All Matter Is Made up of Small Particles Called Atoms

Democritus

Named the atom

Dalton

All matter is made up of small particles called atoms

Atoms of a given element are identical in size and mass

Atoms of different elements can physically/chemically combine to

form compounds

In reactions, atoms combine, separate, or rearrange

Atoms cannot be divided, created, or destroyed

Modern Atomic Theory

Atoms can be divided

Identical elements can have different masses (isotopes)

JJ Thomson

Cathode Ray Experiment

Plum Pudding Model

Discovered the Electron

Robert Millikan

Oil Drop Experiment

Determined the mass of an electron (9.109 x 10-28 kg)

Confirmed the charge of an electron is negative

Rutherford

Gold Foil Experiment

Discovered the proton (mass of proton = 1.673 x 10-27kg)

Discovered the nucleus

Most of the mass is in the center of the atom (positively charged),

electrons orbit the nucleus like planets orbit the sun

Chadwick

Discovered the neutron

Found high energy particles with no charge that had roughly the same mass as the proton (mass of neutron = 1.675 x 10—27kg)

Subatomic Particles

ProtonNucleus-positively charged

NeutronNucleus-neutrally charged

ElectronElectron Cloud in energy levels-negatively charged

Identifying Atoms

Atomic Number

Identifies number of protons in an element

Can be found on the periodic tableperiodic table is

organized according to increasing atomic number

Because an atom is neutrally charged, number of positives has to equal number of negatives; number of protons has to equal number of electrons; once you know the atomic number, you know the number of protons; once you know the number of protons you know the number of electrons. If you know the number of electrons, you know the number of protons. If you know the number of protons, you can identify the specific element by using the periodic table.

Mass Number

Is equal to the number of protons + the number of neutrons. This number is not on the periodic table. If you take the decimal number on the periodic table and round it to a whole number, you will have the mass number. Because atoms of the same element do not have to have the same number of neutrons, atoms of the same elements can have more than one mass number. An atom of the same element with different number of neutrons is the definition of isotopes. Example: Carbon-15 and Carbon-12.

Average Atomic Mass

Since not all atoms of the same element have the same mass (because of the existence of isotopes), the mass on the periodic table (the decimal number) is the average atomic mass

This mass is given in atomic mass units (amu)

To find the average atomic mass: identify all known isotopes of an element and record their masses and determine the average.

Mass given on the periodic table

Mass given in amu (atomic mass units)

1 amu = 1.660540 X 10-27 kg

Identify all known isotopes of an element and average their masses

Example: 1

Name%AbundanceMass (amu)

Hydrogen-199.9851.007825

Hydrogen-20.0152.0140

(%abundance/100)(mass) + (%abundance/100)(mass)

(99.985/100)(1.007825) + (0.015/100)(2.0140) =1.00796 amu

Example 2:

Neon-20 has a mass of 19.992 amu and Neon-22 has a mass of 21.991 amu. In an average sample of 100 Neon atoms, 90 will be Neon-20 and 10 will be Neon-22. Calculate the average atomic mass.

(90/100)(19.992) + (10/100)(21.991) =20.192 amu