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MOODOMETER

Radioactivity

The atom

orbiting electrons

Nucleus (protons and neutrons)

Nuclide notation

Li3

7

Proton number (Z) = number of protons

Nucleon number (A) = number of protons and neutrons

Neutron number (N) = A - Z

Isotopes

Li3

7

It is possible for the nuclei of the same element to have different numbers of neutrons in the nucleus (but it must have the same number of protons)

Li3

6

Isotopes

Li3

7

For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7

Li3

6

4 neutrons3 neutrons

Isotopes of Hydrogen

H1

1

The three isotopes of Hydrogen even have their own names!

H1

2

H1

3

Hi! I’m hydrogen

They call me

deuterium

Hola! Mi nombre es tritium y yo

soy de Madrid!

How do we know the structure of the atom?

The famous Geiger-Marsden Alpha scattering experiment

In 1909, Geiger and Marsden were studying how alpha particles are scattered by a thin gold foil.

Alpha source

Thin gold foil

Geiger-Marsden

As expected, most alpha particles were detected at very small scattering angles

Alpha particles

Thin gold foil Small-angle scattering

Geiger-Marsden

To their great surprise, they found that some alpha particles (1 in 20 000) had very large scattering angles

Alpha particles

Thin gold foil Small-angle scattering

Large-angle scattering

Explaining Geiger and Marsdens’ results

The results suggested that the positive (repulsive) charge must be concentrated at the centre of the atom. Most alpha particles do not pass close to this so pass undisturbed, only alpha particles passing very close to this small nucleus get repelled backwards (the nucleus must also be very massive for this to happen).

nucleus

Rutherford did the calculations!

Rutherford (their supervisor) calculated theoretically the number of alpha particles that should be scattered at different angles. He found agreement with the experimental results if he assumed the atomic nucleus was confined to a diameter of about 10-15 metres.

Rutherford did the calculations!

That’s 100 000 times smaller than the size of an atom (about 10-10 metres).

Stadium as atom YouTube - Structure of the Atom 3: The Rutherford Model

If the nucleus of an atom was a ping-pong ball, the atom would be the size of a football stadium (and mostly full of nothing)!

http://www.youtube.com/watch?v=XBqHkraf8iE

Nucleus (ping-pong ball

Limitations of this model?

• According to the theory of electromagnetism, an accelerating charge (and the orbiting electrons ARE accelerating centripetally) should radiate energy and thus spiral into the nucleus.

Evidence for atomic energy levels

Evidence for atomic energy levels

When a gas is heated to a high temperature, or if an electric current is passed through the gas, it begins to glow.

cathode anode

electric current

Light emitted

Low pressure gas

Emission spectrum

If we look at the light emitted (using a spectroscope) we see a series of sharp lines of different colours. This is called an emission spectrum.

Absorption Spectrum

Similarly, if light is shone through a cold gas, there are sharp dark lines in exactly the same place the bright lines appeared in the emission spectrum.

Some wavelengths missing!Light source gas

Why?

Scientists had known about these lines since the 19th century, and they had been used to identify elements (including helium in the sun), but scientists could not explain them.

Niels Bohr

In 1913, a Danish physicist called Niels Bohr realised that the secret of atomic structure lay in its discreteness, that energy could only be absorbed or emitted at certain values.

At school they called me “Bohr the

Bore”!

The Bohr Model

Bohr realised that the electrons could only be at specific energy levels (or states) around the atom.

The Bohr Model

We say that the energy of the electron (and thus the atom) can exist in a number of states n=1, n=2, n=3 etc. (Similar to the “shells” or electron orbitals that chemists talk about!)

n = 1

n = 3

n = 2

The Bohr Model

The energy level diagram of the hydrogen atom according to the Bohr model

n = 1 (the ground state)

n = 2

n = 3

n = 4n = 5

High energy n levels are very close to each other

Energy eV

-13.6

0

Electron can’t have less energy than this

The Bohr ModelAn electron in a higher state than the ground state is called an excited electron.

High energy n levels are very close to each other

n = 1 (the ground state)

n = 2

n = 3

n = 4n = 5

-13.6

Energy eV

0

electron

Atomic transitions

If a hydrogen atom is in an excited state, it can make a transition to a lower state. Thus an atom in state n = 2 can go to n = 1 (an electron jumps from orbit n = 2 to n = 1)

n = 1 (the ground state)

n = 2

n = 3

n = 4n = 5

-13.6

Energy eV

0

electronWheeee!

Atomic transitions

Every time an atom (electron in the atom) makes a transition, a single photon of light is emitted.

n = 1 (the ground state)

n = 2

n = 3

n = 4n = 5

-13.6

Energy eV

0

electron

Atomic transitions

The energy of the photon is equal to the difference in energy (ΔE) between the two states. It is equal to hf. ΔE = hf

n = 1 (the ground state)

n = 2

n = 3

n = 4n = 5

-13.6

Energy eV

0

electron

ΔE = hf

The Lyman Series

Transitions down to the n = 1 state give a series of spectral lines in the UV region called the Lyman series.

n = 1 (the ground state)

n = 2

n = 3

n = 4n = 5

-13.6

Energy eV

0

Lyman series of spectral lines (UV)

The Balmer Series

Transitions down to the n = 2 state give a series of spectral lines in the visible region called the Balmer series.

n = 1 (the ground state)

n = 2

n = 3

n = 4n = 5

-13.6

Energy eV

0

UV

Balmer series of spectral lines (visible)

The Pashen Series

Transitions down to the n = 3 state give a series of spectral lines in the infra-red region called the Pashen series.

n = 1 (the ground state)

n = 2

n = 3

n = 4n = 5

-13.6

Energy eV

0

UV

visible

Pashen series (IR)

Emission Spectrum of Hydrogen

Which is the emission spectrum and which is the absorption spectrum?

The emission and absorption spectrum of hydrogen is thus predicted to contain a line spectrum at very specific wavelengths, a fact verified by experiment.

Pattern of lines

Since the higher states are closer to one another, the wavelengths of the photons emitted tend to be close too. There is a “crowding” of wavelengths at the low wavelength part of the spectrum

n = 1 (the ground state)

n = 2

n = 3

n = 4n = 5

-13.6

Energy eV

0

Spectrum produced

How do you excite an atom?

1. Heating to a high temperature

2. Bombarding with electrons

3. Having photons fall on the atom

I’m excited!

Limitations of the Bohr Model

1. Can only treat atoms or ions with one electron

2. Does not predict the intensities of the spectral lines

3. Inconsistent with the uncertainty principle (see later!)

4. Does not predict the observed splitting of the spectral lines

Forces in the nucleus

The Coulomb Force

• The repulsive force between protons in the nucleus

+

+

The Strong Force

The nucleons (protons and neutrons) in the nucleus are bound together by the strong nuclear force

The Strong Force

• acts over short distance (10-15 m)

• acts only between adjacent particles in the nucleus

• is carried by gluons

Questions!

Page 372, Questions 1, 4, 6, 8, 10, 11.