Atomic theory timeline by Paige Andersen

2500 years ago

paigeandersen3 — 2024-01-19 22:54:37 UTC

Greek philosopher named Leucippus and Democritus came up with the idea that matter is composed of tiny particles, no one knows now they came up with this concept.

They assumed that if you took a particle and cut it in half enough times, you wouldn't be able to anymore.

Atomos : uncuttable or indivisible

The next 2300 years

paigeandersen3 — 2024-01-19 23:10:47 UTC

The law of conservation of mass. Meaning that even if matter changes shape or form, it's mass stays the same.

In the 1870s

paigeandersen3 — 2024-01-19 23:19:06 UTC

Scientist began looking at what things were made of using discharge tubes. Which was a gas filled tube with electrodes at each end which emit light when electrical current passes through them.

Used to be called a cathode ray because it was originally produced by a negative electrode.

In 1886… German physicist Eugen Goldstein found that the tubes also emitted light from the positive electrode. Discovering that there must also be a positive charge in matter.

paigeandersen3 — 2024-01-19 23:24:57 UTC

English physicist J.J Thompson measured how much heat the cathode rays generate and how much they could be bent by magnets.

He was able to then estimate the mass of the rays, discovering that the mass was 1000X lighter than a hydrogen

He also said the the cathode rays weren’t rays or waves, they were very light and very small negatively charged particles, called corpuscles.

We call them electrons.

paigeandersen3 — 2024-01-19 23:33:48 UTC

1909… Ernest Rutherford, designed an experiment using thin sheet of gold foil and a screen coated with zinc sulfide.

Bombarding sheet with alpha particles, knowing that it was made up of a positive charge. He assumed that they would just fly through the foil with no deflection and most of them did. But not all.

Some particles were deflected at large angles and some straight backwards.

Explanation being that the entire positive charge of an atom, must be concentrated in a very small area called the nucleus.

Concluding that most of the atom was empty space.

paigeandersen3 — 2024-01-19 23:43:02 UTC

Rutherford later discovered that if he bombarded nitrogen with alpha particles it created hydrogen ions.

He then said that these tiny positively charged ions were fundamental particles called protons.

paigeandersen3 — 2024-01-19 23:50:24 UTC

In 1911, Bohr realized that some mathematical principles created by Max Planck and Albert Einstein could be applied to Rutherford atomic model.

He then was able to predict the most likely positions of electrons within the atom.

Bohr’s resulting model was called the planetary model, still familiar today. It represents the electrons in orbits around a small central nucleus, each orbit being able to have a certain number of electrons which correlate to energy levels & orbitals in the modern model of atoms.

paigeandersen3 — 2024-01-20 00:01:35 UTC

German theoretical physicist Werner Heisenberg

Using math discovered that it is not possible to know with being certain the momentum of the electron or any subatomic particle and it’s exact position.

Meaning you cannot say with certainty that all the electrons are neatly aligned in circular orbits like the planetary model.

So they proposed a new theory: the quantum theory

Proposed that electrons weren’t particles or waves. Instead, they had properties of both and neither.

Because it’s based on probability, quantum atoms are often drawn as clouds, the quantum model is often called the cloud model of the atom.