ATOMS
ATOMS, ISOTOPES, AND IONS
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THE ATOM
All elements listed on the periodic table are
made up of atoms. An atom is the smallest
particle of an element.
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Dalton's Atomic Theory
The idea of atoms did not become scientific
theory until 1808.
John Dalton (1766–1844) developed an atomic
theory proposing that atoms were responsible for
the combinations of elements in compounds.
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Dalton's Atomic Theory
1. All matter is made up of tiny particles
called atoms.
2. All atoms of a given element are identical to one
another and different from atoms of other elements.
3. Atoms of two or more different elements combine to
form compounds. A particular compound is always
made up of the same kinds of atoms and the same
number of each kind of atom.
4. A chemical reaction involves the rearrangement,
separation, or combination of atoms. Atoms are
never created or destroyed in a chemical reaction.
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Atoms
Atoms are
• the building blocks of everything around us
• too small to see with the naked eye
Image of
Platinum & Nickel atoms
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Subatomic Particles in an Atom
By the end of the 1880s,
• experiments with electricity showed that atoms
were composed of tiny particles, called
subatomic particles which included protons,
neutrons, and electrons
• it was shown that some subatomic particles in
an atom have charge
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Electrical Charges in an Atom
Electrical charges can be
positive or negative.
• two positive charges
repel each other
• two negative charges
repel each other
• unlike charges attract
each other
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The Structure of an Atom
In 1897, J.J. Thomson discovered the
electron using cathode ray experiments. He
proposed the plum pudding model of an
atom. In this model electrons and protons are
uniformly mixed throughout the atom.
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The Structure of an Atom
In 1911, Ernest Rutherford tested J. J. Thomson’s
hypothesis using his “gold foil” experiment. This
findings were not consistent with Thomson’s
model. Rutherford proposed the planetary model
of the atom. Which states there is:
• a small region in the center with positive
charge called the nucleus
• a region of space around the center of the
atoms occupied by electrons
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The Structure of an Atom
In 1932, James Chadwick discovered that the
nucleus of the atom also contained neutral particles
called neutrons.
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The Structure of an Atom
In an atom, the protons
and neutrons that make
up almost all the mass of
the atom are packed into
the tiny volume of the
nucleus. The rapidly
moving electrons
(negative charge)
surround the nucleus and
account for the large
volume of the atom.
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Mass of the Atom
• The mass of the atom is due to the protons and neutrons in
the nucleus. Electrons have a much smaller mass.
• Chemists use a unit called atomic mass unit (amu), defined
as one-twelfth of the mass of the carbon atom with 6
protons and 6 neutrons.
• The mass of all elements in the periodic table is compared
to the mass of this carbon atom.
On the amu scale, the mass of a proton and a neutron
have a mass of about 1 amu.
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THE PERIODIC TABLE
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ATOMIC NUMBER
The atomic number
• is specific for each element and the same for
all atoms of that element
• is equal to the number of protons in an atom
• typically appears above the symbol of an
element
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ATOMIC NUMBERS AND PROTONS
• Hydrogen has atomic number 1; every H atom
has 1 proton.
• Carbon has atomic number 6; every C atom
has 6 protons.
• Copper has atomic number 29; every Cu atom
has 29 protons.
• Gold has atomic number 79; every Au atom
has 79 protons.
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ATOMIC MODELS
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LEARNING CHECK
State the number of protons in each atom.
1. A nitrogen atom
(a) 5 protons
(b) 7 protons (c) 14 protons
2. A sulfur atom
(a) 32 protons (b) 16 protons (c) 6 protons
3. A barium atom
(a) 137 protons (b) 81 protons (c) 56 protons
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SOLUTION
State the number of protons in each atom.
1. A nitrogen atom
(b) 7 protons
2. A sulfur atom
(b) 16 protons
3. A barium atom
(c) 56 protons
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Atoms Are Neutral
An atom of any element
• is electrically neutral; it has a net charge of zero
• has an equal number of protons and electrons
A neutral atom of calcium, atomic number 20,
contains 20 protons and 20 electrons. It has zero
net charge.
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Mass Numbers
The mass number
• represents the number of particles in the
nucleus
• is equal to the
number of protons + number of neutrons
• does not appear on the periodic table because it
applies to a single atom only
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Calculate Number of Neutrons
We calculate the number of neutrons in an atom
from its mass number and atomic number:
𝑀𝑎𝑠𝑠 𝑁𝑢𝑚𝑏𝑒𝑟 - 𝐴𝑡𝑜𝑚𝑖𝑐 𝑁𝑢𝑚𝑏𝑒𝑟 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑁𝑒𝑢𝑡𝑟𝑜𝑛𝑠
Potassium has a mass number of 39 and an
atomic number of 19. To find the number of
neutrons, subtract the atomic number from its
mass number.
