The Atomic Model Chem 9

Atomic Model Atoms are so small that, even today, direct visual inspection is all but impossible. Our model of the atom is based on indirect experimental data. Because of this, our model of the atom changes as our experimental ability improves

The Greek Model Democritus - A Greek Philosopher around the year 400 BC. Democritus concluded that matter could not be divided into smaller and smaller pieces forever. Eventually, the smallest piece of matter would be found. He used the word "Atomos“ meaning indivisible to describe the smallest possible piece of matter.

The Dalton Model: Billiard Ball Model John Dalton - The English chemist that proposed first Atomic Theory in All elements are composed of indivisible particles. Atoms of the same element are exactly alike. Atoms of different elements are different. Compounds are formed by joining atoms of two or more elements

What did Dalton base his theory on? – Law of Conservation of Mass (Antoine Lavosier, 1789) Atoms are neither created or destroyed (under normal chemical reactions) – Law of Definite Proportion, (Proust's Law) Atom ratio is fixed, so mass must be constant. – Law of Multiple Proportions (Dalton) Applies where two elements, A and B, form more than one compound.

Imagine a penny, made up of copper (Cu) If we ground up the penny into dust, the dust would still have the same properties as copper If you placed atoms of copper in a row, atoms would make a line 1 centimeter long How Big is an Atom

How Big is an Atom? A penny contains around 2.4 x atoms of copper Can we see atoms? To see atoms we need a scanning tunneling electron microscope or an ion beam microscope

A micrograph of a Nickel-Molybdenum (Ni 4 Mo) bi-metallic compound, each dot is a single atom The specimen is in the form of a needle that is over 1000 times sharper than an ordinary household sewing needle. In fact, the end of the needle is so sharp that it cannot be seen by the naked eye or even a standard optical microscope

What’s Wrong with DAT? Most of Dalton’s theory is accepted today However, We now know that atoms ARE divisible Atoms contain sub-atomic particles ELECTRONS PROTONS NEUTRONS

The Thomson Model: Chocolate Chip Cookie Model J. J. Thomson 1856 – The English scientist who discovered Electrons in Thomson determined the charge of an electron to be negative in his cathode ray experiment Thompson initially called electrons CORPUSLES

Electrons High Voltage Negative Positive Vacuum

Robert A. Millikan (1868 – 1953) 1900 – Electron defined as a small charged particle approximately 1/2000 the mass of a Hydrogen atom determined the charge carried by an electron OIL DROP EXPERIMENT

Protons – E. Goldstein Eugene Goldstein found evidence of positively charged particles traveling in the opposite direction to electrons in a cathode ray tube He named them CANAL RAYS He deduced that canal rays were positively charged particles we now know as PROTONS

Neutron Sir James Chadwick –Discovered the neutron in 1932 –Has no charge but a mass close to that of the proton

The Rutherford Model: Gold Foil Experiment Ernest Rutherford - The British physicist who, in 1908, proved the atom had a small, dense, positively charged Nucleus.  Rutherford's model proposed that an atom is mostly empty space. There is a small, positive nucleus with the negative electrons scattered around the outside edge.

 -source Au Foil Flourescent Screen Vacuum c. Rutherford Appleton Laboratories

Gold Foil Experiment

Conclusions Rutherford concluded that the atom is mostly empty space However, the atom contains a nucleus where the mass and positive charge of the atom are concentrated We now know that the nucleus contains PROTONS and NEUTRONS Electrons ORBIT the nucleus but are not part of it

The Bohr Model: The Planet Model  Niels Bohr - The Danish scientist who, in 1913, proposed the Planetary Model of the atom. Electrons move in definite orbits around the nucleus, like planets moving around the nucleus. Bohr proposed that each electron moves in a specific energy level.

Properties of Atoms

The Wave Model  Based on Wave Mechanics, this model proposes that electrons have NO definite path in an atom. The probable location of an electron is based on how much energy it has. The more energy an electron has, the farther from the nucleus. The small, positively charged nucleus is surrounded by a large space in which there are enough electrons to make the atom neutral.

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