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Chapter 4

Ted Ed: Just How Small is an Atom?

Scale
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Atoms are the smallest particle of an element
that still behaves like that element.
Generally, atoms are arranged such that there is
a dense center core (the nucleus) surrounded by
a diffuse field.
In (almost) every atom, there are the following
three key subatomic particles:
◦ Proton
◦ Neutron
◦ Electron
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Below the level of protons, neutrons, and
electrons are other subatomic particles.
Without going into too much depth:
◦ Protons are made of three quarks, two “up” and one
“down.”
◦ Neutrons are made of three quarks, two “down” and
one “up.”
◦ Electrons are made of…electrons.

Protons
◦
◦
◦
◦
Location: Nucleus
Charge: Positive
Mass: 1 amu
Symbol: p+
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Neutrons
◦
◦
◦
◦
Location: Nucleus
Charge: None
Mass: 1 amu
Symbol: n0
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Electrons
◦ Location: A cloud around
the nucleus, moving at
high speeds
 Actually, VERY FAR from
the nucleus, too.
◦ Charge: Negative
◦ Mass: 0 amu (so small
it’s rounded to virtually
nothing)
◦ Symbol: e-
Atomic
Number
Element
Symbol
• The number of protons in the nucleus of each
atom of that element.
• Identifies the element.
• This is also equal to the number of electrons in a
neutral atom.
Element
6
# of
protons
6
# of
electrons
6
Phosphorus
15
15
15
Gold
79
79
79
Carbon
Atomic #
• The total number of protons and neutrons in the nucleus
of an isotope.
• Always round the mass number to the highest/lowest
whole number.
• Can be written as Na-23 or 23Naer Na-23 or 23Na.
Mass # = p+ + n0
16 O
8
16
Arsenic
Phosphorus
8
75
16
8
16
33
75
15
31
How many protons, electrons and neutrons
are in the following atoms?
 9Be
4

◦ Protons = 4 Electrons = 4 Neutrons = 5
 20Ne
10
◦ Protons = 10 Electrons = 10 Neutrons = 10
 35Cl
17
◦ Protons = 17 Electrons = 17 Neutrons = 18

Build the following elements (don’t forget all the
subatomic particles):
◦ Hydrogen, Oxygen, Lithium, Carbon

Questions to ponder:
◦ What happens when you change the number of protons?
◦ What happens when you change the number of neutrons?
◦ What happens with you change the number of electrons?
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Changing the number of protons changes the
element.
In other words, the atomic number of an
element never changes.
◦ It is always equal to the atomic number.


Changing the number of neutrons in an atom
changes the atomic mass, creating a new
isotope.
Isotopes are atoms of the same element that
have different atomic masses.
◦ Example: Carbon always has 6 protons. Sometimes it
has 6 neutrons, sometimes it has 8 neutrons.
◦ Thus, Carbon (mass 12) and Carbon (mass 14) are
isotopes.
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Changing the number of electrons in an atom
changes the charge, creating a ion.
If an atom loses an electron it becomes
positively charged and is called a cation.
If an atom gains an electron it becomes
negatively charged and is called a anion.
Isotopes are atoms of the same element having
different masses due to varying numbers of
neutrons.
Isotope
Protons
Electrons
Neutrons
Hydrogen–1
(protium)
1
1
0
Hydrogen-2
(deuterium)
1
1
1
Hydrogen-3
(tritium)
1
1
2
Nucleus
Atomic mass is the weighted average of all the
naturally occurring isotopes of that element.
Isotope
Symbol
Composition of the
nucleus
% in nature
Carbon-12.00
12C
6 protons
6 neutrons
98.89%
Carbon-13.00
13C
6 protons
7 neutrons
1.11%
Carbon-14.00
14C
6 protons
8 neutrons
<0.01%
Carbon = 12.011
The following table list the isotopes of copper with
the relative abundances. Calculate the average
atomic mass of copper.
Isotope
Symbol
Composition of the
nucleus
% in nature
Copper-63.00
63Cu
29 protons
34 neutrons
69.20%
Copper-65.00
65Cu
29 protons
36 neutrons
30.80%
Step 1
multiply the first isotope
mass by the decimal of the
% in nature. Repeat for any
remaining isotopes.
Step 2
Add to determine the
atomic mass unit for that
element.
63.00 * 0.6920 = 43.60
65.00 * 0.3080 = 20.02
43.60 + 20.02
Carbon = 63.62 amu

Calculate the atomic mass of bromine.
The two isotopes of bromine have
atomic masses and relative
abundances of 78.92 amu (50.69%)
and 80.92 amu (49.31%).

All elements are composed of tiny
indivisible particles called atoms.
 An atom is the smallest particle of an
element that retains the properties of that
element

Atoms of the same element are
identical. The atoms of any one
element are different from those of
any other element.
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Atoms of different elements can
physically mix together or can
chemically combine in simple wholenumber ratios to form compounds.
Chemical reactions occur when atoms
are separated, joined or rearranged.
 Atoms of one element can never be changed
into atoms of another element as a result of
a chemical reaction.
Cathode ray tubes pass electricity through a gas
that is contained at a very low pressure.

Cathode rays have identical properties
regardless of the element used to produce
them.
 All elements must contain identically charged electrons.
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Atoms are neutral, so there must be positive
particles in the atom to balance the negative
charges of the electron.
Electrons have so little mass that atoms must
contain other particles that account for most
of the mass.
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Particles (helium nuclei) were fired at a thin
sheet of gold foil.
Particle hits on the detection screen are
recorded.
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Most of the particles passed right through
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A few particles were deflected
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VERY FEW were greatly deflected
CONCLUSIONS
 The nucleus is small
 The nucleus is dense
 The nucleus is positively charged


Atomic masses were known, but mass of
electron and proton didn’t add up to the
total.
There must be something else in the atom!
Neutrons
 Contained in the nucleus
 No charge
 Mass equals that of a proton
(1.67 x 10-24 g)