30.2 Discovery of the Parts of the Atom: Electrons and Nuclei

Both electrons and nuclei are substructures of the atom.

Some of the basic properties of atoms can be found in the experiments that were used to discover them and can be easily understood using ideas such as magnetic force.

Positive charge is associated with nuclei and negative charge with electrons.

The electric and magnetic forces affect charges.
The discovery of the electron and nucleus as substructures of the atom will be explored.

The gas glows when a high voltage is applied.

Neon lights are the result of these tubes.
They were first studied by a German inventor in the 1860s.
The English scientist William Crookes continued to study what is known as the Crookes tubes, in which electrons are freed from atoms and molecules in the rarefied gas inside the tube and are accelerated from the negative to the positive by the high potential.
The electrons' path is visible as a ray that spreads and fades as it moves away from the cathode, after these "cathode rays" collide with the gas atoms and molecule.

The electrons can make a small paddle wheel rotation.

The normally straight path is bent by a magnet in the direction expected for a negative charge to move away from the cathode.
These were the first signs of charge and electrons.

A gas discharge tube is glowing.

The atoms and molecules in the gas glow in response to the electrons emitted from the cathode.
The name of the tubes used in TVs, computer screens, and x-ray machines is now known as cathode-ray tubes.
The beam bends when a magnetic field is applied.
The negative charge of the rays was verified by magnetic and electric fields.

An excess of negative charge was found when he collected the rays in a metal cup.

The electric field is produced between the charging plates and the tube is placed between the poles of the magnet so that the electric field is in line with the magnetic field of the magnet.

The fields produce opposing forces on the electrons.

Thomson moved the beam up and down by adjusting the electric field after determining the velocity of the electrons.

The schematic shows the electron beam in a CRT passing through electric and magnetic fields and causing phosphor to glow when striking the end of the tube.

The value was not known at the time.

The applied voltage and distance between the plates can be used to determine the deflection.

The measurement can be made by bending the beam of electrons with the magnetic field.
The results are obtained using a magnetic field.

Thomson realized that this is a huge number and that it means the electron has a very small mass.

A factor of 1000 less than the charge per kilogram of electrons is needed to plate a material.
Thomson did an experiment for hydrogen ion and found a charge per kilogram 1000 times smaller than for the electron, implying that the hydrogen ion is more massive than the electron.

The charge per kilogram is 1836 times less than the charge for the electron.

The charges of electrons and protons are the same.

Thomson used different gases in discharge tubes and other methods, such as the photoelectric effect, to free electrons from atoms.

He was able to prove that the electron was an independent particle.
Thomson was awarded the 1906 Nobel Prize in physics for his work, which began in 1897.
It is difficult to remember how amazing it was to find a substructure in the atom.

Thomson's method could not determine the charge of individual electrons due to the order of magnitude expected.

It had been known for a long time that 100,000 C per mole was needed to plate singly ionized strontium.

The charge per ion was calculated to be close to the actual value by dividing it by the number of ion per mole.

One of the most fundamental constants in nature, the charge on electrons, was measured for the first time by the Millikan oil drop experiment.

Fine drops of oil are charged.
There is a potential applied to the metal plates to oppose the force.
The calculation of the charge on a drop can be done with the balance of electric and gravity.
The excess and missing electrons on the oil drops are determined by the charge being quantized in units.

In the Millikan oil drop experiment, fine drops of oil are sprayed.

Some of these are charged by the process and can be suspended between metal plates.

The drop's weight is adjusted to balance the electric field produced by the applied voltage.

The drops can be seen using a microscope, but they are too small to measure their size and mass.

When the voltage is turned off, the mass of the drop is determined.

The more massive drops fall faster than the less massive, and sophisticated calculations can reveal their mass, since air resistance is very significant for these submicroscopic drops.
The mass of oil is nearly constant because it does not evaporate.

He observed that all charges were multiples of the basic electron charge and that sudden changes could occur in which electrons were added or removed from the drops.

Millikan was awarded the 1923 Nobel Prize in physics for his studies of the photoelectric effect.

The mass of the electron can be calculated using the charge of the electron and the charge-to-mass ratio.

The mass of the electron has been verified in many subsequent experiments and is now known to be better than one part in one million.

It is the smallest known mass of any particle.
The calculation gives the mass of other particles.

To prove the existence of one substructure of atoms, the electron, Thomson and Millikan had to show that it had only a tiny fraction of the mass of an atom.

The nature of the nucleus of an atom was completely unexpected.

Another characteristic of quantum mechanics was starting to emerge.

