Atom Theory Timeline

Electrons were discovered in 1897

1897

In 1897 Thomson used a magnet near a tube that would show electrical waves. He discovered that electricity was made up of negative particles (electrons). He realized it was a tiny piece of every atom.

Rutherford team discover the nucleus

1910

Rutherford’s team(which was composed of his students such as Henry Moseley) created a beam from positively charged alpha particles and aimed it at an ultra thin sheet of gold foil. The particles would go through sometimes, but the other times they would bounce back. This meant that the atom was mostly empty space except for a small positive charge at the center. This is known as the nucleus.

X-rays

1912

Around 1921, X-rays were found to have properties like those of light. Henry Moseley discovered that the light of x-ray could be fractured with the help of a crystal.

X-ray spectrum of elements

1912

Henry Moseley and Charles G. Darwin had discovered that the atoms inside the crystal of x-rays were arranged neatly in rows which created spots when it reflected the x-rays. This lead to Henry Moseley testing the elements to determine whether they had a unique x-ray spectrum. He placed samples of elements inside of x-ray tubes. He realized that when a beam of electrons struck the sample, the element gave off x-rays. This gave him a x-ray spectrum of the elements.

All elements are found

1914

The 7 missing elements were found, thus completing the periodic table for that era, as more would be discovered later on.

Finding of other elements

1914

Using the system Moseley created, he was able to determine whether the new finding of new elements where genuine and was also able to determine which new elements were going to be discovered and how many there were.

Atomic number

1914

Henry Moseley arranged the periodic table with the discovery of the x-ray spectrum. By doing so he realized that the nucleus wasn’t only a blob with a positive charge, but rather a collection of positively charged particles that would go up in numbers with each heavier element. Rutherford then discovered the proton using Moseley’s work which helped him determine that each element on the periodic table is defined by the number of protons in its nucleus which is the atomic number.

Neutrons

1932

James Chadwick discovered the neutron.

Creation of other elements

1934

Neutrons wouldn’t be repelled as they approached the nucleus, instead they would go right in. In 1934, Enrico Fermi fired neutrons at uranium atoms, which created a shower of fragments. By analyzing these fragments he realized that the neutrons being fired would chip off a part of the nucleus of the uranium, turning it into a different element. Some of the neutrons would get absorbed by the nucleus and become a proton or a electron.

Creation of an element

1934

Edwin McMillan applied uranium oxide to filter paper and stacked cigarette papers behind the filter paper. When’s the paper was struck with neutrons from the cyclotron, atomic fragments would scatter and some would penetrate the cigarette paper. This lead to him making a new element, element 93.

Element 94

1935

Glenn Seaborg found element 94, through the same method McMillan used. He had to wait for Titanium to break down into a new element, and waited for a month. He discovered plutonium.

Atomic bomb

1941

Plutonium was being tested to create bombs by Seaborg and Emilio Segrè. Many of this tests would produce radiation. They would separate element 93 and would spin it to purify it. This lead to a small amount of plutonium that was fissionable.