Atomic Structure HL – Ionization Energies from Emission Spectra

So, you are around two months into your IB Chemistry course. How is it going? Easy? Difficult? Indifferent?

To start with your teacher or teachers have probably taught you topic 1 (Stoichiometric Relationships) or topic 2 and 12 (Atomic Structure). It doesn’t really matter which was around they have taught you but I would be really surprised if they had started with something different.

Part of the HL atomic structure part of the course involves studying  ionization energies and how they provide evidence for shells and  sub-shells and also studying atomic emission spectra. Further to this the new course has cleverly gelled these two ideas together and you are also expected to be able to calculate ionization energies from emission spectra data.

The calculation can seem a little daunting as the numbers involved are both very large and very small – but don’t let this put you off. I have put together a set of steps that will (I hope!) help you understand this and successfully calculate the value.

1, Take the reciprocal of the wavelength (ie, put 1 over the value)

2, This will give you a wavelength in cm, you need to convert the value into m so divide by 100.

3, Calculate the frequency using c = λ

* c = speed of light (you can get this from the data book). The units of v are s^-1

4, Calculate the energy to remove this single electron using E = hv (h = Planck’s constant – see data book again). This will give you the energy to remove one electron in Joules.

5, Multiply the value by 6.02 x 10 ²³ (ie, Avagadro’s constant) to get the value in J mol ^-1

6, Finally, divide by 1000 to get the answer in KJ mol ^-1.

Phew!

Did you understand this? Do you have any further questions?

Try this question to test your understanding:

Question: Given that the wavelength of convergence for Vanadium is 54411.7 cm ^-1, calculate the first ionisation energy for the element.

Please post your answer below and I’ll let you know if it is correct – I need to see your working out though – don’t go cheating and looking it up in the data book!

Good luck and I look forwards to seeing your answers.

Next month my intention is to pick up on some other ‘new’ areas of the course that you will have been taught / will be taught very soon.