All electrons are the same.
The charge and mass of electrons are unique to all electrons.
This is true of other fundamental entities.
All protons are the same.

The first direct evidence of the size and mass of the nucleus is here.

Basic information on nuclear size and mass is important to understanding the atom, but other aspects of nuclear physics will be examined in later chapters.

Nuclear radioactivity was discovered in 1896 and was the subject of intense study by a number of the best scientists in the world.

After completing his postgraduate studies at the Cavendish Laboratories in England, he moved to Canada where he did the work that earned him a Nobel Prize in chemistry in 1908.
There is a lot of overlap between chemistry and physics in the area of atomic and nuclear physics.
He returned to England in the late 80's and had many future winners as students.
Nuclear radiation was used to look at the size and mass of the nucleus.
A radioactive source that emits alpha radiation was placed in a lead container with a hole in one side to produce a beam of alpha particles, which are a type of ionizing radiation ejected by the nucleus of a radioactive source.
The scattering of alpha particles was observed when they struck a phosphor screen after a thin gold foil was placed in the beam.

The size and mass of the nucleus were shown by scattering alpha particles from a thin gold foil.

Alpha particles with energies of about are emitted from a radioactive source, which is a small metal container in which a specific amount of radioactive material is sealed, and fall upon the foil.
The number of particles that penetrate the foil or scatter to different angles indicates that gold is very small and contains almost all of the gold atom's mass.
The alpha particles that scatter to large angles are similar to a soccer ball bouncing off a goalie's head.

The nucleus of an unstable nuclide can be broken down by the emission of charged particles if the alpha particles are doubly charged.

Nuclear size and mass can be revealed by the way in which the particles scatter from the nucleus.
This is similar to how a bowling ball is scattered by an object.

The atom was supposed to be a small sphere with equal amounts of positive and negative charge.

The incident massive alpha particles wouldn't suffer a lot in the model.
The analysis shows that gold nuclei are very small compared to the size of a gold atom, with almost all of the atom's mass tightly bound.
The gold nucleus is more massive than the alpha particle, so a head-on collision would scatter the alpha particle back to the source.
The larger the nucleus, the less alpha particles that would hit one head on.

The results of the experiment were published by his colleagues in 1909, but it took him two years to convince himself of their meaning.

Like Thomson before him, he was reluctant to accept such results.
Even those at the forefront of discovery are surprised by nature on a small scale.
It was almost as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.
The analysis and model of the atom were published in 1911.
The size of the nucleus was determined to be 100,000 times smaller than the atom.
This means a huge density on the order of the matter.
The existence of previously unknown nuclear forces to counteract the repulsive Coulomb forces in the nucleus is implied.
Huge forces are consistent with the large energies of nuclear radiation.

The small nucleus means that the atom is mostly empty.

Most alphas went through the gold foil with very little scattering, since the atom was mostly empty with nothing for the alpha to hit.
At the time he did his experiments, energetic electrons had been observed to penetrate thin foils more easily than expected.
Most alpha particles are scattered by electrons.
Occasionally, an alpha hits a nucleus head-on and is scattered backwards.

The circles and dots represent the atoms and the nucleus.

The dots are larger than the scale.
Most alpha particles are unaffected by crashes because of their high energy and small mass.
Some head straight toward a nucleus and are scattered back.
The size and mass of the nucleus are given in a detailed analysis.

The planetary model of the atom shows low-mass electrons.

The size of the nucleus is small compared with the size of the electrons.
This picture is similar to how low-mass planets in our solar system circle the large-mass Sun at large distances compared with the size of the sun.
The attractive Coulomb force in the atom is similar to gravitation in the planetary system.
Since the atom is too small to be seen with visible light, a model or mental picture is needed.

The nucleus, electrons, and size of the atom are included in the planetary model of the atom.

This model was the first to recognize the structure of atoms in which low-mass electrons are in a large nucleus.

Our planetary system is similar to the empty atom.

In the next few sections, we will see how the planetary model of the atom was important to understanding the characteristics of atoms.

It was an indication of how different nature is from the classical world on a small quantum mechanical scale.
The discovery of a substructure to all matter in the form of atoms and molecules was being taken a step further to reveal a simpler substructure.
We have been successful in finding deeper substructures, such as those inside the nucleus.
We will look at the direction the search seems to be heading in the later chapters, after we follow this quest in the discussion of quarks and other elementary particles.

He recreated the famous experiment in which he disproved the Plum Pudding model of the atom by observing alpha particles bouncing off atoms and determining that they must have a small core.