How Atoms Were Discovered: Group 3

Progression of Atoms

by Group 3

 

 

 

DEMOCRITUS (460-370 B.C) Democritus expanded the idea to state that matter was composed of small particles called "atoms" that could be divided no further.These atoms were all composed of the same primary matter with the only differences between them being their size, shape and weight. The differences in these characteristics explained the differences in the properties of the matter around us.

 

 

JOHN DALTON (1786-1844) During the 19th century, a vast amount of data on how substances react with each other was collected. From this data, some simple laws of chemical reactivity had been devised. Dalton's theory can be summarized as follows:

1. Matter is composed of small particles called atoms
2. 1. All atoms of an element are identcial, but are different from those of any other element.
3. During chemical reactions, atoms are neither created nor destroyed, but are simply rearranged.  
4. Atoms always combine in whole number multiples of each other 

 

 

Dmitri Mendeleev (1834 - 1907) Dmitri Mendeleev attempted to classify the elements not by some "accidental, or instinctive reasons, but by some exact principle. The only unchanging numerical data available at this time was the atomic weight. By arranging the elements in order of increasing atomic weight he discovered that there existed a periodicity of the elemental properties. He used this periodicity to create a table in which that elements with similar properties were vertically aligned with each other.

 

 

First periodic table (1869)

 

J.J THOMPSON(1856-1940) At approximately the same time as radioactivity was being investigated, J.J. Thomson and others were performing experiments with cathode ray tubes.

 

 

Cathode Ray tube - It is an evacuated tube that contains a small amount of gas between two metallic plates. When a potential is placed between the cathode (the negatively charged plate) and the anode (the positively charged plate) a "ray" of electric current passes from one plate to the other.

Thomson discovered that this ray was actually composed of particles. Thomson proposed that an atom was composed of a spherical ball of positive charge with "corpuscles" of negative charge imbedded in it. The corpuscles would later become known as electrons.

   This is how to represent an atom

ERNEST RUTHERFORD(1871 - 1937) HANS GIEGER(1882 - 1945)

In 1909, Rutherford set a fellow scientist, Hans Geiger, and a student, Ernest Marsden, to work on this problem. They devised a system that allowed alpha particles (the nuclei of helium atoms) to be shot at a very thin piece of gold foil and the trajectory of the particles monitored.

 

In order to account for the fact that many of the alpha particles passed through the gold film, Rutherford discounted Thompson's solid ball model of the atom, and believed that the central positive charge of the atom represented only a small fraction of the atom's size, and that the remainder was primarily empty space. He calculated that, while an individual atom was about 1x10-10 meters in diameter, the nuclear diameter was only about 1x10-14 meters.

 Robert Millikan  (1868 - 1953)
It was not until the work of Robert Millikin that the number value of this charge could be determined.
 

Millikin was able to calculate the charge on each of the droplets he tested. The calculated charges on the droplets all turned out to multiples of a single number. Millikin therefore reasoned the elementary charge, or the smallest of charge, must be equal to this value. By combining his new information with the mass to charge ratio for the electron determined by Thomson, the mass of an electron was calculated for the first time.

 



 

 

 

 

SUMMARY ABOUT ATOMS:

 

• Atoms are composed of three elementary particles: the electron, the proton and the neutron.
• Most of the mass of the atom is concentrated in the nucleus of the atom. The protons and neutrons reside in the nucleus while the electrons exist outside of the nucleus.
• The number of protons is equal to the number of electrons.
• The type of element each atom is determined by the number of protons it has.
• The number of protons in an element is equal to the atomic number.
• Atoms do not have the same atomic mass. Atoms of the same element with different masses are called isotopes.
• The sum of the number of protons and neutrons in a particular atom is called the mass number. The mass number is different for different isotopes of the same element.

 

 

 

 

Members:
Leslie Constantino
Xyrill Cruz
Tish Darroca
Monica Dayo