Energy Levels

Quantum Physics Lesson 4

Learning Objectives

Explain what is meant by the term Explain what is meant by the term energy level.energy level.

Describe what happens when excited Describe what happens when excited atoms de-excite.atoms de-excite.

Calculate the energy of emitted photons Calculate the energy of emitted photons using the equation hf = Eusing the equation hf = E11 - E - E22..

Explain how a fluorescent tube works Explain how a fluorescent tube works HWKHWK

Homework

Use books and the internet to research how fluorescent tubes work.

How do fluorescent tubes work? Why are they used? Use the words electrons, atoms, photons,

ionisation and excitation in your answer. Do not copy and paste put in your own

words! Reference your work properly.

What is Quantisation?

Calculate the gravitational potential energy of your sweet to win it.

Notice that each sweet can only have a certain specific energy determined by the flat surfaces in the room.

When something can only take certain discrete values we say it is quantised e.g. shoe size.

Definitions

Ground State

Excited State

An energy input raises the electrons to higher energy levels.

The lowest energy level of an atom.

When one or more of an atom’s electrons moves to an outer shell at higher energy.

Energy Levels The electrons in an atom are trapped because

they are attracted to the protons in the nucleus.

Their energies are determined by which shell they are in: nearer the nucleus = less energy.

But just like the sweets resting on the desk, each electron can only take certain discrete energies which correspond to the shells or orbits.

VideoVideo

BBC The Atom – Jim Al-Khalili 3/6 BBC The Atom – Jim Al-Khalili 3/6 0:00-9:100:00-9:10

QuantisationQuantisation A quantum mechanical system or particle that is A quantum mechanical system or particle that is

bound, confined spatially, can only take on bound, confined spatially, can only take on certain discrete values of energy, as opposed to certain discrete values of energy, as opposed to classical particles, which can have any energy.classical particles, which can have any energy.

These values are called These values are called energy levelsenergy levels. . The term is most commonly used for the energy The term is most commonly used for the energy

levels of electrons in atoms or molecules, which levels of electrons in atoms or molecules, which are bound by the electric field of the nucleus.are bound by the electric field of the nucleus.

The energy spectrum of a system with energy The energy spectrum of a system with energy levels is said to be quantised.levels is said to be quantised.

Energy LevelsEnergy Levels

“Why do the states have negative energy?” •This is because the zero of energy is considered to be that of a free electron 'just outside' the atom. •All energy states 'below' this – i.e. within the atom are therefore negative. •Energy must be put into the atom to raise the electron to the 'surface' of the atom and allow it to escape.

Well Analogy

Imagine you are stuck down a hole…with a number of ledges.

You could try to jump to get out but unless you jump high enough to reach one of the ledges you will just fall back down releasing energy as sound energy.

It’s a bit like this in the atom the electrons need the ionisation energy to get out.

De-excitationDe-excitation

Whenever these is a vacancy in a shell an electron from a outer shell moves down to fill it.

When this happens the electron emits a photon.

The photon has an energy equal to the difference between the energy levels.

De-excitationDe-excitation

Equation

In words:-

In Symbols:-

levelsenergybetweendifferencephotonemittedofEnergy

21 EEhf Note that E1 and E2 refer to the energies of the energy levels and hf is the energy of the photon emitted when an electron falls from the higher level to the lower level.

Excitation using Photons An electron in an atom can absorb a

photon and move to a higher energy level But only if the photon energy matches

the difference between the energy levels.

This is not true for ionisation; as long as the energy of the incoming photon is greater than the ionisation energy, the electron can escape.

Electron in orbit round a nucleus in an atom

Energy input. The electron is excited and rises to a higher energy level (shell).

The electron falls back to its original energy level and energy is emitted in the form of radiation. The bigger the drop the greater the energy emitted and the shorter wavelength the radiation has (blue light).

Fluorescence When an electron in a higher energy

level de-excites it can either:-

Go straight down to the ground state one photon.

Cascade down several energy levels several photons, each of lower energy.

An atom can absorb UV but emit visible light.

The energy levels and The energy levels and spectra seriesspectra series