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
Of?

Protons: positively charged particles in the
nucleus.
 The mass of a proton is about 1.7 x 10E
-24g.
 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
zero.
Ion(s)

The charges of protons and electrons are
opposite but equal, so their charges cancel
out.

Because an atom has no overall charge, it is
neutral.

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
way.

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.
Isotopes

Isotope: atoms that have the same number
of protons but have different numbers of
neutrons.

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.

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
number.

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
small.
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
Neutrons

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
neutrons

12 mass number – 6 atomic number = 6
neutrons
Calculating the mass of an
element

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
Mass

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
chlorine?

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
FORGET TO LABEL YOUR ANSWER IN
AMU’s.
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
FORCE.

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
change.