1. • Overview of Atomic Structure
• Atomic Number and Mass Number
• Isotopes
The Structure of the Atom
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2. Structure of an Atom
Elements, such as helium, depicted here, are made up of atoms.Atoms are made up of protons and neutrons located within the nucleus, with electrons in
orbitals surrounding the nucleus.
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3. Protons, Neutrons, and Electrons
Both protons and neutrons have a mass of 1 amu and are found in the nucleus.However, protons have a charge of +1, and neutrons are
uncharged.Electrons have a mass of 0 amu, orbit the nucleus, and have a charge of -1.
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4. • An atom is composed of two regions: the nucleus, which is in the center of the
atom and contains protons and neutrons, and the outer region of the atom which
holds its electrons in orbit around the nucleus.
• Protons and neutrons have approximately the same mass, about 1.67 × 10-24
grams, which scientists define as one atomic mass unit (amu) or one Dalton.
• Each electron has a negative charge equal to the positive charge of a proton.
• Neutrons are uncharged particles found within the nucleus.
Overview of Atomic Structure
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Structure of an Atom
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5. • Despite having different numbers of neutrons, isotopes of the same element have
very similar physical properties.
• Some isotopes are unstable and will undergo radioactive decay to become other
elements.
• The predictable half-life of different decaying isotopes allows scientists to date
material based on its isotopic composition, such as with Carbon-14 dating.
Isotopes
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Application of carbon dating
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6. Atomic number, Chemical Symbol, and Mass Number
Carbon has an atomic number of six, and two stable isotopes with mass numbers of twelve and thirteen, respectively.Its atomic mass is 12.11.
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Which of the following statements correctly describes the relative
masses of the three subatomic particles?
A) Protons and electrons weigh the same, neutrons weigh much less.
B) Neutrons and electrons weigh the same, protons weigh much less.
C) Protons and neutrons weigh the same, electrons weigh much less.
D) Protons, neutrons and electrons all weight the same.

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Atoms, Molecules, and Ions
Which of the following statements correctly describes the relative
masses of the three subatomic particles?
A) Protons and electrons weigh the same, neutrons weigh much less.
B) Neutrons and electrons weigh the same, protons weigh much less.
C) Protons and neutrons weigh the same, electrons weigh much less.
D) Protons, neutrons and electrons all weight the same.

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Atoms, Molecules, and Ions
Which of the following statements correctly describes the relative
charges of the three subatomic particles?
A) Neutrons are positive, electrons are neutral, protons are negative.
B) Electrons are positive, protons are neutral, neutrons are negative.
C) Protons are positive, neutrons are neutral, electrons are negative.
D) Protons and neutrons have a charge of +1, while electrons have a
much smaller charge.

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Atoms, Molecules, and Ions
Which of the following statements correctly describes the relative
charges of the three subatomic particles?
A) Neutrons are positive, electrons are neutral, protons are negative.
B) Electrons are positive, protons are neutral, neutrons are negative.
C) Protons are positive, neutrons are neutral, electrons are negative.
D) Protons and neutrons have a charge of +1, while electrons have a
much smaller charge.

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Which of the following statements accurately describes the
locations of the three subatomic particles that make up an atom?
A) The protons and neutrons are in the nucleus, while the electrons orbit
the nucleus.
B) The protons and electrons are in the nucleus, while the neutrons orbit
the nucleus.
C) The neutrons and electrons are in the nucleus, while the protons orbit
the nucleus.
D) The protons, neutrons and electrons are all in the nucleus.

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Atoms, Molecules, and Ions
Which of the following statements accurately describes the
locations of the three subatomic particles that make up an atom?
A) The protons and neutrons are in the nucleus, while the electrons orbit
the nucleus.
B) The protons and electrons are in the nucleus, while the neutrons orbit
the nucleus.
C) The neutrons and electrons are in the nucleus, while the protons orbit
the nucleus.
D) The protons, neutrons and electrons are all in the nucleus.

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If an element has a mass number of 18, what can be said about
the number of protons and neutrons it contains?
A) It has 8 protons and 10 neutrons.
B) It has 10 protons and 8 neutrons.
C) It cannot be determined from the information given.
D) It has 9 protons and 9 neutrons.

