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Unit 2 Lesson 6 The Atom
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Unit 2 Lesson 6 The Atom
As a Matter of Fact …
What is matter made of?
• The Greek philosopher Democritus thought matter
could be divided into smaller and smaller units he
called atomos.
• In 1803, John Dalton proposed that all substances
are made of atoms that cannot be divided.
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Unit 2 Lesson 6 The Atom
What is matter made of?
• In 1897, J. J. Thomson performed experiments
that detected smaller particles within atoms.
• In the early 1900s, Ernest Rutherford and James
Chadwick revealed the nature of the dense center
of an atom.
• Today we have the electron cloud model.
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Unit 2 Lesson 6 The Atom
What are atoms?
• An atom is the smallest particle into which an
element can be divided and still be the same
substance.
• In 1808, John Dalton published an atomic theory
that said all atoms of a particular element are
identical.
• Dalton also said that atoms of an element differ
from atoms of other elements.
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Unit 2 Lesson 6 The Atom
What are the parts of an atom?
• Atoms are made of protons, neutrons, and
electrons.
• Protons are positively charged particles.
• The mass of a proton is given in the atomic
mass unit (u). One proton has a mass of 1 u.
• Help make up the nucleus of the atom
• Help identify the atom (could be considered an atom’s DNA)
+
+++
+ +
+
+
• Equal to the atomic number of the atom
• Equal to the number of electrons
+
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Unit 2 Lesson 6 The Atom
What are the parts of an atom?
• Neutrons are particles that have no electrical
charge.
• In most atoms, there are at least as many
neutrons as protons.
• The mass of a neutron is slightly more than a
proton but is still considered to be 1 u.
-
+
+
+++++
+
-
-
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Unit 2 Lesson 6 The Atom
What are the parts of an atom?
• The nucleus is the at the center of the atom and
contains the protons and neutrons.
• The overall charge of the nucleus is equal to the
number of protons in the nucleus.
• The nucleus is small but very dense.
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Unit 2 Lesson 6 The Atom
• Electrons are negatively charged particles.
• Electrons move around the nucleus very quickly in
a region called the electron cloud.
• Electrons are very small compared to protons and
neutrons.
• Found outside the nucleus of the atom, in the
electron orbits/levels; each orbit/level can hold a
maximum number of electrons ( 1st = 2, 2nd = 8,
3rd = 8 or 18, etc…)
• Move so rapidly around the nucleus that they
create an electron cloud
• Equal to the number of protons
• Involved in the formation of chemical bonds
+
+++
++++
-
-
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-
Unit 2 Lesson 6 The Atom
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Unit 2 Lesson 6 The Atom
What are the parts of an atom?
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Unit 2 Lesson 6 The Atom
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Hydrogen (H) Atom
Notice the one electron in the first orbital
+ =1
-
=0
-
=1
+
Even though there are no neutrons present,
Hydrogen is still considered an atom
How many
more
electrons
can fit in
the 1st
orbital/
level?
Oxygen (O) Atom
Notice the two electrons in the first orbital/level
and the six in the second
+ =8
-
=8
-
-
-
=8
++ +
+ + +
+
+
-
-
-
-
How many
more
electrons
can fit in
the 2nd
orbital/
level?
Sodium (Na) Atom
Notice the two electrons in the first orbital/level,
eight in the second, and one in the third
-
+ = 11
-
-
-
= 12
-
= 11
++ +
+ + +
+
+
-
-
-
-
-
How many
more
electrons
can fit in
the 3rd
orbital/
level?
Unit 2 Lesson 6 The Atom
What are the parts of an atom?
• The number of protons and electrons in an atom
are the same, so the charges are balanced.
• An atom can gain or lose electrons to become an
ion, which has a net positive or negative charge.
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Unit 2 Lesson 6 The Atom
•
An atom that carries an electrical charge is called
an ion
•
If the atom loses electrons, the atom becomes
positively charged (because the number of
positively charged protons will be more the
number of electrons)
•
If the atom gains electrons, the atom becomes
negatively charged (because there are more
negative charges than positive)
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Unit 2 Lesson 6 The Atom
The number of protons does not change in an ion.
The number of neutrons does not change in an ion.
So, both the atomic number and the atomic mass
remain the same.
This atom has lost an electron. Now it has one
more proton than electron.
One more proton means one more positive charge.
This makes the total charge of the atom POSITIVE.
Cation = positively charged particle
This atom has gained an electron. Now it has one
less proton than electron.
One less proton means one less positive charge.
This makes the total charge of the atom
NEGATIVE.
Anion = negatively charged particle
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Unit 2 Lesson 6 The Atom
Take a Number!
How can we describe atoms?
• Different combinations of protons, neutrons, and
electrons produce atoms with different properties.
• The number of each kind of particle within an
atom determines its unique properties.
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Unit 2 Lesson 6 The Atom
How can we describe atoms?
• The number of protons distinguishes the atoms of
one element from the atoms of another.
• The number of protons in the nucleus of an atom
is the atomic number of that atom.
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Unit 2 Lesson 6 The Atom
How can we describe atoms?
• Atoms of an element have the same number of
protons, but the number of neutron may differ.
• Isotopes are atoms of the same element that have
different numbers of neutrons.
• The total number of protons and neutrons in an
atom’s nucleus is its mass number.
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Unit 2 Lesson 6 The Atom
Describing Atoms
• Atomic Number = number of protons
• In a neutral atom, the # of protons = the # of
electrons
• Atomic Mass= the number of protons + the number
of neutrons
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Mass Number
The total number of protons and neutrons in an atom’s nucleus
Expressed in Atomic Mass Units (amu)
Each proton or neutron has a mass of 1 amu
What would be the mass
number of this atom?
+
-
 3
 4
+
++
3 protons + 4 neutrons =
a mass number of 7 amu
Why did we not account for the
electrons when calculating the
mass number?
-
-
Building Atoms
Using the whiteboard and the proton, neutron,
and electron pieces, build the following atoms,
and determine their atomic and mass numbers.
Atoms
Carbon
Beryllium
Oxygen
Lithium
Sodium
Protons
6
4
8
3
11
Neutrons Electrons
6
6
5
4
8
8
4
3
12
11
Unit 2 Lesson 6 The Atom
Isotopes
The number of protons for a given atom never changes.
The number of neutrons can change.
Two atoms with different numbers of neutrons are called
isotopes
Isotopes have the same atomic #
Isotopes have different atomic Mass #’s
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Unit 2 Lesson 6 The Atom
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