HISTORY, MODELS AND SUBATOMIC PARTICLES

The Atom

History of the Atom

Speculations of what matter is made up was prominent in Ancient Greece.

They thought matter was made up of what they call “Primal Matter”.

Thales thought that “WATER” was the Primal Matter because this was found everywhere.

Anaximenes thought that “AIR” was the Primal Matter.

History of the Atom

Heraclitus had the idea that “FIRE” must be primal since it represents changing world.

Aristotle claimed that substances came from all 4 elements. Earth, Air, Fire, Water.

History of the Atom

Democritus and his teacher Leucippus proposed that matter came from bits of matter that could not be broken down into smaller pieces.

This gave rise to the term “Atom” that came from the Greek word Atomos meaning Indivisible.

Introduction to Atoms

Atoms of an element are spheres stacked together like marbles. This was proven by viewing the atoms of gold in a scanning tunneling microscope but the images were fuzzy. Thus they still don’t have an actual view of an atom.

Introduction to Atoms

Two or more atoms joined together are called molecules.

Combination of two or more different elements chemically in the same fixed proportion by mass forms a compound.

Electrically charged atoms or molecules are called ions. They can be positively (cation) or negatively (anion) charged.

Atomic Theory of John Dalton

In any chemical reaction, atoms are rearranged or transferred to produce different substances. The identity and the mass of the atoms are unchanged.

Laws Governing Atoms and Matter

Joseph ProustHe claimed that a given compound always contained the same elements in the same proportion by mass.

Law of Definite Proportion

Laws Governing Atoms and Matter

Antoine LavoisierHe claimed that there was no detectable loss or gain of mass during a chemical reaction. Matter could neither be created nor destroyed.

Law of Conservation of Mass

Atomic Theory of John Dalton

All elements are made up of tiny, indivisible particles called atoms

Atoms combine only in certain whole number combinations to produce molecules.

Compounds are composed of molecules. All molecules of a compound are identical to each other.

Thomson’s Model: Plum Pudding Model

J. J. Thomson proposed that all atoms have a negative electrical charge, he identified the electrons which are confined in the atom. The atom is a positively charged blob and inside are the stationary electrons. Making the atom neutrally charged.

Rutherford’s Gold Foil Experiment

Ernest Rutherford, made an experiment with a gold foil with a +2 charged alpha particle. He noticed that the beam passed through, some bounced back and some were deflected. This means that there are spaces in between atoms and at the same time, a positive charged particle deflected the particles and scattering them around.

The Nuclear Model

The Bohr’s Model

Devised by Neils Bohr, electrons exist in specific regions of the atom at various distances from the nucleus. The electrons are quantized, meaning a certain number of electron could stay in an energy level, hence other electrons would tend to jump to the next energy level. Quantum comes from the latin word “Quantus” meaning how much.

Modern Atomic Model

An atom has a small, dense, positively charged nucleus surrounded by a large region in which there are electrons to make the atom neutral

Electrical Nature of Matter

The Cathode-Ray Tube Experiment:Proved the electrical character of the atom.

Cat ions: positively-charged ions that move toward the negatively charged electrode “cathode”.

An ions: negatively-charged ions that move toward the positively charged electrode “anode”.

Cathode (-)

Anode(+)

Zinc sulfide screen

The Subatomic Particles

The Electron:Was discovered by J.J. Thomson in 1897The mass of an electron is approximately

9.110×10−31 kilograms, or 5.486×10−4 atomic mass units.

1 amu is 1.660 × 10-24gThe electron has no known substructure.The size of an electron is not yet determined

but it is believed to be less than 10-12 cm.

Subatomic Particles

The Proton:Observed by Eugene Goldstein through the

canal rays, but what he observed where only positive ions. The discovery were credited to Earnest Rutherford wherein he observed the + nuclei of a nitrogen gas.

It has a mass of 1.673×10−24 gIt has a diameter of approximately 10-14 M. It

is 1836 times more massive than an electron and very slightly less massive than a neutron.

Subatomic Particles

The Neutron:Discovered by James Chadwick in 1932Bears neither a positive or negative chargeIt has a mass of 1.675×10−24 gThe size is 10-15 mIt is contained in the Nucleus of the atom

together with the proton making up its mass.