Lecture 2

The Atom : elements, discovery of electron and nucleus

Throughout the centuries, alchemists and chemists had gained a substantial body of knowledge about the properties of materials. In particular, it was known that some materials could be combined together to form materials with different properties, and, vice-versa, some material could be broken down into more elementary components.

Materials that, within the know-how of the times, could not be separated into its components were considered to be rather fundamental and were called elements. Combination of different elements were then referred to as compounds.

Obviously, scientists of the time had no way to know whether elements were really "elementary", nor there was any clear idea why different materials existed with different properties. In fact, one of the popular scientific ventures was a search for the "philosopher's stone", endowed with the virtue of transforming one element into another (e.g. Lead into Gold...).

Incidentally, why Lead? Lead and Gold have similar physical (although not chemical) properties : they are "soft" (i.e. they can be rolled into very thin sheets), they have a low melting temperature, they are both "heavy", etc.

Parenthesis on Heaviness

What does it mean that a given substance is heavier than another?


Which one is heavier?

A : a pound of iron




B : a pound of feathers

We then learn the following lesson : to judge about relative heaviness, you need to compare equal volumes.

The commonly used quantity is the density, defined as the weight of a unit of volume. Density should therefore be expressed in kg/m³, although more commonly one uses g/cm³ (a cubic meter is a rather large volume..).

Examples :

Water : 1.0 g/cm³

Aluminum: 2.70 g/cm³

Lead : 11.35 g/cm³

etc.

Density can be defined also for gases. It is usually measured in grams/liter, and one also needs to specify the gas pressure and temperature. For example, at 1 atmosphere and 20 Centigrades:

Helium : 0.125 g/l

Air : 1.205 g/l

And, just in case you do not know it, a material will "float" in a given medium if its density is less than that of the medium: wood (density : oak = 0.8, pine = 0.5) floats in water, Helium floats in air, etc.

Let us come back to elements, atoms, etc. In the 19th century, thanks to the progress in the study and applications of electricity, many more elements could be isolated and identified by means of electrolysis: due to some processes that we'll understand better in future classes, sending the current from an electric battery through a liquid where a given compund is in solution, can result in the accumulation, at the battery terminals, of the elements of which the compound is made.

Standard example : electrolysis of water results in the formation of two gases, recognized as Hydrogen and Oxygen.

Moreover, careful measurements always showed that the volume of produced Hydrogen was always twice the volume of Oxygen, although the weight of the Oxygen produced was 8 times larger than the Hydrogen's weight.

The fact that elements combined in well definte proportions was one of the main motivations for the Atomic Theory of Matter (J. Dalton, 1808) :


The fundamental building blocks of the elements are some microscopic elements called atoms. Different elements correspond to different atoms. Atoms, always according to Dalton, are indivisible (hence the name). Individual atoms of different elements combine in well defined proportions, to form individual units of the various compounds. Such units are called molecules.

In our previous example, a molecule of water will then be formed by two atoms of Hydrogen and one of Oxygen. Moreover, one can infer that the Oxygen's atom is 16 times heavier than the Hydrogen atom.

As we will see, Dalton was right to a very large extent, except for the indivisibility of atoms...


The next chapter in our brief historical review is the discovery of the Periodic Table of Elements.

Thanks to continuous progress during the past century, more and more new elements were identified, and scientists were hard at work trying to put some order in this rapidly increasing family.

Success was reached by Dmitri Mendeleev who, recognizing recurrent regularities in the element properties, was able to come forward with his now famous chart.

Still, as is often the case in Science, one scientific breakthrough, while answering many questions, in turn gives birth to many new questions:

• why were the elements organized in that particular way rather than any other one?
• why, for instance, there were only two elements in the first row?
• why elements of a given family reacted readily with elements of some, but not some other family? Why were the elements of the family in the rightmost column very non-reactive? etc. etc.

The answers came from the discoveries of our century, and the consequent advent of the atomic (and sub-atomic) era.


It all started with the discovery of the electron, in 1897....

Let us do the following, very simple for today's standards, experiment:

Heated Filament Zinc Sulphide screen






Cathode (-) $\longleftarrow\Delta V\longrightarrow$ Anode (+)



Heating the filament and applying a voltage results in a bright spot on the screen. Deduction : the heated filament emits "cathode rays", which appear to be negatively charged paricles.

The experiment can be refined :









Applying a voltage between plates D and F, one can deflect the path of the cathode rays.

Do you know of which modern commodity the above setup is the grand-grand father?

A more elaborate setup can provide even more information on the "cathode rays" .

Facts to remember : electric and magnetic fields affect the motion of charged particles. More specifically:

• a constant electric field (constant Voltage) provides a constant force (therefore constant acceleration)
• a particle moving in a constant magnetic field will be forced to move in a circle of constant radius (the radius depending upon the actual strength of the field)

e/m DEMO

By performing such types of measurements, J.J. Thomson was able to determine the value of the ratio between the cathode rays charge and mass ( i.e. e/m). The result obtained showed that these entities could not be consistent with being whole atoms, but they had to be some much smaller particle. They were called electrons.

A few years later, an american physicist, R. Millikan, was able to perform an independent measurementr of the mass of the electrons, so that, based on the previous knowledge, one could then know the separate values of e and m:

$e = 1.6\times 10^{-19} C, m = 9.11\times 10^{-31} kg$

DISCOVERY of the ATOMIC NUCLEUS

At the beginning of the century, the following facts were known above the atoms:

• atoms are electrically neutral
• within the atom, there are tiny (and very light, when compared to the total atom mass) negatively charged particles, the electrons

Consequently, one had to infer that the bulk of the atoms was formed by some distribution of heavy, positively charged, matter. The assumption of the times was that this matter was uniformly distibuted over the atom's volume, with the electrons inside like "raisins in a bun".

NOTE: there was no real reason for adopting such a picture for the atom, it was just the most "natural" hypothesis.

A completely different picture was revealed when Rutherford (together with Geiger and Marsden) performed his now famous experiment.


[Note : the figures following are extracted from: http://pdg.lbl.gov/ cpep/adventure.html]


















To explore the distribution of electric charges within the atom, Rutherford sent a beam of $\alpha$ particles (positively charged products of radioactivity) against a gold foil target. A Zinc Sulphide screen was measuring the deflection of the projectiles after traversing the foil.

Why did Rutherford use gold as a target?


A Gold was considered the purest element, therefore it would give the cleanest results

B Manchester University didn't care about price, when it was coming to Physics research

C Gold was known to be rather chemically inert, therefore its composition would not be modified by the $\alpha$ particles

D Gold could be stretched into extremely thin sheets

According to the accepted model of the atom, the result of the experiment should have been





















What was found was instead:


















(historical note: after a while, Geiger got fed up of observing tiny light pulses on a fluorescent screen, and he invented an electronic particle detector, the Geiger counter