Unit III Objectives 3 - 5

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Subatomic Particles
Atomic Number | Isotopes | Ions
Subatomic Particles
Learning Objectives
• Understand the importance of atomic numbers
• Find the numbers of subatomic particles in a given isotope
• Calculate the average atomic mass of an element using
isotopic composition
• Determine the charge of an ion based on numbers of
subatomic particles
Atomic Number
• Neutrons
– Neutral charge, 0
– 1 amu
• Protons
– Positive charge, +1
– 1 amu
• Atomic number – the number of protons in an atom,
which determines the identity of an element
– Modern periodic table is arranged by atomic number
• Ex) All atoms containing 6 protons are carbon atoms
Isotopes
• Isotopes – the atoms of the same element that
contain different numbers of neutrons and
therefore have differing masses
• Mass number – the sum of the number of
neutrons and the number of protons in an
isotope
Isotopes
• Isotopic notation
– Mass number written as a superscript in front of the element’s
symbol
– Atomic number written as a subscript in front of the element’s
symbol
Finding the Number of a Subatomic Particle
Ex) How many neutrons does 178 O contain?
I.
II.
Strategy
Given and unknown
III. Formula
so
IV. Conversions needed? No.
V. Intermediates needed? No.
VI. Solve
IV. Does the result make sense? Yes.
Average Atomic Mass
• Value given for element’s mass on periodic table
• Isotopic composition – the relative amounts of each
isotope of an element found in a sample
• Percent abundance – the percentages that describe isotopic
compositions
Average Atomic Mass
• Average atomic mass – the weighted average of the masses
of all stable isotopes of an element
• Atomic mass unit (amu) – a unit of mass equal to onetwelfth the mass of a carbon-12 atom
Finding Average Atomic Mass
Step 1 Change the given percent abundances into decimal form.
Step 2 Multiply the given mass of each isotope by its percent
abundance in decimal form.
Step 3 Find the sum of these values.
Finding Average Atomic Mass Example Step 1
Ex) Use the information in the data table to find the average
atomic mass of silver.
Isotope
Silver-107
Silver-109
Step 1
Percent
51.839%
48.161%
Atomic Mass (amu)
106.91
108.90
Change the given percent abundances into decimal form.
Finding Average Atomic Mass Example Step 2
Ex) Use the information in the data table to find the average
atomic mass of silver.
Isotope
Silver-107
Silver-109
Step 2
Percent
51.839%
48.161%
Atomic Mass (amu)
106.91
108.90
Multiply the given mass of each isotope by its percent abundance in
decimal form.
Finding Average Atomic Mass Example Step 3
Ex) Use the information in the data table to find the average
atomic mass of silver.
Isotope
Silver-107
Silver-109
Step 3
Percent
51.839%
48.161%
Find the sum of these values.
Does the result make sense? Yes.
Atomic Mass (amu)
106.91
108.90
Ions
• Electrons
– Outside nucleus
– Negative charge, –1
• Atoms containing equal numbers of protons and electrons
are neutrally charged
• Ion – an atom that has an overall charge because it does
not contain the same number of protons and electrons
– Cation – a positively charged ion formed when an atom loses one
or more electrons
– Anion – a negatively charged ion formed when an atom gains
one or more electrons
– Charge equals sum of charges of protons and electrons
Finding the Charge of an Ion
Step 1 Find the sign of the charge by determining whether
protons outnumber electrons or electrons outnumber
protons in the ion.
Step 2 Find the magnitude of the charge by determining the
difference between the number of protons and electrons
in the ion.
Finding the Charge of an Ion
Ex) What is the charge on an ion that contains 53 protons and 54
electrons?
Step 1 Find the sign of the charge by determining whether protons
outnumber
electrons or electrons outnumber protons in the ion.
Electrons outnumber protons, so this is an anion. The ion will be
negatively charged.
Step 2
Find the magnitude of the charge by determining the difference
between the number of protons and electrons in the ion.
There is one more electron than proton, so the magnitude of the
charge is 1. The charge of the ion is –1.
Does the result make sense? Yes.
Subatomic Particles
Learning Objectives
• Understand the importance of atomic numbers
• Find the numbers of subatomic particles in a given isotope
• Calculate the average atomic mass of an element using
isotopic composition
• Determine the charge of an ion based on numbers of
subatomic particles

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