Atomic Structure and Theory

What do we currently know about the atom?

• Atoms contain particles. The best-known particles are the proton, neutron, and electron.

Location in the atom Charge Mass in amu

Proton

Neutron

Electron

What do we currently know about the atom?

• And more recently, scientists have been discovering lots of interesting particles like:

• Quarks – scientists believe that protons and neutrons are made up of particles called quarks. These have names like up, down, charm, strange, top, and bottom.

• Baryons – a particle made up of two quarks. (Protons and neutrons are baryons)

What do we currently know about the atom?

• Mesons – a particle made up of a quark and an antiquark.

• Leptons – point-like particles of which there are six types. Electrons are one type of lepton. There is also muon, tau, and three types of neutrinos.

So how did we get here?

Democritus (460 – 370 BC)

• Stanford Encyclopedia of Philosophy:

• Democritus, known as the ‘laughing philosopher’ because of his emphasis on the value of ‘cheerfulness,’ was one of the two founders of ancient atomist theory.

• Democritus’ theories about atoms were a response to another Greek philospher who said that it is impossible for there to be change without something coming from nothing.

• Democritus argued that change was possible because there were unchanging material principles (atoms) which persist and merely rearrange themselves to form the changing world of appearances.

Democritus (460 – 370 BC)

• How did he figure it out?

• Democritus was a philosopher who thought about nature and morals. He used his observations of the natural world to write about what he believed it was made up of.

“By convention there is color, By convention sweetness, By convention bitterness, But in reality there are atoms and space.”   -Democritus (c. 400 BCE)

Democritus (460 – 370 BC)

John Dalton (1766-1844)

• Dalton proposed several postulates about the atom:

1. All atoms of the same element are identical

2. Atoms of different elements have different masses

3. Compounds consist of different combinations of elements in fixed ratios.

4. Chemical reactions are rearrangements of these atoms.

5. Atoms can not be broken down into smaller particles.

John Dalton (1766-1844)

• How did he figure it out?• In the course of his studies

on meteorology, Dalton wondered how water and air could occupy the same space at the same time, when solid bodies could not. If the water and air were made of discrete particles, this type of mixing would be possible.

John Dalton (1766-1844)

• How did he figure it out?• Dalton performed

experiments on mixtures of gases and noticed that certain gases maintained the same ratios of mixture regardless of amount. This law of proportions led him to believe that the ratios remained the same because they were consistent down to the smallest particle or atom.

J.J. Thomson (1856-1940)

• J.J. Thomson concluded that atoms contained a particle that was 1,000 times lighter than an atom, and that it always had the same charge, no matter what atom it came from.

• He called these particles “corpuscles”. Needless to say, this was later changed to electron, a name suggested by George J. Stoney a few years earlier.

J.J. Thomson (1856-1940)• How did he do it?

• Cathode ray tube experiments:• Physicists in the 19th century found out that if

they constructed a glass tube with wires inserted in both ends, and pumped out as much of the air as they could, an electric charge passed across the tube from the wires would create a fluorescent glow.

J.J. Thomson (1856-1940)

• J.J. noticed that when a magnetic field was applied to the cathode rays, the rays were deflected (bent).

• He then noticed that when he applied an electrical field, the rays were deflected in the opposite direction.

J.J. Thomson (1856-1940)

• Since the electricity that deflected them was believed to be made up of negative particles, he concluded that the cathode ray particles must also be made of negative particles.

• Video: Cathode ray tube

J.J. Thomson (1856-1940)

• He also used electrical and magnetic fields of varying strengths to measure the charge-to-mass ratio of these particles. He found that the ratio was very high, so the particles either had a huge charge or a very small mass.

J.J. Thomson (1856-1940)

• J.J. Thomson explained the fact that the atom was neutral by proposing the “plum pudding model”, in which corpuscles were orbiting in a sea of positive charges.

Thomson’s plum

Pudding model:

Ernest Rutherford (1871-1937)• Soon after the discovery of

radioactivity in 1896 by the French physicist Antoine Henri Becquerel, Rutherford identified the three main components of radiation and named them alpha, beta, and gamma rays.

• He also concluded that atoms are made up of a dense inner core (the nucleus) that has a positive charge.

Ernest Rutherford (1871-1937)• How did he do it?

• The Geiger-Marsden experiment (or Gold foil experiment)

• Alpha particles from a radioactive gas were shot at a thin layer of gold atoms.

• Alpha particles are made up of two protons and two neutrons. (a helium nucleus)

• If J.J. Thomson’s plum pudding model was correct, the rays should have been deflected at small angles as they reached the positive particles in the atoms.

If plum pudding model was correct:

Alpha particle:

Ernest Rutherford (1871-1937)• Rutherford and his researchers were amazed to see that

many of the alpha particles were barely deflected at all, and a few particles were actually deflected backwards!

• This led Rutherford to conclude that there must be a very small, very dense area of positive charge.

• He also concluded that electrons must orbiting this dense positive core.

• Website

Ernest Rutherford (1871-1937)“It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you…It was then that I had the idea of an atom with a minute massive center, carrying a charge.”—Ernest Rutherford

Niels Bohr (1885-1962)

• During Bohr’s time, it had been suggested that electrons orbited the nucleus of the atom in circular orbits.

• Niels Bohr postulated that electrons move to a higher orbit by absorbing radiation (a photon) and emit a photon of energy when they move to a lower orbit. He used many of Max Planck’s ideas about light to reach this conclusion.

Niels Bohr (1885-1962)

• Bohr also showed that these energy levels are quantized – only certain energy levels are allowed.

• This is similar to the idea of rungs of a ladder. You can not step between the rungs. An electron can not exist between levels.

QUANTIZED NOT QUANTIZED

Niels Bohr (1885-1962)• How did he do it?• By studying atomic emission spectra.• To create an emission spectra, a sample of gas is energized with heat

or electricity. The heated gas gives off light.• On passing through a very thin slit and then through a prism, the light

emitted by the excited atoms is separated into spectral lines. • Excited electrons

James Chadwick (1891-1974)

• James Chadwick discovered the neutron, a neutral particle in the nucleus of the atom.

• Chadwick’s discovery made it possible to create elements heavier than uranium in the laboratory.

James Chadwick (1891-1974)• How did he do it?• Chadwick smashed alpha particles into beryllium, a rare

metallic element, and allowed the radiation that was released to hit another target: paraffin wax.

• When the beryllium radiation hit hydrogen atoms in the wax, the atoms were sent into a detecting chamber.

• The experiment results showed a collision with beryllium atoms would release massive neutral particles, which Chadwick named neutrons. This provided the answer for hidden mass in atoms.

To be continued…

• The story of the atom doesn’t stop here. • Coming up next: Heisenberg’s uncertainty

principle, multiple universes, and quantum mechanics.