The Atom

The Atom
Mr. McMartin
Beta Pod Science
How Small is an Atom?
A PENNY contains about 2 x 10Ε22 atoms
or 20,000,000,000,000,000,000,000
That’s 20 thousand billion billion atoms- over
3,000,000,000,000 times more atoms than
there are people on earth.
An aluminum atom has a diameter of about
. 00000003 cm.
What is the Nucleus Made
Protons: positively charged particles in the
 The mass of a proton is about 1.7 x 10E
 Because atoms masses are so small
scientists made a new unit for them.
 Protons = 1amu
Neutrons: particles of the nucleus that have
no charge.
 Neutrons= 1amu
Atomic mass unit (amu): the SI unit used to
express the masses of particles in atoms.
What’s Outside the Nucleus?
Electrons are found outside the nucleus in
the electron clouds.
They are negatively charged.
It takes 1,800 electrons to equal the mass of
one proton. They are so small they are
usually thought to have a mass of almost
The charges of protons and electrons are
opposite but equal, so their charges cancel
Because an atom has no overall charge, it is
If the numbers of protons and electrons are
not equal, then the atom becomes a charged
particle called an ION.
 An atom that loses one or more electrons
is a positively-charged ion.
 An atom that gains one or more electrons
becomes a negatively-charged ion.
How Do Atoms of Different
Elements Differ?
There are more than 110 different
elements… they are all different in some
You can tell the difference between atoms of
separate elements by the number of protons
in the nucleus of an atom (atomic number).
Atomic number: the number of protons in the
nucleus of an atom.
Isotope: atoms that have the same number
of protons but have different numbers of
You can have an isotope of the same atom.
Ex. Hydrogen atom vs Hydrogen Isotope
Hydrogen atom= 1proton & 1electron
Hydrogen isotope= 1proton, 1nutron, &
1 electron
Properties of Isotopes
Each element has a limited number of
isotopes that are found in nature.
Some isotopes have special properties
because they’re unstable. These are
Radioactive atoms spontaneously fall apart
after a certain amount of time.
As they do, they give off smaller particles,
as well as energy.
Properties of Isotopes cont.
Isotopes of an element share most of the
same chemical and physical properties. For
example, the most common oxygen isotope
has 8 neutrons in the nucleus. Other
isotopes of oxygen have 9 or 10 neutrons.
All three isotopes are colorless, odorless
gasses at room temperature. Each isotope
has the chemical property of combining with
a substance as it burns. Different isotopes
of an element even behave the same in
chemical changes in your body.
Telling Isotopes Apart
You can tell each isotope apart by it’s mass
Mass Number: the sum of the protons and
neutrons in an atom.
Electrons are not included in an atom’s
mass number because their mass is so
Naming Isotopes
You can tell the difference between isotopes
by finding how many neutrons they have.
To identify a specific isotope of an element,
write the name of the element followed by a
hyphen and the mass number of the isotope.
 Ex. Hydrogen-1 or carbon-12
You can find the number of neutrons in an
element by using the following equation:
 Mass number – Atomic number = number
of neutrons
Example of Finding Amount of
Find the amount of neutrons in Carbon-12.
It has an atomic number of 6.
Step 1: Identify mass number (12)
Step 2: Identify atomic number (6)
Step 3: Plug into equation
12 mass number – 6 atomic number =
Step 4: solve equation to find amount of
12 mass number – 6 atomic number = 6
Calculating the mass of an
Most elements contain a mixture of two or
more isotopes.
 Ex. All copper is composed of copper-63
atoms and copper-65 atoms.
Atomic mass: the weighted average of the
masses of all the naturally occurring isotopes
of that element.
To find the atomic mass:
 Multiply the mass number of each isotope by
it’s percentage abundance in decimal form.
 Add these amounts together to find the
atomic mass.
Example of Finding Atomic
Chlorine-35 makes up 76% of all the
chlorine in nature, and chlorine-37 makes up
the other 24%. What is the atomic mass of
Step 1: Multiply the mass number of each
isotope by it’s percentage abundance in
decimal form.
(35 x 0.76) =
(37 x 0.24) =
Step 2: Add these answers of the amounts
together to find the atomic mass. DON’T
Forces of Atoms
Forces are “pushes” and “pulls.” You have
seen the make-up of individual atoms but we
have not discussed the forces acting
between more than one atom.
There are four basic forces at work
everywhere between and around an atom.
 Gravitational force
 Electromagnetic force
 Strong force
 Weak force
These forces work together to give atoms
their structure an properties.
Gravitational Force
Gravitational force acts between all objects
all the time.
The amount of gravitational force between
objects depends on their masses and the
distance between them.
Because the masses of particles in atoms
are so small, the gravitational force within
atoms is very small.
Electromagnetic Force
Objects that have the same charge repel
each other, while objects with opposite
charge attract each other.
This is due to electromagnetic force.
Protons and electrons are attracted to each
other because they have opposite charges.
The electromagnetic force holds the
electrons around the nucleus.
Strong Force
Protons push away from one another
because of the electromagnetic force.
A nucleus containing two or more protons
would fly apart if it were not for the STRONG
At the close distances between protons and
neutrons in the nucleus, the strong force is
greater than the electromagnetic force, so
the nucleus stays together.
Weak Force
The WEAK FORCE is an important force in
radioactive atoms.
In certain unstable atoms the neutron can
change into a proton and an electron.
The weak force plays a key role in this

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