Atomic Structure
Protons, Neutrons, and Electrons
Section 4.3
4.3
Distinguishing Among Atoms
• Just as apples come
in different varieties,
a chemical element
can come in different
“varieties” called
isotopes.
Protons
• Protons- determine the identity of an atom
– Elements are different because they each
have different #s of protons
• The atomic number of an element is the
number of protons in the nucleus of an
atom of that element
– You can find this number by looking at your
periodic table
• In a neutral atom, the number of protons is
equal to the number of electrons
Atomic Number
Practice Finding # of protons
and electrons
Finding the number of Neutrons
• 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.
– How do I find the mass number?
• If given, add the protons and neutrons together
• If not, round the mass from the periodic table to the
nearest whole number. This will provide the mass
number of the most common isotope.
Finding Protons, Neutrons,
Electrons
• You may just be given the name of an
element and be expected to look it up on
the periodic table.
• You may be given a symbol with mass
numbers and atomic numbers beside it.
Writing and Interpreting
Chemical Symbols
Mass of an atom
• Nucleus- contains most of the mass of
an atom
– Protons & Neutrons contain most of an
atom’s mass
• Mass of proton = mass of neutron = 1amu
– Electrons- 1/2000th of the mass of a proton
Atomic structure
• Grams are not used to measure mass
of atoms because atoms are too small
– Amu is used instead
– Amu = atomic mass unit
• Amu – 1/12th the mass of a carbon-12
atom, which contains 6 protons and 6
neutrons
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.
Isotopes
• To indicate the identity of an isotope, write the
element name – mass #
– Carbon-12 Carbon-14
– Isotopes can also be indicated using the method we
saw earlier
Calculating the Average atomic
mass of an element
• Average atomic mass- a weighted average of
the masses of the isotopes of an element
Average atomic mass = (% x mass) + (% x mass) + (% x mass)….
Example of average atomic mass
for carbon
mass number
13
exact weight
percent
abundance
12
12.0000 amu
98.90
13
13.003355 amu 1.10
(12.0000amu) (0.9890) + (13.003355amu) (0.0110) = 12.011 amu
Check your answers
• C has an average atomic mass of
12.011 amu.
• This is closer to 12 than to 13, which
means most isotopes must be carbon12.
• This matches our known data, which
say that 98.90% of carbon atoms are C12.
How do we know the mass of
isotopes??
• Use a mass spectrometer
Results from mass spectrometer
Throughout this section, we’ve
been using the periodic table
• 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).
Periodic Table
• 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.
– This occurs because they have a different
number of outer shell (or valence) electrons
4.3
The Periodic Table—A Preview
• A Period
Periodic Table
• Each vertical column of the periodic table
is called a group, or family.
• Elements within a group have similar
chemical and physical properties.
– This occurs because they have the same
number of valence (or outer shell) electrons
4.3
The Periodic Table—A Preview
• A Group or Family