Structure of an Atom
What Is an Atom?

An atom is often referred to as the
building block of matter. Atoms have
a nucleus surrounded by an electron
cloud
Atoms are composed of smaller
subatomic particles in an atom. The
proton, neutron, and electron.
 Each element is composed of one
type of atom and can not be broken
down into a simpler substance

Inner Structure of an Atom
• Nucleus - Small, dense, positively
charged center of the atom which
contains most of the atom’s mass
• The nucleus contains subatomic
particles that have certain
characteristics:
Protons
(p+) - positively charged
particles in the nucleus of the atom,
contribute to the atom’s mass
Neutrons (no)- particles in the nucleus
that have no charge, but contribute to
the atom’s mass NEUTRAL
Outer Structure of an Atom
●Electron cloud - an area encircling
the nucleus where electrons are
likely to be found
●The cloud contains energy levels
 electrons (e-)are negatively charged
particles located in specific energy levels
energy levels completely surround the
nucleus
there is a specific order for the electrons
to fill the energy levels
Outer Structure of an Atom
●Energy levels completely surround
the nucleus
Maximum
of 2
electrons
Nucleus
Maximum
of 50
electrons
Maximum
of 8
electrons
Maximum
of 18
electrons
Maximum
of 32
electrons
Outer Structure of an Atom
●valence electrons are in the
outermost energy level and are the
electrons involved in bonding
valence electrons determine the
number of bonds an atom can
form
 an atom is most stable when its
outermost energy level is full
Atom Characteristics
The number of protons in the nucleus is
the atomic number of that atom.
 The number of protons (+) equals the
number of electrons (-) in a neutral atom
 The mass number is the mass of the
protons plus the mass of the neutrons
mass is reported in the SI units atomic
mass units (amu)
Protons and neutrons each are given
an amu of 1
Electrons have a mass of nearly zero

Decoding Atom Information
# of
protons
Atomic
mass
6
C
carbon
12.011
Symbol
Name
Decoding Atom Information:
# of protons =
# of electrons
in a neutral
atom
Atomic mass is a
weighted average
of all the isotopes
of an element
6
C
carbon
12.011
The mass number of an atom is for
a particular isotope of that element
 Mass number is the total number of
protons and neutrons in the
nucleus
 Example for C-12:

◦ 6 protons + 6 neutrons= 12
◦ 12 is the mass number for Carbon-12
*13 is the mass number for Carbon-13
and comes from C-13 having 6 protons
and 7 neutrons