Chapter 4 Section 3
Distinguishing Among Atoms
4.3
• Just as apples come in
different varieties, a
chemical element can
come in different
“varieties” called
isotopes.
4.3
• Atomic Number
– What makes one element different from another?
4.3
• 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.
4.3
Atomic Number
4.3
• Mass Number
– How do you find the number of neutrons in an
atom?
4.3
– 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.
4.3
• Au is the chemical symbol for gold.
4.3
• Isotopes
– How do isotopes of an element differ?
4.3
• Isotopes
–atoms that have the same number of
protons but different numbers of
neutrons.
–different numbers of neutrons =
different mass numbers.
4.3
• isotopes are chemically alike
– because they have identical numbers of
protons and electrons.
4.3
• 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.
4.3
• 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
• Remember, it is asking which is more abundant.
• You do not have to tell by how much
for Conceptual Problem 4.3
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?
4.3
– 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).
4.3
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.
4.3
• A Period
4.3
• Group (or family)
– each vertical column of the periodic table
– Elements within a group have similar chemical and
physical properties.
4.3
• A Group or Family
END