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Parts of an Atom
What is an atom?
 Atoms
are the smallest parts to
which an element can be divided
yet still be that element.
 Atoms are the building blocks of all
matter.
How small is an atom?

An atom is so small that a penny
contains about 2 x 1022 or
20,000,000,000,000,000,000,000
atoms!!!
But there’s more…

Atoms consist of even smaller particles.
These include:
– Protons
– Neutrons
– Electrons
Parts of an Atom
The protons and
neutrons make up
the center or
nucleus.
The Nucleus
 The
center of an atom
 Small, dense (packed tightly)
 Contains protons and neutrons:
most of the atom’s mass
 So dense, if it were the size of a
grape, it would weigh over 9 million
tons!
Protons




Located in the nucleus
Positive charge (+)
Mass of 1 atomic
mass unit (amu)
Atoms of the same
element have same
number of protons!
protons = positive
Neutrons
 Located
in the
nucleus
 Have no charge
- neutral
 Mass of 1 amu
neutrons = neutral
Electrons
Found outside of the nucleus in electron
cloud
 Negatively charged (-)
 Mass of almost zero
 Number of electrons equals the numbers
of protons to cancel out the charges
(think of -1+1=0)

electrons = negative
Valence Electrons


Electrons in the “outer energy level”
Important in determining an element’s
properties and its bonding ability
Review: Parts of an Atom
Parts
Charge
Location
Mass
Proton
Positive (+)
Nucleus
1 amu
Neutron
No charge
Nucleus
1 amu
Electron
Negative (-)
Electron
Clouds
Almost zero
ATOMIC NUMBER
 The
atomic number is the number
of protons in the nucleus of an
atom
Atomic Mass
 The
weighted average of all the
isotopes of an element that exists
in nature
Mass Number


An element’s mass number is the atomic
mass rounded to the nearest whole number
To find the number of NEUTRONS in an
atom:
Mass Number - Atomic number = # of Neutrons
ex: Fluorine atomic mass 18.998, atomic #9
19 – 9 = 10 neutrons
ISOTOPES
 Atoms
that have the same atomic
number but DIFFERENT atomic
masses
 The number of protons is the
same, but the number of neutrons
is not!
ISOTOPES: Example

Carbon-12 : Atomic # = 6 Mass # = 12
–

Has 6 protons, 6 neutrons
Carbon-14: Atomic # = 6 Mass # = 14
–
Has 6 protons, 8 neutrons