Atomic Theory!

Theory Development:

Who was John Dalton?

John Dalton was an English scientist. He is often referred to as the father of atomic theory. In 1803, he proposed the theory of the atom. He was also the first scientist to develop an atomic theory based on scientific observation. 

He believed that...

1. All matter is made of atoms.

2. Atoms cannot be broken down further.

3. Atoms within an element are the same;  atoms from different elements are not. For example, hydrogen atoms are different from oxygen atoms.

The Law of Conservation of Mass says that atoms are rearranged during a chemical reaction, but are not lost.

The Law of Multiple Proportions says that compounds are formed when atoms from two or more elements combine. In any one compound, the ratio of the number of atoms to one another is a whole number. In other words, if two elements combine to form more than one compound, the masses of one element will combine with the other elements in a whole number ratio. For example, in CO2, the ratio of carbon to oxygen atoms is 1:2.

Dalton's theory didn't get everything right. For example, it was later confirmed that atoms can be broken down into subatomic particles (known as electrons, neutrons, and protons), but it was a great start because atoms and molecules are still the smallest particles a substance can be and still retain its chemical and physical properties. 

Who was J.J Thomson?

J.J Thomson was an English physicist who is credited with the discovery of the electron and proposed the "plum pudding" model of the atom. 

Part of Dalton's theory was disproved when Sir Joseph John (J.J) Thomson discovered the electron in 1897. Thomson used the idea of radiation energy that is transmitted in the form of waves, particles, or rays. 

Using electromagnetic radiation theory, Thomson built a Cathode Ray Tube to prove that negatively charged particles (electrons) were present in atoms. A cathode-ray tube is a closed glass cylinder in which most of the air has been removed. Inside the tube are two electrodes, a cathode, which is the negatively charged electrode, and an anode, which is the positively charged electrode. When a high voltage is applied between the electrodes, a beam of electrons travels from the anode to the cathode. Thomson was able to determine the ratio of electrical charge to the mass of a single electron.

-1.76 * 10^8 Coulomb (c)/g

Coulomb is the unit of electric charge.

Thomson imagined that atoms looked like a "bowl of plum pudding", meaning that the electrons just "sat" in a pudding of protons. The negative charges of the electrons were canceled out by the positive charges of the protons. J.J Thomson believed that electrons were like plums inside a positively charged "pudding".

Who was Ernest Rutherford?

Ernest Rutherford was a British physicist. He took Thomson's idea a step further by using radioactive particles. He shot positively charged alpha particles (particles made up of two protons and two neutrons) through a piece of gold foil. He noticed that some of the alpha particles went straight through the foil, but others bounced back. What would cause the alpha particles to bounce back? Rutherford theorized that...

1. Atoms are made of mostly empty space (which is why most particles went "straight through" the gold foil.

2. Atoms have a positively charged center (which caused the positively charged particles to "bounce back").

His second idea came from the theory that because "like repels like", (meaning positively charged alpha particles were most likely hitting the center of the atom, which was also positively charged). He named that center of the atom the nucleus.

The nucleus is a positively charged central core of an atom; contains nearly all of its mass.

The Development of Theory of Electron Charges:

Electrons are matter. All matter has mass;  therefore, electrons must have mass. But how do you measure the mass of something as small as an electron?

Who was R.A. Millikan?

Robert Andrews Millikan was an American physicist who received the Nobel Prize in Physics in 1923 for his work on the measurement of the elementary electric charge. In 1909, R.A Millikan preformed an experiment in which he was able to find the size of the charge on an electron.

Millikan placed a negative charge onto a drop of oil. He then determined how much electric charge must be applied to suspend the oil drop in an electronic field between two metal plates. By balancing the gravitational and electric forces on the oil drop, he could determine the charge. After multiple experiments, he found that the charge on an electron was -1.602 * 10^-19 C.

Using Thomson's charge/mass ratio of the electron, Millikan determined the mass of an electron.

Mass of an electron = charge divided by charge/mass

The charge of both the electron and proton known as the Elementary Electric Charge (E) or (Q) is -1.6022 * 10^-1 C.

Because Rutherford knew the charge of the proton, he could plug it into his equation and calculate it in reverse to determine that the mass of the proton is -1.672 * 10^-27 kg.

Who was James Chadwick?

James Chadwick was a British physicist who discovered the neutron in 1932. He was awarded the Nobel Prize in Physics in 1935. He spent his life studying the nucleus of the Rutherford atom and found the mass of the nucleus to be twice as large as previously thought. In addition, Chadwick noticed that the Rutherford atom was neutral, yet the protons had a positive charge. These pieces of information led Chadwick to theorize that there must be small particles other than protons present in an atom's nucleus: ones without a charge. He named these subatomic particles neutrons.

Electron: charge = -1.60 * 10^-19 C and mass = 9.1 * 10^-31 kg

Proton: charge = 1.60 * 10^-19 C and mass = 1.672 * 10^-27 kg

Neutron: charge = 0.00 C and mass = 1.674 * 10^-27