4.3
Atomic Number
Atomic Number
What makes one element different from
another?
4.3
Atomic Number
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
for Conceptual Problem 4.1
4.3
Mass Number
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 is called the mass number.
The number of neutrons in an atom is
the difference between the mass
number and atomic number.
4.3
Mass Number
Au is the chemical symbol for gold.
4.1
for Sample Problem 4.1
4.3
Isotopes
Isotopes
How do isotopes of an element differ?
4.3
Isotopes
Isotopes are atoms that have the same number
of protons but different numbers of neutrons.
Because isotopes of an element have
different numbers of neutrons, they
also have different mass numbers.
4.3
Isotopes
Despite these differences, isotopes are
chemically alike because they have identical
numbers of protons and electrons.
for Conceptual Problem 4.2
4.3
Atomic Mass
Atomic Mass
How do you calculate the atomic mass
of an element?
4.3
Atomic Mass
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.
An atomic mass unit (amu) is defined as one
twelfth of the mass of a carbon-12 atom. It is
based on the mass of a proton or neutron.
1 gram = 6.02 x 1023 amu’s (Avagadro’s number)
1 amu = 1.67 x 10-24 grams
4.3
Atomic Mass
Some Elements and Their Isotopes
4.3
Atomic Mass
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.
4.3
Atomic Mass
Weighted Average Mass of a Chlorine Atom
for Conceptual Problem 4.3
for Conceptual Problem 4.3
4.3
Atomic Mass
To calculate the atomic mass of an
element, multiply the mass of each
isotope by its percent relative
abundance, expressed as a decimal, and
then add the products.
4.3
Atomic Mass
For example, carbon has two stable isotopes:
• Carbon-12, which has a relative abundance of
98.89%, and
• Carbon-13, which has a relative abundance of
1.11%.
4.3
The Periodic Table—A Preview
The Periodic Table—A Preview
Why is a periodic table useful?
4.3
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).
4.3
The Periodic Table
The Periodic Table—A Preview
4.3
The Periodic Table—A Preview
• 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.
4.3
A Period
The Periodic Table—A Preview
4.3
The Periodic Table—A Preview
• Each vertical column of the periodic table is
called a group, or family.
• Elements within a group have similar chemical
and physical properties.
4.3
A Group or Family
The Periodic Table—A Preview
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.
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
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.
END OF SHOW