Download The Atom: Quick Note Guide

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
%Abundance
Mass (amu)
Hydrogen-1
99.985
1.007825
Hydrogen-2
0.015
2.0140
(%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