

Waves• All EM waves travel at the same speed (c) but have different

wavelengths (λ). (E.g. Red is longer than blue)• Number of waves passing a particular point in 1s is called the

frequency (v)

Continuous spectrum

Continuous spectrum of all the colours of the visible light region.

Atomic Absorption and Emission Line Spectra – The hydrogen atom spectrum

• When EM radiation is passed through a group of atoms some radiation is absorbed.

• This will excite atoms from a lower energy level to a higher energy level.

• A spectrometer analyses the transmitted radiation to the incident radiation.

Absorption and Emission Spectrum• When white light is passed through a sample of

hydrogen gas, an absorption spectrum is produced.

• If high voltage is applied to the gas, a corresponding emission line spectrum is produced.

Different elements have different line spectra, so they can be used like barcodes to identify unknown elements.

Line spectrum

Energy is supplied to individual elements, a spectrum of particular

wavelengths/colours is seen.

Line spectrum consists of discrete lines

Lines converge towards the higher end (violet) of the spectrum.

Ultra violet and infrared region

A similar series of lines at an even higher energy occurs in the UV region of the spectrum, this also happens at lower

energy in the IR region of the spectrum.

Notice that the lines converge at higher energies.

Hydrogen line spectra

Explanation of the emission spectra

When energy is supplied to an atom electrons are excited (gain

energy) from their lowest (ground) state to an excited state.

Explanation of the emission spectra

• Electrons can only exist in a certain energy level. When the electron jumps to a higher level it is UNSTABLE.

• When the electrons drop from a higher level to a lower level they emit energy.

• This energy corresponds to a particular wavelength and shows up as a line spectrum.

Explanation of the emission spectra

When electrons return to the first energy level (n=1) the series of lines occurs in the

UV region, as this involves the largest energy change.

Explanation of the emission spectra

The visible region spectrum is formed by electrons dropping back to n=2

level.

Explanation of the emission spectra

The lines in the IR region is due to electrons dropping to the n=3

level.

Explanation of the emission spectra

The lines in the spectrum converge because the energy levels themselves converge.

As we have learned, electrons exist in very specific energy levels. And when these electrons absorb energy…They get energized up to higher levels. Actually, the jump to higher levels is not a gradual transition as was just shown.

It is a “quantum” jump, and looks more like this:

Quantum means it happens all at once – instantaneously – because the electron can never exist between levels – not even for a second.

Once it is at this higher level (excited state), it doesn’t stay there long. It quickly drops down to a lower level – again as a quantum leap – and as it does, it gives off a distinct band of light energy.Also, notice how the electron doesn’t have to drop all the way back down to the lowest level. It can get energized up to any level, and from there it can drop to any lower level. AND the different drops each produce different frequencies of light.

See how an electron dropping from the 3rd level to the 2nd level produced red lightA 4 2 electron drop produces blue lightAnd a 5 2 drop produces violet light

How the lines arise in an emission spectrum

Why do we get a line spectrum?

• Due to the quantum jump between energy levels we get the line spectrum. It is a discrete change.

• If electrons could move between energy levels we would instead get a continuous spectrum.

Convergence

• Eventually, at the convergence limit, the lines merge to form a continuum.

• Beyond this point the electron can have any energy.

• It is free from the influence of the nucleus• It is now outside the atom

The Hydrogen Spectrum

Gives out energy when electrons drop energy levels

n=1 UV spectrumn=2 Visible light

spectrumn=3 IR spectrum

Why do the lines converge on the emission spectrum?

• Lines converge at higher energies because higher energy levels inside the atom are closer together.

• n= ∞ means the electron is not inside the atom, it has been ionised.

• Ground state to n= ∞ is called the IONISATION ENERGY.

NOTE:Any electron in a hydrogen atom that has been promoted and then falls back down to the nth level will be: n=1 (Lyman series) n=2 (Balmer series) n=3 (Paschen series)

Energy of the Photon of Light

Energy of photon= (E2-E1)

E2 (High energy)

E1 (Low energy)

Complete the test yourself questions

• Page 66• Questions 8-10• Check your answers on page 559