2. What is an atom?
• Atom: the smallest unit of matter
that keeps the identity of the
substance
• Each element has its own unique atom.
• The basic building block of matter.
3. Atomic Structure
• Atoms are composed of 2 regions:
• Nucleus: the center of the atom
• Electron cloud: region that surrounds
the nucleus
Nucleus
Electron
Cloud
4. What’s in the Nucleus?
• The nucleus is made of 2 of the 3
subatomic particles:
• Protons: positively charged particles
• Mass of 1 atomic mass unit (amu or μ)
• Neutrons: neutrally charged particles
• Mass of 1 atomic mass unit (amu or μ)
5. What’s in the Electron Cloud?
• The 3rd
subatomic particle exists
outside of the nucleus in the electron
cloud
• Electron: particle with a negative charge
and relatively no mass (1/2000 μ)
• Electrons are in constant motion, and
• they are scattered throughout the
electron cloud.
6. More about the Electron Cloud
• Some electrons are found in the outer
energy level so they are called valence
electrons.
• These electrons can be lost, gained or
shared with other atoms.
• For most atoms, the outer energy level can
hold a maximum of 8 electrons.
• Exceptions: some atoms can only hold 2
electrons
• (Ex: Helium & Hydrogen)
7. • Electron Cloud Model of the atom:
How exactly are the particles
arranged?
All of the
protons and
the neutrons
Outer energy
level that usually
can hold up to 8
electrons
Electrons that are
not in the outer
energy level
8. How do these particles interact?
• Protons and neutrons live compacted
in the tiny positively charged nucleus
accounting for most of the mass of
the atom
• The negatively charged electrons are
small and have a relatively small mass
but occupy a large volume of space
outside the nucleus
9. How do the subatomic particles
balance each other?
• In an atom:
• The protons = the
electrons
• If 12 protons are in an
atom, then 12 electrons
are there to balance the
overall charge of the
atom—atoms are neutral
10. How do the subatomic particles
balance each other?
• In an atom:
• The neutrons have no charge;
therefore they do not have to equal
the number of protons or electrons.
12. How do we know the number of
subatomic particles in an atom?
• Atomic number: this number shows the
number of protons in an atom
• Ex: Hydrogen’s atomic number is 1
• So hydrogen has 1 proton
• Ex: Carbon’s atomic number is 6
• So carbon has 6 protons
**The number of protons identifies the atom.
(The # of protons never changes.)
Ex. 2 protons = He, 29 protons = Cu
13. • Mass number: the number of protons and
neutrons in the nucleus
• Ex: hydrogen can have a mass of 3.
Since it has 1 proton it must have 2 neutrons
# of neutrons = mass number - atomic number
How do we know the number of
subatomic particles in an atom?
14. Determining the number of
protons and neutrons
• Li has a mass number of 7 and an
atomic number of 3
• Protons = 3 (same as atomic #)
• Neutrons= 7-3 = 4 (mass # - atomic #)
• Ne has a mass number of 20 and an
atomic number of 10
• Protons = 10
• Neutrons = 20 - 10= 10
15. • Atomic mass: the average mass of all
isotopes of one element
• The atomic mass can be rounded to find
the most common total number of
protons and neutrons.
atomic mass = # of neutrons + # of
protons
(the average)
Determining the number of
protons and neutrons
16. What about the electrons?
• In an atom, the electrons are equal to
the number of protons
• So e-
= p = atomic #
• Ex: He has a mass # of 4 and an
atomic # of 2
• p+
= 2
• no
= 2
• e-
=2
17. What does Lithium look like?
Mass # = 7 atomic # = 3
p+
= 3 no
= 4 e-
= 3
3 p and 4 n live
in the nucleus
18. Determine the number of
subatomic particles in the
following:
• Cl has a mass # of 35 and an atomic #
of 17
• p+
= 17, no
= 18, e-
= 17
• K has a mass # of 39 and an atomic #
of 19
• P+
= 19, no
= 20 e-
= 19
19. What does carbon look like?
Mass # = 12 atomic # = 6
p+
= 6 no
= 6 e-
= 6
6 p and 6 n live
in the nucleus
20. What about ions?
• In an ion, the electrons are not equal
to the number of protons.
• Atoms that gain electrons have more
negative charges and become negative
ions.
• Atoms that lose electrons have more
positive charges and become positive
ions.
