Isotopes:

• Recall that there are two subatomic particles that determine the mass of an atom (atomic mass, mass number, or atomic weight) - protons and neutrons.
• Atoms typically have the same number of protons and neutrons within their nucleus; however, this is not always the case.
• Isotopes are atoms of the same element that have different numbers of neutrons; the number of protons will not change.

Determining the Number of Subatomic Particles in an Atom:

• Atomic number = # of protons
• Mass number = protons + neutrons
• In a neutral atom - # of protons = # of electrons

• Helium - 4

Determining the Identity of an Element:

• You established that atoms are identified by the number of protons or the atomic number.
• If the number of protons changes then the identity of the atom and the element also will change.
• Changing the number of neutrons or electrons does not change the name of the atom/element.

Determining charge:

• Subtract the number of protons and the number of electrons (charge = #p+ - #e- ).
• Neutral atom = charge of 0
• Positive charge = number of protons that exceed the number of electrons.
• Negative charge = number of electrons that exceed the number of protons.

Periodic Table of Elements:

• Atomic number is always a whole number.
• Atomic mass is usually displayed as an average of all of the possible isotopes of that element type.
• Example - the actual atomic mass of Carbon is 12.011 amu
• Some periodic tables round the value of the atomic mass, most do not.