Analía Bellizzi – Chemistry Classes

Ronald Reagan High School

The Atomic Models Through History

From the Ancient Greeks to the actual models. All you need to know. 


Everything begun with the Greeks (460 BC)

  • The concept of the smallest particle was conceived in the 5th century BC by Leucippus of Miletus.
  • His pupil, Democritus of Abdera (picture) developed five major points that their theory was based upon.
  • Historians have discovered this from the quotations of other Greeks (most of the original documents by Leucippus and Democritus have been lost).

The 5 points of Democritus’ conclusion

    • All matter is made up of indivisible particles that they called a-tomos. which means indivisible
    • There is a void, which is empty space between atoms.
    • Atoms are completely solid.
    • Atoms are homogeneous, with no internal structure.
    • Atoms vary in:
      • Size
      • Shape
      • Weight

Aristotle and beyond (380-322 BC)

  • In the 4th century BC, the well known philosopher Aristotle vehemently argued that the atomic theory was completely incorrect and was therefore dismissed by scientists for many decades. According to him, there were 4 elements: fire, water, earth and air.
  • In fact, the Catholic Church agreed with Aristotle’s position and announced that atomistic ideas were equivalent to those of Godlessness. “Democritus of Abdera said that there is no end to the universe, since it was not created by any outside power.”

A gap of around 2000 years passed by...

Lavoisier is widely considered as the “father of modern chemistry”

He mostly worked with gases and was very precise in his measurements. 

He discovered that, although matter may change its form or shape, its mass always remains the same.

This is known as the LAW OF CONSERVATION OF MASS. 

John Dalton’s Model (1803)

John Dalton, an English teacher, is now called the father of modern atomic theory. His model of the atom can be represented by a compact sphere.

      1. All elements are made of atoms.
      2. Atoms of an element are identical in their  masses.
      3. Atoms of different elements have different masses.
      4. Atoms only combine in small, whole number ratios.
      5. Atoms can be neither created nor destroyed

Interesting Fact: The unit for atomic weight was called a “Dalton” for many years. Today, it is used in biochemical circles, (e.g. “The atomic weight of that protein is 35,000 daltons.”)

John Dalton

So far, we had an idea on how atoms behaved, but


Joseph John Thomson (1898-1904)

Joseph John Thomson, an English physicist, discovered that: 

    • Cathode rays were negatively charged particles which he called ‘corpuscles.’ (we call them electrons)
    • This particles had a mass about 1000 times smaller than hydrogen
    • These corpuscles were the things from which atoms were built up.

Thomson designed the famous “plum pudding model” with a positive mass (represented as white in the picture) and negative particles inserted (red spheres)

Thomson's experiment using cathodic rays

Ernest Rutherford (1910)

  1. Ernest Rutherford studied under J. J. Thomson.
    • He was the first scientist to propose the theory of the nuclear atom.
    • His famous gold foil experiment, (watch the video below) proved that the atom had 
      1. a positive nucleus
      2. where the mass was concentrated ,
      3. and the electrons were surrounding it.
      4. most of the atom is made of empty space
      5. the nucleus is 10,000 times smaller than the atom.
    • He was not able to specify how electrons were arranged around the nucleus but he imagined a sort of planetary system around the nucleus.

The experimental setup for Rutherford’s gold foil experiment:

  • A radioactive element that emitted alpha particles was directed toward a thin sheet of gold foil that was surrounded by a screen which would allow detection of the deflected particles

According to the plum pudding model (top) all of the alpha particles should have passed through the gold foil with little or no deflection. 

Rutherford found that a small percentage of alpha particles were deflected at large angles, which could be explained by an atom with a very small, dense, positively-charged nucleus at its center (bottom).

Niels Bohr’s model (1915)

Niels Bohr came up with the orbit model or planetary model in 1915. It is referred to as the “planetary model”.

  • The electrons orbit the nucleus like the planets orbit the Sun.
  • Each orbit can have a fixed number of electrons given by the formula

  • Electrons belong in their natural state (ground state)
  • They jump to a higher energy level if they receive energy, and jump back to the ground state releasing the energy in the form of light.

Rutherford again(1917)

The discovery of the proton is credited to Ernest Rutherford, who proved that the nucleus of the hydrogen atom (i.e. a proton) is present in the nuclei of all other atoms in the year 1917.

Erwin Schrodinger (1920-1925) Quantum Model

In 1926 Erwin Schrödinger used math to describe the probability of finding an electron in a certain position around the nucleus. This atomic model is known as the quantum  mechanical model of the atom.

    • Electrons do not “ORBIT” around the nucleus like planets around the sun.
    • They can be anywhere in a “cloud like” zone around the nucleus.
      An electron will occupy levels and sublevels of energy with different shapes.
    • Every one is called an “ORBITAL” and can hold only 2 electrons.
      • The first level has only 1 sublevel (s)
      • The second level has 2 sublevels (s & p)
      • The third level has 3 sublevels (s, p & d)
      • Etc….

Heisenberg's Uncertainty Principle

He described the atom’s structure using pure mathematics. He created matrix mechanics, where the order of multiplication matters. He published his work in 1925 in a breakthrough paper. In the subsequent series of papers with Max Born and Pascual Jordan, during the same year, this matrix formulation of quantum mechanics was substantially elaborated. He is known for the Heisenberg uncertainty principle, which he published in 1927 which says that it is impossible to know the position and the momentum (velocity) of an electron at the same time. When you think you know one of them, the other changes. Heisenberg was awarded the 1932 Nobel Prize in Physics “for the creation of quantum mechanics”

The Solvay Conference:
There Are 17 Nobel Laureates In This Photograph

James Chadwick (1932)

James Chadwick confirmed the existence of the neutron, which Ernest Rutherford predicted but he could not prove.

By shooting alpha particles (helium nuclei) at beryllium in 1930, scientists noticed that a strong, penetrating radiation was emitted.

James Chadwick proved that this radiation consisted of a neutral particle with about the same mass as a proton.

Because it did not have any charge, it was called a neutron

Here you have a review of the History of the atomic theory

This page contains all the information you need for the second chapter: ATOMS.

Video a Volatile history Part 1

History of the atom: 

Atomic Structures and Introduction to Periodic Tavle

Summary of Chapter topics

matter-pt-and-atom-review.pdf   ANSWERS



Atomic Models through history






Review for the end of the quarter test (District test on 10/21 or 10/24)

-ATOMS, IONS: Its parts, where particles are located, charge

-Watch the following video to understand Rutherford’s experiment and :

His conclusions can be summarized as follows:

  1. Almost all ALPHA PARTICLES passed through the GOLD FOIL without any deflection
  2. Atoms are not compact as in Dalton’s or Thompson’s models
  3. Atoms have a nucleus where the positive charge and mass are concentrated.
  4. Electrons are moving around in almost empty space and form the majority of the volume of the atom
  5. THE MOST IMPORTANT CONCLUSION: Atoms are almost empty space

 -Principles of the Scientific Method

-Homogeneous and Heterogeneous mixtures

-Solutions – solvent, solute,

-Elements and compounds

-Separation methods

Density as INTENSIVE PROPERTY . How to calculate it

Periodic table