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Section 4.2—Atomic Structure
Objectives:•determine the number of protons, neutrons, & electrons in atoms•distinguish between the properties of protons, neutrons, & electrons•define isotopes•differentiate between mass number and average atomic mass•calculate weighted average atomic masses
What are atoms?
An atom is the smallest piece of an element that has all the chemical properties of that element.
What is in an atom?
An atom is made of three sub-atomic particles
Particle Location
Nucleus
Nucleus
Outside the nucleus
Mass
1 amu(1.6710-
27 kg)
1 amu (1.6710-27 kg)
0.00055 amu
(9.1010-31 kg)
Charge
+1
0
-1
Proton
Neutron
Electron
What gives an atom its identity?
What makes an atom “carbon” as opposed to “oxygen”?
Every atom has a different number of protons.
The number of protons determines the identity of the atom
On a periodic table, the atomic number shows the number of protons.
The Nucleus & Mass
Since the nucleus has protons & neutrons, and the mass of each one is 1 amu…
The mass of the nucleus (called the mass number) is the number of protons + neutrons
Since electrons have relatively no mass (0.054% of one proton or neutron), we don’t need to worry about them when determining mass of an atom.
The mass number (protons + electrons) represents the mass of an atom.
Charges in an Atom
Protons have a positive charge (+)Electrons have a negative charge (-)Neutrons have no chargeOverall charge of an atom = 0 because
the number of protons equals the number of electrons in any atom.
Ions
An ion is any charged particle; specifically, if an atom LOSES or GAINS electrons, it will become an ion.
If an atom loses electrons, there will be more protons than electrons. It will become a positively-charged ion.
If an atom gains electrons, there will be more electrons than protons. It will become a negatively-charged ion.
Element symbols
Charge# protons - # electrons(assumed to be “0” if blank)
Element Symbol1 or 2 letters, found on
the periodic table
Mass number# protons + # neutrons
Atomic number # of protons
Another element symbolIt is possible for atoms to have the same
number of protons but different numbers of neutrons. Such atoms are called isotopes.isotopes.
To make it easy to identify the various isotopes of an element, scientists add a number after the element’s name.
This number is the mass numbermass number - the sum of the number of protons and neutrons in the nucleus of an atom of the isotope.
Let’s Practice
Example:Fill in the missing values
Symbol Name Atomic #
Mass #
Charge Proton Neutron Electron
Magnesium-25 +2
83Li
15 16 18
53 127 -1
What are isotopes?
Isotopes - n. Atoms of the same element with a different number of neutrons
Some isotopes are radioactive—but not all…many are quite stable!
Isotopes Example
If they have different number of neutrons, and neutrons have a mass of 1 amu…
Then isotopes of the same element will have different masses!
But because their protons are the same, they are the same element!
Hydrogen-2Hydrogen-1
Mass # = 1 amu Mass # = 2 amu
Identifying Isotopes
12C 13C
Carbon-12 Carbon-13
Isotopes can be differentiated by their different mass numbers in the element symbol or by the mass number following their name.
Isotopes
Elements are found in nature as a mixture of all of their isotopes.
The percent abundancepercent abundance of the isotopes (percent of each isotope in a sample) is constant no matter where the sample is found.
Isotopes have the same chemical behavior because chemical behavior is determined by the number of ELECTRONS an element has.
Atomic Mass
The atomic massatomic mass of an element is the weighted average mass of the isotopes of that element.
Atomic mass, therefore, takes into account the percent abundance of each isotope as well as the mass of an atom of that isotope.
The isotope in greater abundance will have a greater effect on determining the atomic mass.
Analyzing Atomic Mass
Analyzing an element’s atomic mass can SOMETIMES give you a clue as to what the most abundant isotope for that element is.
The atomic mass of fluorine is 18.998 amu. From this you can conclude that most fluorine occurs in the form of fluorine-19.
Mass Number versus Atomic Mass
Mass Number Average Atomic Mass
# of protons + # of neutrons Average of actual masses
For one specific isotope only
Is not found on the periodic table
Weighted average of all isotopes
Is found on the periodic table
Always a whole number Not a whole number
Calculating Atomic Mass
1 Multiply the % abundance of each isotope by its mass. (75%)(35) = 26.25 amu
(25%)(37) = 9.25 amu
2 Add the mass contributions of all the isotopes together.
35.5 amu is the calculated atomic mass of chlorine.
Chlorine-35 Chlorine-37
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