CHAPTER 17SECTION 2

Masses of Atoms

Atomic Mass Remember that Protons

and Neutrons are much bigger than an Electron.

Because of this, the nucleus contains the majority of the mass in any atom.

The mass of a Proton and Neutron are approximately the same.

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Add em up!

The mass of one Proton or one Neutron is considered to be 1 atomic mass unit.1 amu =

By counting the number of Protons in an atom we get the atomic number.

Atoms of the same element will all have the same atomic number.

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Mass Number

If we add up the number of Protons and Neutrons in an atom we get the mass number of the atom.Some books call this the atomic mass.

Number of Protons + Number of Neutrons = Mass Number

Number of Protons = Atomic Number

Atomic Number ≠ Mass Number

Practice

HydrogenHelium

Lithium

Carbon

Isotopes

Elements are defined by how many protons are in the nucleus of the atom.

Atoms that come from the same element but have a different number of Neutrons are called Isotopes.

Do Isotopes of the same element have the same atomic number?

Do Isotopes of the same element have the same mass number?

ExampleAll threes isotopes have the same number of … PROTONS!

All three isotopes have a different number of… NEUTRONS!

All three isotopes have the same atomic numberbut different mass numbers.

Isotope Examples

Isotope Examples

Average Atomic Mass

Because most elements have more than one type of isotope, the masses of each isotope are used to make a weighted-average.Average atomic mass

Isotopes of CarbonCarbon-12 99%

Carbon-13 1%

Carbon 14 0. 0000000001%

Weighted-Average Take the mass of each isotope

and multiple it by the % of that isotope that exists in nature.

Next, add them together.

The average atomic mass of Carbon is…

12.01 amu

(12)X(0.99) (13)x(0.01) (14)x(0.000000000001)+ +

Weight lose Atoms with small nuclei are very stable.

So, what is the most stable element in the universe?

Atoms with large nuclei become less stable. Atoms with very large nuclei are unstable.

If a nucleus is unstable it will begin to lose Protons and Neutrons. These particles will shoot off as radiation leaving a smaller nucleus behind.

Basically the atom looses weight!

Remember, Protons define the element.

If an atom is losing protons that means it is turning into another type of element.This process is known as radioactive decay.

Different elements and their isotopes will decay at different rates. Some are very fast…. MicrosecondsSome are very slow…. Billions of years

Half-Life NO! not the game! The time that it

takes for half of a substance to disappear due to radio-active decay is called the materials half-life.

Number of half-lives Fraction remaining % remaining

0 1 100%

1 1/2 50%

2 1/4 25%

3 1/8 12.50%

4 1/16 6.25%

Half-Lives of certain isotopes

Excellent! Find a partner. You will work

together to identify different atoms.