Atomic Structure

IB Physics Power Points

Topic 07 and 13

Atomic and Nuclear Physics

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Part 1 Development of an Atomic Model

Democritus to Niels Bohr

Early ideas about atomsDemocritus (~ 400 BC) proposed that matter was composed of indivisible particles he called “atomos”.

This idea was largely ignored until John Dalton (1766-1844) proposed his atomic theory as a model for matter

While this theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today instead of alchemy

Dalton’s Atomic Theory - Summary

1. matter is composed, indivisible particles (atoms)

2. all atoms of a particular element are identical

3. different elements have different atoms

4. atoms combine in certain whole-number ratios

5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.

Dalton’s theory states

1. matter is composed of indivisible particles called atoms. (Dalton visualized atoms as dense spheres – not unlike billiard balls)

However, the discovery of the electron by J.J. Thomson in the late 1800’s introduced the idea that there might be something smaller than atoms.

To include electrons, scientists had to modify the Dalton atomic model. The new vision was a positive sphere that had the newly discovered negative electrons embedded in it. This was called the plum pudding model.

In the early 1900’s, Ernest Rutherford tested the plum pudding model with his famous gold foil experiment.

a

More great info @ http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

Predicted results of gold foil experiment if the Plum Pudding model had been correct.

This is NOT what was observed.

What was observed?

To explain the results, Rutherford proposed a new model that included a positive center for the atom called the nucleus.

Adding the evidence from emission spectra

Colored lights do not emit all the wavelengths of the visible light spectrum. For example, a red light emits mostly wavelengths from the red end of the spectrum. An energized gas sample will emit light of specific wavelengths characteristic of the gas. This is called a line spectrum

Emission spectra are unique for each element

The Bohr model of the atom • developed using information from

hydrogen emission spectrum studies.

• a central dense positive nucleus composed of protons and neutrons.

• negative electrons orbit the nucleus like planets around the Sun (but not flat like the solar system)

• mostly empty space. Nucleus is 10-5 times smaller than atom.

Orbiting electrons occupy discrete energy levels!

Electrons can only “jump” between energy levels if they absorb or emit a specific amount of energy.

Atoms have quantized energy states!

The line spectrum of hydrogen as a direct result of energized electrons releasing a specific amount of energy by emitting a photon of light of a certain wavelength.

The different lines in the hydrogen spectrum where evidence for a number of different energy levels.

lower energylonger wavelength

higher energyshorter wavelength

Visible spectrum for

hydrogen atomconvergence

Limitations of the Rutherford Bohr Nuclear Model

Why do electrons orbit the nucleus? i.e. model does not explain why the electrons orbit rather than spiral towards the nucleus.

Why is there one mass of positive charge in the nucleus? i.e. or if there are multiple positive particles why does the nucleus not fly apart?

Part 2 Atomic Structure – The Basics

Nuclide A specific atomic structure as determined by the number of protons and neutrons in the nucleus

Nucleon A nuclear particle (protons and neutrons)

Mass Number (A)

The total number of nucleons (protons + neutrons)

Atomic Number (Z)

The number of protons in the nucleus

Isotope Two atoms of the same element (same Z) but with different masses (A). The mass difference between isotopes is due to varying numbers of neutrons.

Atomic Notation

Show the mass number and atomic number

in A-Z notation (atomic number often

omitted)

Show the name of the element, and the

mass number in hyphen notation

sodium-23

atomic number Z

Na1123mass number

A

Forces in the Nucleus