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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?
4.3
• 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.
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• Weighted Average Mass of a Chlorine Atom
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• Remember, it is asking which is more abundant.
• You do not have to tell by how much
• 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.
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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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• 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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• 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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