39 (mass number) – 19 ( atomic number) = 20 neutrons
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Composition of Elements
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Learning Check
An atom of zinc has a mass number of 65.
1. How many protons are in the nucleus?
(a) 30
(b) 35 (c) 65
2. How many neutrons are in the nucleus?
(a) 30
(b) 35 (c) 65
3. What is the mass number of a zinc atom
that has 37 neutrons?
(a) 37
(b) 65 (c) 67
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Solution
An atom of zinc has a mass number of 65.
1. How many protons are in the nucleus?
(a) 30
2. How many neutrons are in the nucleus?
(b) 35
3. What is the mass number of a zinc atom
that has 37 neutrons?
(c) 67
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Learning Check
An atom has 14 protons and 20 neutrons.
1. Its atomic number is
(a) 14
(b) 16
(c) 34
2. Its mass number is
(a) 14
(b) 16
(c) 34
(b) Ca
(c) Se
3. The element is
(a) Si
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Solution
An atom has 14 protons and 20 neutrons.
1. Its atomic number is
(a) 14
2. Its mass number is
(c) 34
3. The element is
(a) Si
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ISOTOPES
Isotopes
• are atoms of the same element that have different
mass numbers
• have the same number of protons, but different
numbers of neutrons
• most elements have two or more isotopes that
contribute to the atomic mass of that element.
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Atomic Symbols
An atomic (nuclear) symbol
• represents a particular isotope of an element
• gives the mass number in the upper left corner
and the atomic number in the lower left corner
mass number
atomic
number
24
12
Mg
Chemical Symbol
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Atomic Symbols, Subatomic Particles
The atomic symbol indicates the number of
protons, neutrons and, electrons in a specific
isotope of an element.
16
8
O
31
15
P
65
30
Zn
8 protons
8 neutrons
15 protons
16 neutrons
30 protons
35 neutrons
8 electrons
15 electrons
30 electrons
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Atomic Mass
Atomic mass is the average of all the naturally occurring isotopes
of that element.
The atomic mass of an element is
• listed below the symbol of each element on the
periodic table
• calculated based on the weighted average of all
naturally occurring isotopes
• based on its comparison
to the mass of 12C
• not the same as
the mass number 11
Atomic Mass
Na
22.99
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VALENCE ELECTRONS
Electrons orbit the nucleus in various energy
levels or shells.
Electrons in the outermost energy shell are called
valence electrons.
• All elements in the same group have the same
number of valence electrons.
• An elements valence electrons determines the
chemical properties
• Atoms transfer or share valence electrons in order to
obtain a stable electron configuration. Typically,
eight or zero valence electrons.
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Ions
Ions, which have electrical charges, form when
atoms lose or gain electrons to form a stable
electron configuration.
• Metals lose valence electron to form ions with
positive charge.
• Non-metals gain valence electrons to form
ions with negative
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Positive Ions: Loss of Electrons
A sodium atom (Na) will lose its valence electron to
form a sodium ion (Na+).
Positively charged ions of metals are called cations.
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Positive Ions: Loss of Electrons
Magnesium, a metal in Group 2A (2), obtains
a stable electron configuration by losing two
valence electrons, forming an ion with a
2+ charge.
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Negative Ions: Gain of Electrons
An atom of chlorine with seven valence
electrons gains one electron to form an octet.
Because it now has 18 electrons and not
17 electrons, it becomes a chloride ion (Cl−)
with a charge of 1−.
Negatively charged ions of nonmetals are called
anions.
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Transfer of Electrons
The metal transfers its valence electron(s) to the
non-metal.
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Formula and Names of Some Common Ions
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Summary of Important Concepts
General Rules
𝐴𝑡𝑜𝑚𝑖𝑐 𝑁𝑢𝑚𝑏𝑒𝑟 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑃𝑟𝑜𝑡𝑜𝑛𝑠
𝑀𝑎𝑠𝑠 𝑁𝑢𝑚𝑏𝑒𝑟 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑁𝑒𝑢𝑡𝑟𝑜𝑛𝑠 + 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑃𝑟𝑜𝑡𝑜𝑛𝑠
𝐶ℎ𝑎𝑟𝑔𝑒 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑃𝑟𝑜𝑡𝑜𝑛𝑠 − 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠
𝑁𝑢𝑐𝑙𝑒𝑎𝑟 𝐶ℎ𝑎𝑟𝑔𝑒 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑃𝑟𝑜𝑡𝑜𝑛𝑠
For Atoms
𝐶ℎ𝑎𝑟𝑔𝑒 = 𝑍𝑒𝑟𝑜
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑃𝑟𝑜𝑡𝑜𝑛𝑠 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠
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Questions
Prepared and Compiled from various sources by
D. Leonard (Learning Specialists)
The Academic Support Center @ Daytona State College
http://www.daytonastate.edu/asc/ascsciencehandouts.html
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