**Atomic Mass and "AMU"**

** Atomic Masses** - In 1961 a system was devised to assign atomic masses to each element.

To do this, a "standard" was needed...

An isotope of carbon, carbon-12 (^{12}C), was used as the standard and was assigned a value of *exactly 12 amu*.

➞ __amu__ = atomic mass units

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**Atomic Mass and Isotopes**

*Question: * Why is carbon's

__atomic mass__equal to 12.01 amu on the Periodic Table

_________

**answer:**➞ most elements occur as a mixture of isotopes and natural carbon is found as 98.89% ^{12}C and 1.11% ^{13}C atoms:

As you can see in the graphic above, __atomic mass__ is a "weighted average" of isotopes.

Notice that * not* a single carbon atom weighs 12.01 amu!! This is an average weight.

The math involved in calculating "weighted averages" is why atomic masses of elements are expressed as decimals on the Periodic Table.

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** ex:** Lithium (Li) = 6.941amu

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**The Mole and Avogadro's Number**

** The Mole** = 1 mole of any substance contains

__exactly__6.022 x 10

^{23}items of that substance.

The mole is often abbreviated as "mol."

__examples__:

"1 __dozen__ eggs" = 12 eggs

"1 __mol__ of eggs" = 6.022 x 10^{23} eggs

__examples__:

1 mole Fe atoms = 6.022 x 10^{23} atoms of Fe

1 mole bananas = 6.022 x 10^{23} bananas

1 mole H_{2}O molecules = 6.022 x 10^{23} molecules H_{2}O

**6.022 x 10**^{23} is Avogadro's Number (N_{A})

^{23}is Avogadro's Number (N

_{A})

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****** The __mass (g)__ of __1 mole__ of an element is equal to its __atomic mass (amu)__.

➞ So for carbon...

**12.01 amu** = 12.01 grams per mole = **12.01 g/mol**

And this is true for every element in the Periodic Table.

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##
**Atoms to Grams Calculation:**

*ex:* Find the mass in grams of a sample of 6 Fe atoms.

**"Start with what you're given, and put it over 1" **

Remember?...

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**Grams to Moles Calculation:**

*ex:* Find both the number of moles of atoms and the actual number of atoms in a 10.0 g sample of aluminum.

And to calculate the number of aluminum atoms, we have:

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**Grams to Atoms Calculation:**

*ex:* A silicon chip has a mass of 5.68 mg. How many silicon atoms are present?

**Atoms to Moles, Atoms to Grams Calculations:**

*ex: * Calculate the number of moles and the mass of a sample of cobalt containing 5.00 x 10

^{20}atoms of Co.

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In my next post on **SECTION 3 - Chemical Quantities and Stoichiometry**,

We'll discuss Molar Mass, Stoichiometry, and Grams-to-Moles Practice Problems...