UNIT 2.4The Early Discoveries that led to

the Atomic Theory

Cathode Ray

J.J. Thompson used the Cathode Ray to determine the mass/charge ratio of an electron.

He discovered that atoms were divisible into smaller parts.

Robert Millikan’s Oil Drop Experiment

Used this to discover the charge of an electron

JJ Thompson’s Theory

JJ Thompson came up with a theory that electrons are embedded into a positively charged sphere

Ernest Rutherford’s Gold Foil Experiment

Rutherford’s experiment showed him that something dense and positively charged was in an atom

Unit 2.5

Atomic Theory Today

Structure of an Atom Today

Atom is electrically neutral Composed of a positively charged central

nucleus surrounded by one or more electrons

Atomic Mass,Number,Symbol

A is the mass number (protons plus neutrons)

Z is the atomic number (protons) X is the atomic symbol An isoptope has the same number of protons

but a different number of neutrons

AZX

Atomic Mass

Atomic Mass Unit is 1/12 of carbon atom (1.008 amu for a hydrogen atom)

Mass spectrometry is used to determine the atomic mass of an element

First divide the element’s mass number by 12 to get the atomic mass unit

Then multiply your answer by the percent abundance of the element to get the amu for that isotope

After you do this for all the isoptopes, you must add all of the amu’s together and you will get your atomic mass

The Atomic Theory

All matter is composed of atoms Atoms can not change into other atoms in

a reaction All atoms have the same number of

protons and electrons which determines the behavior of the element

Compounds are formed by a combination of two or more elements in specific ratios.