Atoms, Ions, and Molecules

Atoms, Ions, and Molecules

The atomic theory of matter

-         Daltons theory of matter

1.     Each element is composed of extremely small particles call atoms.

2.     All atoms of a given element are identical; atoms or different element are different.

3.     Atom of an element are not changed into different types of atom by a chemical reaction.

4.     Compounds are formed when atoms of more than one element combine.

·        Law of constant composition

In a given compound the relative numbers and kind of atoms are constant.

·        Law of multiples proportions

When two elements from different compounds, the mass ratio of the element in one compound are related to the mass ratio in the other by a small whole number.

Example :

          Carbon Monoxide ( CO )           12g C and 16g O

          Carbon Dioxide    ( CO₂ )                     12g C and 32g O₂

-         The Discovery of Atomic Structure    -

-         Cathode rays and electrons

·        A Cathode Rays Tube ( CRT ) is hollow vessel with an electrode at either one.

·        The tube is evacuated and then partically filled with a gas.

·        A high voltage is applied a cross the electrodes and the gas glows.

·        Since glowing gas originates from the cathode, it is reffered to as a “ Cathode Rays “

-         Characteristic of cathode rays

·        The rays are deflected by magnetic fields

·        Metal plates exposed to cathode rays acquire a negative charge

·        The rays emitted are independent gas in the cathode tube

-         Based on this information, it was determined that cathode rays must be some fundamental particles. In 1897, Thomson determined the change to mass ratio of an electron to be 1.76 x 10⁸  c/g. Milikan determined the charge on an electron to be 1.60 x 10^-19 colombs.

The modern view of atomic structure

          Major components of the atom :

·        Proton                            positive ( + ) charge                           1.0073        amu

·        Neutron                no charge                                 1.0087        amu

·        Electron                negative ( - ) charge                           0,000548    amu

-         The proton and neutron reside in the nucleus     -

The nucleus atom

·        The particles were shot through a piece of gold foil.

·        Most of the particles went straight throught the foil without deflection.

·        A few of the particles were scaterred.

·        A very few of the particles were deflected back to ward the source.