Atomic Structure and the Atomic Structure and the Periodic Table ContinuedPeriodic Table Continued

Atomic MassAtomic Mass

Mass = the amount of matter in an object Mass = the amount of matter in an object What does an atom weigh?What does an atom weigh?

Unit of Measurement = Atomic Mass Units or uUnit of Measurement = Atomic Mass Units or u

1u = 1.67 X101u = 1.67 X10-24-24 g g

oror

0.00000000000000000000000167 g0.00000000000000000000000167 g

This is a REALLY SMALL NUMBER!!!!!This is a REALLY SMALL NUMBER!!!!!

Mass NumberMass NumberMass Number = the sum of the number of Mass Number = the sum of the number of protons and neutrons in the nucleus of an protons and neutrons in the nucleus of an atomatom

Protons and Neutrons each have a mass Protons and Neutrons each have a mass around 1 uaround 1 u

Example: An atom has 6 protons and 6 Example: An atom has 6 protons and 6 neutrons: What is the mass number?neutrons: What is the mass number?

6 protons + 6 neutrons = 12 u Carbon6 protons + 6 neutrons = 12 u Carbon

How Do You Know How Many How Do You Know How Many Neutrons Are In An AtomNeutrons Are In An Atom

Subtract the Atomic # (which equals the # Subtract the Atomic # (which equals the # of Protons) from the Mass #of Protons) from the Mass #

Important Point: Atomic mass and thr Important Point: Atomic mass and thr mass # are not the mass # are not the same thingsame thing

Atomic Mass

HUH!!!!!!HUH!!!!!!

To get the mass number from the atomic To get the mass number from the atomic weight, either round the atomic weight up weight, either round the atomic weight up or down (.0 - .49 round down; .5 - .99 or down (.0 - .49 round down; .5 - .99 round up):round up):

Example: OxygenExample: Oxygen Atomic Mass = 15.9994Atomic Mass = 15.9994

Mass Number = 16Mass Number = 16

Back to the original question:Back to the original question:

For Oxygen the mass number is 16 and For Oxygen the mass number is 16 and the number of protons is 8the number of protons is 8

16 mass # – 8 Protons = 8 Neutrons16 mass # – 8 Protons = 8 Neutrons

Do all atoms of the same element Do all atoms of the same element have the same number of neutrons?have the same number of neutrons?

No, they can be differentNo, they can be different

Atoms of the same element that have Atoms of the same element that have different number of neutrons are called:different number of neutrons are called:

Isotopes!!!Isotopes!!!

IsotopesIsotopesHow are isotopes written:How are isotopes written:

The top number is the mass # and the bottom The top number is the mass # and the bottom number is the atomic numbernumber is the atomic number

To get the # of neutrons just subtract the two To get the # of neutrons just subtract the two numbersnumbers

Why are isotopes important?Why are isotopes important?

Isotopes are used in radiometric datingIsotopes are used in radiometric dating

Used in ecological studiesUsed in ecological studies

Biochemical StudiesBiochemical Studies

Cancer therapiesCancer therapies

ForensicsForensics

What does having different # of What does having different # of neutrons mean for the element?neutrons mean for the element?

This means the different isotopes of the This means the different isotopes of the same element have different masses!!!!same element have different masses!!!!

Average Atomic Mass:Average Atomic Mass: The average mass of a mixture of its isotopesThe average mass of a mixture of its isotopes

This is the number we find on the periodic This is the number we find on the periodic table of elementstable of elements