Download Ch. 3.4 ppt. Isotopes

Isotopes
As techniques for finding the masses of atoms has
improved, we have learned that not all atoms of the
same element are identical.
Isotopes –
• atoms of the same element that have different masses
• vary in the number of neutrons they contain in the
nucleus
• almost all elements have more than one isotope.
• Chemically, isotopes act exactly the same.
Two Isotopes of Sodium
Average Atomic Mass
• Decimal number listed on periodic
table.
• Weighted average of all the naturally
occurring isotopes of that element.
• No atom has this exact mass.
Think About It
• Hydrogen has two natural isotopes
hydrogen-1 and hydrogen-2. The atomic
mass for hydrogen is 1.007 amu. Explain
why the atomic mass is not 1.50.
• Chlorine has two naturally occurring
isotopes, chlorine-37 and chlorine-35. Which
isotope is more abundant? How do you
know?
Counting Atoms and the Mole
Counting Atoms
• SI unit for amount of substance is called
mole.
• A mole measures the number of particles
within a substance.
• A mole refers to a specific number of
particles. (Counting Unit)
• 1 mole = 6.02 x 1023 particles
• 6.02 x 1023 is also known as Avogadro’s
Number
1 mole magnesium = 6.02 x 1023 Mg atoms
1 mole zinc
= 6.02 x 1023 Zn atoms
1 mole lead
= 6.02 x 1023 Pb atoms
Molar Mass
• Although 1 mole always contains the same number
of particles, the mass of one mole varies depending
on the substance.
• Molar Mass – mass of one mole of a substance.
• Mass of one mole of an element is equal to its
ATOMIC MASS expressed in grams.
– 1 mole of magnesium = 24.31 grams
– 1 mole of zinc
= 65.39 grams
– 1 mole of lead
= 207.2 grams
One Minute Paper
• You have one minute to answer these two
questions concerning today’s lesson.
– What was the most important thing you
learned?
– What is still muddy?