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Distinguishing Among Atoms >

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Atomic Mass

Atomic Mass

How do you calculate the atomic mass of an element?

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>Distinguishing Among Atoms Atomic Mass

It is useful to to compare the relative masses of atoms to a standard reference isotope. Carbon-12 is the standard reference isotope. Cabon-12 has a mass of exactly 12 atomic mass units.

An atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom.

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Some Elements and Their Isotopes

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The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element.

A weighted average mass reflects both the mass and the relative abundance of the isotopes as they occur in nature.

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Practice Problems

for Conceptual Problem 4.3

for Conceptual Problem 4.3

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For example, carbon has two stable isotopes:

• Carbon-12, which has a natural abundance of 98.89%, and

• Carbon-13, which has a natural abundance of 1.11%.

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To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.


SAMPLE PROBLEM

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4.2


SAMPLE PROBLEM

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SAMPLE PROBLEM

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Practice Problems for Sample Problem 4.2

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>Distinguishing Among Atoms The Periodic Table—A Preview

A periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties.

A periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements).

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The Periodic Table—A Preview

Why is a periodic table useful?

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The Periodic Table

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• Each horizontal row of the periodic table is called a period.

• Within a given period, the properties of the elements vary as you move across it from element to element.

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A Period

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• Each vertical column of the periodic table is called a group, or family.

• Elements within a group have similar chemical and physical properties.

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A Group or Family (Groups 1-18 regents tables)

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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

4.3 Section Quiz

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4.3 Section Quiz

1. Isotopes of an element have

a. the same mass number.

b. different atomic numbers.

c. the same number of protons but different numbers of neutrons.

d. the same number of protons but different numbers of electrons.


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4.3 Section Quiz

2. How many neutrons are in sulfur-33?

a. 16 neutrons

b. 33 neutrons

c. 17 neutrons

d. 32.06 neutrons


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4.3 Section Quiz

3. If sulfur contained 90.0% sulfur-32 and 10.0% sulfur-34, its atomic mass would be

a. 32.2 amu.

b. 32.4 amu.

c. 33.0 amu.

d. 35.4 amu.

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