The International School of Panama

General Chemistry

Chapter 4 notes

Early Theories of Matter

The development of the concept of the atom is fascinating.  It is a clear example of how different hypotheses, which help explain observations, must be changed when new things that cannot be explained with them, are discovered.

 Several thousand years ago, knowledge was not acquired scientifically, in the way we do it today. People thought about the mysteries of life but they did not experiment to test their ideas. The philosophers were scholarly thinkers. The Greek philosophers thought that all matter was made of 4 elements: air, fire, water and earth.  

 Democritus, Leucipus and Anaxagoras were among the Greek philosophers who proposed that matter was made of tiny particles. Democritus atomic theory was more elaborated. He used the word “atomos” (origin of the English word atom) and it means indivisible.

  Born: about 460 BC in Abdera, Thrace, Greece

 Died: about 370 BC

http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Democritus.html

Democritus ideas:

·        Matter is composed of empty space through which atoms move

    ·        Atoms are solid, homogeneous, indestructible and              indivisible

·        Different kinds of atoms have different sizes and shapes

·        The differing properties of matter are due to shape, size and movement of atoms.

·        Apparent changes in matter are due to changes in grouping of atoms and not to changes in the atoms themselves.  

This theory is really amazing for the time…. Remember they did not experiment and they did not have tools to measure and observe things…

However Democritus idea was criticized by other philosophers, including Aristotle, who rejected the idea totally. He could not believe the “nothingness” of empty space.

One thing that Democritus could not explain was what holds the atoms together so for 2000 years his theory was denied….

In the 18^th century the concept was revives but it was in the 19^th century (~1803) when John Dalton (1766-1844), and English schoolteacher revised Democritus idea, based on scientific research and propose his atomic theory.

http://results.searchscout.com/content/429/20462-1/content20462-0.html?b=15868&m=Mjc1MDMxNjg0&t=1000131616&d=0&k=term+paper&c=20462

 Main points of Dalton’s atomic theory

·        All matter is composed of extremely small particles called atom

·        All atoms of a given element are identical, having the same size, mass and chemical properties. Atoms of specific elements are different from those of any other element. 

·        Atoms cannot be created, divided into smaller particles or destroyed.

    ·        Different atoms combine in simple whole number ratios to form          compounds.

·        In a chemical reaction, atoms are separated, combined or rearranged.

Dalton’s atomic theory was accepted and it marked the beginning of the development of the modern theory of the atom. However certain things were wrong about Dalton’s theory:

1.    Atoms are not indivisible and they are made of even smaller particles.

2.    Even though the atoms of the same element are almost identical, atoms of the same element may have slightly different mass.

 Important definitions:

Atom: The smallest particle of an element that retains  the properties  of the element

Atoms are so small that it is impossible to see them with only using your eyes.  It is even impossible to see them with the microscopes you know. You need a Scanning Tunneling Microscope to see the atoms.

Subatomic Particles and the Nuclear Atom

Discoveries in Science may occur by accident. Goodyear for example, heated by accident a mixture of natural rubber and sulfur and obtained a compound that revolutionized the rubber industry, used in automobile tires. Such is the case of the discovery of subatomic particles.

Scientists wanted to know what the relationship was between electricity and matter.  With that purpose, Sir Williams Crookes invented a tube in which a vacuum was created. He put two electrodes (anode and cathode) at the end of the tube and connected them to a battery (positive and negative respectively). To his surprise, when he connected the tube, a glowing part was observed in the tube. With other tests, he could notice that the beam producing the light was coming from the cathode. For this reason the beam was called Cathode Ray and the tube Cathode Ray Tube or CRT. Televisions and computer monitors use this invention to produce the images, when the radiations from the cathode strike the chemicals that produce light at the back of the screen.

http://cwx.prenhall.com/bookbind/pubbooks/blb/chapter2/medialib/blb0202.html

In 1890s J.J .Thomson used the CRT to study the nature of the Cathode Rays. He added electric and magnetic fields and observed how the beam was deviated. This allowed him to determine that the beam was a stream of negatively charged particles. He had discovered the electron. He could also determine the charge-to-mass ratio. He compared to other charge to mass ratios and concluded that the mass of this particle was a lot less than that of the lightest atom (hydrogen).

What does this mean for the atomic theory? It meant that Dalton was wrong, the atom was not the smallest particle, the atom was divisible into smaller subatomic particles. It was hard to believe but Thomson was right.

In 1909, Robert Millikan, an American physicist performed a clever experiment, known as the oil drop experiment and measured the charge of an electron very accurately.  Using the charge to mass ratio he calculated the mass of an electron, 9.1 x 10^-28 g which is approximately 1/1837  the mass of a hydrogen atom.

Matter is neutral so to account for these negative charges Thomsom proposed his model of the atom, known as the “plum pudding model” (today it would be “chocolate chips cookie dough” model)in which the electrons are embedded in a positive sphere.

The electrons are like raisings in a pudding

or chocolate chips in the cookie dough.

  http://cwx.prenhall.com/bookbind/pubbooks/blb/chapter2/medialib/blb0202.html

Ernest Rutherford from New Zealand (1911) performed the gold foil experiment to test Thomson’s model.

He expected all the massive and energetic alpha particles go through the thin gold foil like a baseball thrown against a tissue paper would go through it.

http://cwx.prenhall.com/bookbind/pubbooks/blb/chapter2/medialib/blb0202.html

Unexpected results: Most particles went through, some were deviated and very few bounced back.

He then realized that Thomson’s plum pudding model was wrong.  He suggested the nuclear model of the atom. According to Rutherford, the atom is mostly empty space, with a tiny dense central region called the nucleus. This nucleus contains the positive charge and most of the mass of the atom, being very dense. The electrons are around this nucleus, moving rapidly through a lot of empty space.

http://www.sci.tamucc.edu/pals/morvant/genchem/atomic/page7.htm

By 1920, Rutherford refined his theory on atoms and concluded that in the nucleus there are particles called protons with a charge equal but opposite to the charge of the electron. In other words protons have a positive charge. In 1932, James Chawdick showed that the nucleus also contained another particle without charge. This particle was called the neutron and its mass is equal to that of the proton. 

Summarizing, the atoms contains three main subatomic particles: protons, neutrons and electrons. The neutrons and protons have the same mass.

The electrons and protons have equal but opposite charges.

Properties of Subatomic Particles