Atomic Structure
Dr. Steve Badger
As we work through this…
• Find a Periodic Table of the Elements in
the text and keep it handy for reference
• Use your copy of the handout entitled
“Atomic Structure” to follow along and
take notes
Consider a simple model
• The nucleus is in the center of the atom
and contains
– Positively charged protons (p+)and
– Neutrons (n) with no charge
• Outside the nucleus, and at a great
relative distance from the nucleus, we
find negatively charged electrons (e—)
Atoms have a net charge of zero
• It is not correct to say, “atoms have no
charge.” The protons and electrons that
makeup the atom have a charge.
• Atoms can have a net charge of zero if
and only if the number of electrons (—)
equals the number of protons (+).
Atomic Number = #p+
• The atomic number is the number of protons.
• While it is true that the number of electrons
in an atom is equal to the number of protons,
it is not correct to say that the atomic
number is the number of electrons.
• Look in the Periodic Table Find and find the
atomic number of hydrogen, carbon,
nitrogen, neon, calcium, and iron.
Protons
• The number of protons an atom contains
will determine which element it is.
• Thus, an atom is nitrogen if and only if it
has seven protons.
• Another way of saying this is, “an atom
is nitrogen because it has seven
protons.”
Electrons
• Electrons have a negative charge and are
outside the nucleus
• Electronic structure determines reactivity
• That is, the number and arrangement of
electrons—especially the outermost
electrons—determines how an atom reacts
Electronic Structure
• The electronic structure determines
whether an atom usually forms covalent
bonds or ionic bonds
• Elements in the same family (column)
typically react similarly
Three Subatomic Particles
• The mass of the proton and the mass of
the neutron are essentially the same:
1 amu (atomic mass unit)
• Electrons have a mass so small
(compared to that of the proton and
neutron) that we can ignore their mass
• We say their mass approaches zero
Masses of subatomic particles
Particle
Mass
Proton
1 amu
Neutron
1 amu
Electron
—> 0 amu
The mass of an atom
• So the mass of an atom is essentially
the sum of the masses of the protons
and the masses of the neutrons
Consider these three atoms
1st atom
2nd atom
3rd atom
1 proton
1 proton
1 proton
1 electron
1 electron
1 electron
0 neutrons 1 neutrons 2 neutrons
What element is represented in the first column?
What element is represented in the second column?
What element is represented in the third column?
How do you know the answer?
Why hydrogen?
• So all three of them stand for hydrogen
• Will all three atoms react the same?
• Yes. Why?
What’s different about these 3 atoms?
•
•
•
•
The masses are different.
The 1st atom has a total mass of 1 amu
The 2nd atom has a mass of 2 amu
The 3rd atom has a mass of 3 amu
Atoms of the same element with different
masses are called….?
Isotopes
• Isotopes have the same number of
protons, but different number of
neutrons—thus different masses
• Most elements exist in nature as two or
more isotopes
• Are you familiar with any isotopes?
Sure! Name one
Carbon-14
• Chemists write 14C but say “carbon-14”
• The 14 means that it has a total mass
of 14 amu
• Since it is carbon, you know that it has
six protons.
(Verify this in the Periodic Table.)
Atomic Mass = #p+ + #n
• How many neutrons does 14C have?
• Think like this: The sum of the number
of protons and the number of neutrons
equals the atomic mass.
• Thus, 14 minus 6 equals 8…so
14C has 8 neutrons.
Bonding
• Most atoms form either ionic bonds or
covalent bonds.
• A few atoms can form either type
depending on what it is bonded to.
• And metals form “metallic bonds”
Ionic bonds
• Ionic bonds are formed when one atom
loses an electron(s) and another atom gains
an electron(s)
• Now the number of protons is not equal to
the number of electrons, and this “atom”
has a charge…
• …but we no longer call these atoms—they
have a charge—so we label them ions
Ionic bonds
• Oppositely charged ions attract each
other, and we call this an ionic bond
• Sodium chloride (NaCl) is an example of
an ionic bond (Na+Cl—)
• Elements on the far left side of the
Periodic Table typically form ionic bonds
with elements on the far right side of
the Periodic Table
Covalent bonds
• When two atoms share a pair of
electrons, each atom behaves as if
the pair of electrons belongs to it
• This shared pair of electrons is called
a covalent bond
• The atoms that make up water (H2O)
are covalently bonded to each other
Some of you may need to
• Read and read this handout
• Spend extra time learning the terms