Atomic Structure
• ATOM: the smallest particle that has the
properties of an element.
• From the early Greek concept of the atom
to the modern atomic theory, scientists
have built on and modified existing models
of the atom.
• Atoms are composed of a positively charged
nucleus surrounded by an electron cloud.
– Nucleus (99% of atom’s mass): uncharged
neutrons and positively charged protons.
– Electron cloud: negatively charged electrons in
constant motion creating a “cloud” like a fan.
DEMOCRITUS
• In 400 B.C., this Greek
philosopher suggested
that the universe was
made of indivisible units.
• “Atom” (atomos)– Greek
word meaning “unable to
be divided”
JOHN DALTON
In 1808, this English
schoolteacher proposed his
own atomic theory that
became widely accepted.
Dalton’s Atomic Theory:
–
–
–
–
All elements are made of tiny atoms.
Atoms cannot be subdivided.
Atoms of the same element are exactly alike.
Atoms of different elements can join to form
molecules.
Do these ideas still hold true today?
• As it turns out, the atom can be divided into
subatomic particles.
• Thompson and Millikan are given credit for the
first discoveries relating to electrons.
http://www.shs.d211.org/science/faculty/hlg/history%20of%20atom%20sanjay%20karan/millikanpage.htm
•Rutherford discovered the positively charged nucleus
(GOLD FOIL EXPERIMENT)
NIELS BOHR
• In 1913, this Danish scientist
suggested that electrons “orbit” the
nucleus.
• In Bohr’s model, electrons are placed
in different energy levels based on
their distance from the nucleus.
Electrons are like books in a book case.
They can only exist on certain levels.
Only certain orbits are allowed
The Bohr Atom was a “Solar
System” model.
MODERN ATOMIC
MODEL
• By 1925, Bohr’s model of the atom no longer
explained all observations. Bohr was correct
about energy levels, but wrong about electron
movement.
• Electrons occupy the lowest energy levels
available.
• Energy increases as distance from the nucleus
increases.
• Electrons move in patterns of “wave functions”
around the nucleus.
• It is impossible to know an electrons velocity and
location at any moment in time (Schrödinger).
Schrödinger
This is very
difficult stuff!
In 1926 Schrodinger proposes
an equation that gives the
probability of finding an
Don’t worry, this
electron at any place in the won’t be on the
test.
atom.
  


 V  i
2
2m x
t
2
2
VALENCE ELECTRONS
• Electrons in the outermost energy level are
called VALENCE ELECTRONS.
• Valence electrons determine how an atom
will act in a chemical reaction.
• Atoms with equal numbers of valence
electrons have similar properties.
Review Questions
What part of the atom is much smaller than
the atom, yet contains most of the mass?
a) the nucleus
b) the electron cloud
Not drawn to scale
While its diameter is very small compared to
that of the entire atom, 99% of the mass of an
atom comes from the protons and neutrons in
the nucleus.
Which statement is true according to Dalton’s
atomic theory?
a)
b)
c)
d)
Atoms of different elements join to form larger
atoms.
Atoms can be subdivided into smaller particles.
Atoms of the same element differ in electric
charge.
Atoms of the same element are exactly alike.
This statement was part of Dalton’s atomic
theory. While this statement was holds true if
we remove the word “exactly”, we have since
found that atoms of an element often differ in
number of neutrons (isotopes).
According to Bohr’s model of the atom,
electrons behave like
a)
b)
c)
d)
planets rotating on their axes.
planets orbiting the sun.
light energy in a vacuum.
waves vibrating on a string.
While Bohr’s model of energy levels of
Schrödinger
electrons around an atom remains supported,
his model of electron movement has been
revised. Schrödinger and other scientists
have since found that electrons travel in
wave-like patterns around the nucleus.
1869: created first periodic table of elements.
Arranged elements in order of increasing
atomic mass.
One of Rutherford’s students.
1914: Arranged the elements in order of increasing
atomic number
(responsible for TODAY’S periodic table).
Organization of the Periodic Table
• PERIODICITY: regular variations (or
patterns) of properties with increasing
atomic number. Both chemical and
physical properties vary in a periodic
(repeating) pattern.
• PERIOD: horizontal row of elements
•GROUP (FAMILY):
vertical column of elements
Periodic Key
# of protons = Z
# of electrons = # of protons (in a neutral atom)
6
# of neutrons = A-Z
Atomic number (Z)
C
Element’s symbol
Carbon
Element’s name
12.011
Atomic mass (A)
Determining # P+, N, and Efrom chemical symbols:
Mass #
No net
charge
14
6
Atomic #
• Example 1: atom
# protons = 6
#electrons= 6
#neutrons= 14-6 = 8
C
ISOTOPES
• Isotopes are atoms that have the same #
of protons, but a different # of neutrons.
• Example:
Carbon-12 vs. Carbon-14
12C
Mass # = 12; Atomic # = 6 (6P, 6E, 6N)
14C
Mass # = 14; Atomic # = 6 (6P, 6E, 8N)
IONS
• Ionization: the process of adding or
removing electrons from an atom or group
of atoms.
• An ion has a net electric charge.
• Cation: ion with a positive charge.
Ex: Na+
• Anion: ion with a negative charge.
Ex: O2-
Valence electrons &
electron dot diagrams
• Review: The valence electrons are the
outermost electrons in an atom. These are
the electrons that are involved when there
is a chemical reaction.
• Looking at a periodic table, you can quickly
determine how many valence electrons an
atom has by what column it is in.
# of Valence electrons
2
1
2
3
4
5
6 7 8
ELECTRON DOT DIAGRAMS:
visual representations of elements and their valence electrons
Standard form:
36
4
7
Order of electron/dot
placement
R
58
1
2
element symbol
Example:
Oxygen
O
6 valence
electrons
How many dots are in an electron dot
diagram of chlorine?
a)
b)
c)
d)
1
7
17
35
Chlorine and all of the other
halogens (column 17) have
seven valence electrons.
Cl
Review Questions…
How many protons, electrons and
neutrons are in one atom of oxygen-17?
a)
b)
c)
d)
17 p, 17 e, 17 n
17 p, 17 e, 1 n
8 p, 8 e, 8 n
8 p, 8 e, 9 n
While most oxygen atoms have a mass of 16 g/mol,
oxygen-17 is an isotope with a mass of 17 g/mol.
The number of protons in an element is the same
for every atom of that element.
# electrons = # protons if element has no charge.
The periodic law states that when elements
are arranged in order of increasing
__________ _________, similarities in their
properties occur in a regular pattern.
a) Atomic mass
b) Atomic number
c) Atomic radius
Moseley created the modern periodic
table when he determined that elements
should be placed in order of increasing
atomic number (# of protons). It’s a
shame that WWI took the life of this
brilliant scientist when he was so young.
Carbon-12 and carbon-14 are
a)
b)
c)
d)
isomers
isotopes
radioactive elements
different elements
Isotopes are atoms of the same element with
different numbers of neutrons, and therefore
different atomic masses.
While carbon-14 is used in radioactive dating,
carbon-12 has a more stable nucleus and
therefore is not used in this capacity.
How many protons, electrons and
neutrons are in O2-?
a)
b)
c)
d)
10 p, 8 e, 8 n
8 p, 8 e, 8 n
8 p, 10 e, 8 n
8 p, 6 e, 8 n
Every atom or ion of oxygen has 8 protons.
The net charge of -2 indicates that the ion has two
more negative charges (electrons) than positive charges
(protons)
All atoms and ions of standard oxygen-16 have 8
neutrons.