Components of the Atom• Nucleus:

– Small densely packed area located at the center of the atom

• Nuclear Forces:– Forces that allow protons and neutrons to

exist close together in the nucleus• Electron Cloud (orbitals)

– The large volume area that surrounds the nucleus

• Subatomic particles– Particles that make up the atom– Quantity of these parts determine an

element’s chemical/physical properties

Subatomic Particles

Particle Charge Symbol Location Grams AMU*

Proton Positive p+ Nucleus 1.673e-24 1

Neutron Neutral n0 Nucleus 1.675e-24 1

Electron Negative e- Cloud 9.11e-28 0

* AMU = atomic mass unit

Characteristics of Atoms

• All matter is composed of atoms• Atoms of one element differ in

properties from atoms of another element

• Atoms are neutral (p+ = e-)• Atoms are mostly made of empty

space.

Element Symbols

C12

Mass Number

6

Atomic Number

C-12Mass Number

Examples:

Atomic number

Mass number

p+ e- n0

O

30

238 146

30

16 8 8 8

Zn 65 3530

8

U 92 92 92

Difference between an Atom, Ion, and Isotope

• Atom: p+ = e-

• Ion: Charged particles (positive or negative) # of electrons change (p+ ≠ e-)

• Isotope: # of neutrons change Masses are different since the # of neutrons

are different

** If the # of protons change, the element changes **

Isotopes

• Where to find the mass number of isotopes: Information will be given either 3 ways

Name: carbon-14 or C-14 Symbol: 14C Adding given # no to p+ (or atomic #)

The Atomic Mass of an Isotope will be given. It can not be taken from the periodic table.

Atomic Masses• The weighted average of all the isotopes

masses known for an element• Masses found on periodic table are

atomic masses• All elements have isotopes: some are

radioactive• What you are given when trying to

calculate atomic mass Each known isotope and it’s atomic mass The percent abundance (how often it occurs)

for each element

Atomic Masses• Calculate the atomic mass of Si based on the following

isotopes of Silicon:

Si-28 Mass: 27.989 92.21%

Si-29 Mass: 28.977 4.70%

Si-22 Mass: 21.988 3.09%

1) Multiple the mass of each isotope by the abundance

6794.0309.988.21

362.10470.977.28

81.259221.989.27

Note: the percent

must be in decimal form

2) Add the products above to find the atomic mass

85.276794.362.181.25

The atomic mass is 27.9 amu