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Atoms, Molecules, and Ions
If an element has a mass number of 18, what can be said about
the number of protons and neutrons it contains?
A) It has 8 protons and 10 neutrons.
B) It has 10 protons and 8 neutrons.
C) It cannot be determined from the information given.
D) It has 9 protons and 9 neutrons.

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Atoms, Molecules, and Ions
If an atom has _______ protons, what is its atomic number?
A) It depends on the number of neutrons in the atom.
B) It depends on the number of electrons in the atom.
C) 8
D) 16

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Atoms, Molecules, and Ions
If an atom has _______ protons, what is its atomic number?
A) It depends on the number of neutrons in the atom.
B) It depends on the number of electrons in the atom.
C) 8
D) 16

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Atoms, Molecules, and Ions
Which of the following statements concerning isotopes is true?
A) Isotopes are atoms of the same element that have the exact same
atomic mass.
B) Isotopes are atoms of different elements that have the same atomic
mass.
C) Isotopes are atoms of different elements that have different atomic
masses
D) Isotopes are atoms of the same element that have different atomic
masses.

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Atoms, Molecules, and Ions
Which of the following statements concerning isotopes is true?
A) Isotopes are atoms of the same element that have the exact same
atomic mass.
B) Isotopes are atoms of different elements that have the same atomic
mass.
C) Isotopes are atoms of different elements that have different atomic
masses
D) Isotopes are atoms of the same element that have different atomic
masses.

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Which of the following measurements used in radiometric dating
can be used to determine the age of fossils?
A) The decrease in the amount of an unstable radioisotope according to
its known half-life
B) The increase in the amount of an unstable radioisotope according to
its known half-life
C) The current half-life of a stable radioisotope in the sample
D) The amount of a radioisotope leached into surrounding environments
according to its known half-life

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Atoms, Molecules, and Ions
Which of the following measurements used in radiometric dating
can be used to determine the age of fossils?
A) The decrease in the amount of an unstable radioisotope according to
its known half-life
B) The increase in the amount of an unstable radioisotope according to
its known half-life
C) The current half-life of a stable radioisotope in the sample
D) The amount of a radioisotope leached into surrounding environments
according to its known half-life

21. Key terms
• atom the smallest possible amount of matter which still retains its identity as a chemical element, consisting of a nucleus
surrounded by electrons (CC BY-SA 3.0)
• atomic mass the quantity of matter in an atomic particle, sub-atomic particle, or molecule (CC BY-SA 3.0)
• atomic number The number of protons in an atom. (CC BY-SA 3.0)
• electron The subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor
constitutes electricity. (CC BY-SA 3.0)
• isotope any of two or more forms of an element where the atoms have the same number of protons, but a different number of
neutrons within their nuclei (CC BY-SA 3.0)
• neutron A subatomic particle forming part of the nucleus of an atom and having no charge.It is equal in mass to a proton. (CC BY-SA
3.0)
• proton Positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an
element. (CC BY-SA 3.0)
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22. Attribution
• Connexions. "Fundamental force types." CC BY 3.0 http://cnx.org/content/m14044/latest/
• Connexions. "Atoms, Isotopes, Ions, and Molecules: The Building Blocks." CC BY 3.0
http://cnx.org/content/m44390/latest/?collection=col11448/latest
• Connexions. "Atoms, Isotopes, Ions, and Molecules: The Building Blocks." CC BY 3.0
http://cnx.org/content/m44390/latest/?collection=col11448/latest
• Wikibooks. "A-level Chemistry/OCR/Atoms, Bonds and Groups/Atoms and Reactions/Atoms." CC BY-SA 3.0
http://en.wikibooks.org/wiki/A-level_Chemistry/OCR/Atoms,_Bonds_and_Groups/Atoms_and_Reactions/Atoms
• Connexions. "Atoms, Isotopes, Ions, and Molecules: The Building Blocks." CC BY 3.0
http://cnx.org/content/m44390/latest/?collection=col11448/latest
• Wikipedia. "Radiocarbon dating." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Radiocarbon_dating
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