Chapter 4 Section 3 Distinguishing Among Atomsrdibler.net/Chemistry/Notes -1/Chapter 4 Notes/chem ch...

Chapter 4 Section 3

Distinguishing Among Atoms

• Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes.

4.3

4.3

• Atomic Number

– What makes one element different from another?

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• Elements are different because they contain different numbers of protons.

• The atomic number of an element is the number of protons in the nucleus of an atom of that element.

Atomic Number 4.3

• Mass Number

– How do you find the number of neutrons in an atom?

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– mass number

• The total number of protons and neutrons in an atom

• The number of neutrons in an atom is the difference between the mass number and atomic number.

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• Au is the chemical symbol for gold.

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• Isotopes

– How do isotopes of an element differ?

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• Isotopes

–atoms that have the same number of protons but different numbers of neutrons.

–different numbers of neutrons = different mass numbers.

• isotopes are chemically alike

–because they have identical numbers of protons and electrons.

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• Atomic Mass

–How do you calculate the atomic mass of an element?

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• It is useful 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.

• atomic mass unit

– Amu

– defined as one twelfth of the mass of a carbon-12 atom.

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• Some Elements and Their Isotopes 4.3

• atomic mass

–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.

4.3

• Weighted Average Mass of a Chlorine Atom

4.3

• Remember, it is asking which is more abundant.

• You do not have to tell by how much

for Conceptual Problem 4.3

• 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.

4.3

• 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%.

4.3

• The Periodic Table—A Preview

– Why is a periodic table useful?

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–periodic table • an arrangement of elements in which the elements are

separated into groups based on a set of repeating properties.

• 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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4.3

• Period

– each horizontal row of the periodic table

– 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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• Group (or family)

– each vertical column of the periodic table

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

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• A Group or Family

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