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**Atoms: The building blocks of Matter**

Counting Atoms

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**Atomic Number The number of protons in the nucleus of each atom**

Identifies element Periodic table – elements are arranged in order of increasing atomic #

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**Isotopes Atoms of the same element, with different numbers of neutrons**

Only the masses are different. Mass # = # protons plus the # neutrons

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**Designating Isotopes Hyphen notation/Nuclear symbol Carbon-12**

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Sample Problems How many protons, electrons, and neutrons are in an atom of Chlorine-37? Write the nuclear symbol for oxygen-16. Write the hyphen notation for the element whose atoms contain 7 electrons and 9 neutrons.

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**Atomic Mass Masses of atoms are measured in atomic mass units (amu)**

One Oxygen-16 atom = x 10-23g Masses of atoms are measured in atomic mass units (amu) By definition: 1 Carbon-12 atom = 12 amu so… amu = 1/12 mass of a Carbon-12 atom

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**Atomic Mass Some examples: Oxygen-16: 15.994915 amu**

1 proton: amu 1 neutron: amu 1 electron: amu How can carbon-12 have a mass of 12 amu?

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**Average Atomic Mass Atomic mass of carbon: 12.0111 amu (on p.t.)**

The atomic masses on the periodic table are weighted averages of the isotopes of each element. Isotope Percent abundance Atomic mass (amu) Carbon-12 98.90 % 12 Carbon 13 1.10 %

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**Isotope Percent Abundance Atomic Mass (amu)**

Oxygen 16 Oxygen-17 Oxygen-18 99.762 0.038 0.200 Copper-63 Copper-65 69.17 30.83 Uranium-234 Uranium-235 Uranium-238 0.005 0.720 99.275

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**The Mole Mole – SI unit for amount By definition…**

One mole equals the number of atoms in exactly 12 grams of Carbon-12 Avogadro’s Number: x 1023

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**Molar Mass Molar Mass – the mass of one mole of a pure substance**

The molar mass of an element is numerically equal to the atomic mass. 1 atom Hydrogen: amu 1 mole Hydrogen: g

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Sample Problems: What is the mass in grams of 3.6 mol of the element Carbon? What is the mass in grams of mol of the element Chlorine? How many moles of copper are in 3.22 g of copper? How many moles of lithium are in 2.72 x 10-4 g of lithium?

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Mole Conversions How many moles of carbon are in 2.25x1022 atoms of carbon? How many moles of oxygen are in 2,000,000 atoms of oxygen? How many atoms of sodium are in 3.80 mol of sodium? What is the mass in grams of 5.0 x 109 atoms of neon? How many atoms of carbon are in g of carbon? What mass of silver contains the same number of atoms as 10.0g of boron?

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