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Stephanie Siu

Chapter 4
Atomic Structure:

You cannot see the tiny particles that make up matter. All matter is composed of such particles, called atoms. An atom is the smallest particle of an element that retains its identity in a chemical reaction. Several thousand years ago, 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.

Democritus ideas:

Matter is composed of empty space through which atoms move. Atoms are solid, homogeneous, indestructible, and indivisible. Different kindsof atoms have different sizes and shapes. The differing properties of matter are due to shape, size, and movement of atoms.

One thing that Democritus could not explain was what holds the atoms together.

In the 18th century the concept was revived but it was in the 19th century (∼1803) when John Dalton (1766-1844), and English school teacher revised Democritus idea, based on scientificresearch and propose his atomic theory. By using experimental methods, Dalton transformed Democritus’s ideas into a scientific theory.

Main points of Dalton’s atomic theory:

1. All elements are composed of extremely small particles called atoms
2. All atoms of a given element are identical, having the same size, mass and chemical properties.
Atoms of specific elements are different fromthose of any other element.
3. Atoms cannot be created, divided into smaller particles or destroyed.
4. Different atoms can physically mix together or can chemically combine in simple whole number ratios to form compounds.
5. In a chemical reaction, atoms are separated, joined or rearranged. Atoms of one element, however, are never change into atoms of another element as a result of achemical reaction.

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 : Atoms are not indivisible and they are made of even smaller particles. Even though the atoms of the same element are almost identical, atoms of the same element may have slightly different mass.

Despitetheir incredible small size, individual atoms are observable today with instruments such as: SCANNING TUNNELING MICROSCOPES

Atom: The smallest particle of an element that retains the properties of the element. Example: Hydrogen atom

Molecule: A group of atoms that are bonded together and act as a unit.
Example: Oxygen molecule

Subatomic Particles:
Much of Dalton's Atomic Theory isaccepted today, except that atoms are now known to be divisible. They can be broken down into even small particles, called subatomic particles.

Three kinds of subatomic particles are:

Scientists wanted to know what was the relationship between electricity and matter. With that purpose, Sir Williams Crookes invented a tube in which a vacuum was created.

He put twoelectrodes (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.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 called these particles CORPUSCLES, but they were later called electrons.

Thomson performed other experiments that involved passing electric...
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