Atomic Theory

PS 9.26

PS 9.27

What is the nature of matter?• The ancient Greeks

pondered that question.

• Empedocles in 460 B.C.suggested that all matter was made up of four elements, earth, air, fire and water.

• Democritus and Leucipus in 370 B.C. suggested that if you cut a piece of matter into smaller and smaller pieces, eventually, you will find that you will come to a particle which you cannot divide any smaller.

• He called these particles atomos, which means indivisible. The word atom is derived from this.

Continuous Theory

• Aristotle in 320 B.C. disagreed with Democritus

• He thought that you could keep cutting a piece of material indefinitely.

• The argument went back and forth for over 2000 years.

A New Atomic Theory

• In 1803, the next great development in Atomic Theory came to be.

• John Dalton developed a theory which has come to be known as the “Billiard Ball Model”.

Reasons for Dalton’s Theory

• The Law of Definite Proportions. If matter exists as atoms, the atoms always combine in the same ratios in any given compound.

Principles of Billiard Ball Model• All elements are composed of tiny

indivisible particles called atoms• Atoms of the same element are

identical. Atoms of any one element are different from those of any other element.

• Atoms only combine in small, whole number ratios to form compounds

• Atoms cannot be created or destroyed or changed.

Discoveries of the 19th Century• Michael Faraday made an important

discovery in 1825.• One of his discoveries involved

hooking a high voltage source up to a glass tube from which he removed most of the air.

• He observed an eerie glow in the tube. Other scientists investigated.

Building on Faraday’s Work• Geissler developed a better

vacuum pump in 1875, enabling scientists to remove even more of the air from the tubes.

• They studied the glow in the tube and concluded that the glow was caused by rays coming from the cathode.

• They called the rays Cathode Rays.

• William Crooks developed tubes which led to some major discoveries in 1873.

• One tube that he created looked like this.

• The cathode rays struck a metal piece shaped like a cross.

• The cathode rays left a shadow, proving that the cathode rays travelled in straight lines unless acted upon by other influences.

• He also discovered that when he put a small metal paddle wheel in a cathode ray tube that the cathode rays caused it to spin.

Here is a close-up of the

paddle wheel

• He concluded from this that the cathode rays must have mass.

• Other experiments showed that the cathode rays were deflected by electrical fields and magnetic fields.

• From this, scientists deduced that the cathode rays were, in fact, tiny positively charged particles, streaming from the cathode.

• JJ Thomson studied these particles.

• On April 30, 1897, Joseph John (J.J.) Thomson announced that cathode rays were negatively charged particles which he called 'corpuscles.'

• We now call them electrons. He devised experiments to study the properties of these electrons.

Thomson developed a new theory called the Raisin Bun Model (Or Plum Pudding)

• Thomson described the atom as being like a raisin bun. The electrons were little, (-) raisins embedded in a (+) dough.

• The total (-) charge and total (+) charge must balance out

• The next development in atomic theory came about as a result of an amazing discovery in 1895.

• In that year, working with specialized cathode ray tubes, a scientist by the name of Wilhelm Roentgen discovered a marvelous new ray which he called X-rays.

• A French scientist named Becquerel decided to see if he could find any substances which would spontaneously give off X-rays.

• One day, in 1896, he inadvertently left uranium sitting on a covered photographic plate. When he exposed the plate, he discovered a strong image on the plate.

Henri Becquerel

• Pierre and Marie Curie worked to discover other elements which gave off these rays 1898.

• They called these rays radioactivity.

• While working at McGill in 1911, Ernest Rutherford discovered that there were three types of radioactivity, which he called alpha, beta and gamma. Alpha released a helium atom. Beta releases electrons. Gamma rays releases photons.

Gold Foil Experiment• He set up an

apparatus using a source of alpha radiation.

• The alpha particles were directed through a narrow slit at a gold foil.

The Nucleus

• Rutherford devised a new theory for the atom.

• He said that the atom consisted mostly of empty space. At the center of this empty space is a small, hard, dense positively-charged central particle which he called a nucleus.

The Nuclear Model

A scientist named Moseley in 1914, showed that each element has a unique number of protons. A hydrogen nuclei has one proton, a helium nuclei has two and so on. The number of protons in an atom’s nucleus is known as its atomic #.

It has been discovered that the amount of charge is equal and opposite to the amount of charge on an electron. Therefore, any neutral atom must have equal numbers of protons and e-

• Neils Bohr in 1922, discovered that the electrons in an atom were not just randomly placed around the nucleus but instead were in orbits. This was the beginning of the quantum theory.

• James Chadwick in 1932 discovered a neutral subatomic particle that had almost the exact same mass as the proton and therefore explained why an atom’s atomic mass did not match the weight of all the protons and electrons.