4.2
THE STRUCTURE OF AN
ATOM
Atomic Structure
• Atoms are composed of 2 regions:
• Nucleus: center of atom that contains mass
of atom
• Electron cloud: region that surrounds
nucleus that contains most of space in atom
Electron
Cloud
Nucleus
What’s in the Nucleus?
• Nucleus contains 2 of 3 subatomic
particles:
• Protons: subatomic particle w/ 1+ charge (p+)
• Rutherford - 1911
• Neutrons: subatomic particle w/ no charge
(no)
• James Chadwick - 1932
What’s in the Electron Cloud?
• The 3rd subatomic particle resides
outside nucleus in electron cloud
• Electron: subatomic particle w/ 1- charge
(e-) and virtually no mass
• JJ Thomson - 1897
How do these particles interact?
• Protons and neutrons live compacted in tiny
nucleus
• most atom’s mass
• electrons small and reside outside nucleus
• small mass (2000 e- = 1 p+ or no)
• occupy large volume of space outside nucleus
Atoms
How do the subatomic particles balance
each other?
• In atoms:
• protons = electrons
• If 20 protons are present in atom then 20
electrons balance overall charge of atom—atoms
are neutral
• The neutrons have no charge; therefore they
do not need to (and often times don’t) equal
protons or electrons
How do we know the number of
subatomic particles in an atom?
• Atomic #: indicates # of protons in
atom
• Ex: Hydrogen’s atomic # is 1
• hydrogen has 1 proton
• Ex: Carbon’s atomic # is 6
• carbon has 6 protons
**Number of protons identifies element
similar to how your fingerprint ID’s you.
Ex. 2 protons = He, 29 protons = Cu
ALWAYS!!
How do we know the number of
subatomic particles in an atom?
• Mass number: number of protons and
neutrons in nucleus (p+ + no)
• Ex: hydrogen can have a mass # of 3.
Since it has 1 proton it must have 2 neutrons
• # of neutrons = mass # - atomic #
What are Isotopes?
• Atoms of same element with different #
of neutrons
• Same atomic #
• Different mass # (b/c neutrons are
different)
• Ex. Carbon 12, Carbon 13, and Carbon 14 all
naturally occurring isotopes of Carbon.
• Each has 6 p+ and 6 e-, but each has
different # of neutrons (therefore,
different mass#)
Determining the number of protons and
neutrons
• Li has mass # of 7 and atomic # of 3
• Protons = 3 (same as atomic #)
• Neutrons= 7-3 = 4 (mass # - atomic #)
• Ne has a mass # of 20 and an atomic # of 10
• Protons = 10
• Neutrons = 20 - 10= 10
What about the electrons?
• electrons are equal to protons
• So e- = p+ = atomic #
• Ex: He has mass # of 4 and atomic # of 2
• p+ = 2
• no = 2
• e- = 2
Basic Atomic Structure 1:57
Determine the number of subatomic
particles in the following:
• Chlorine has a mass # of 35 and an
atomic # of 17
• p+ = 17, no = 18,
e- = 17
• Potassium has a mass # of 39 and an
atomic # of 19
• P+ = 19,
no = 20
e- = 19
Candy Atoms
• Atom #1 - mass # of 5 and an atomic # of 3.
• Atom #2 – 5 protons and 7 neutrons.
• Atom #3 – Atomic # of 7 and 8 neutrons.
Candy Atoms
• Atom #4 – mass # 18 and 9 electrons
• Atom #5 – build your own candy atom using the candies
that you have. You should be able to accurately
determine:
• Atomic #
• Mass #
• # of protons, neutrons, and electrons
4.3 Modern Atomic
Theory
Bohr Model of the Atom
• Agreed with Rutherford
• Small nucleus w/ lots of
space
• Devised planetary model
• trying to show why e-
were not sucked into p+ in
nucleus of atom.
• e- in specific energy levels
Misconceptions from the Bohr Model
• Bohr model good for diagramming atoms
and energy levels
• e-
do NOT move like planets in
predictable orbits
• Mathematics determine probable
location of e-
Energy Levels
• Possible energies e- can have
• Like floors in hotel
• Floor nearest nucleus - ground floor
The Electron Hotel
• Levels nearer nucleus have lower energy
(ground floor of hotel)
• Electrons fill energy levels from inside -
outside. (ground floor - top floor of
Electron Hotel)
Electron distribution in an Atom
3d
3p
Energy
Level
3
ENERGY
3s
2p
2
2s
1
1s
NUCLEUS
Energy Levels
• Can’t stand “in between”
steps in hotel stairwell
• e-’s can’t exist “in
between” energy levels
• Must absorb right amt of
energy in order to move up
energy levels
• Must lose right amt to
move down
Evidence of Energy Levels
• Energy gains & loses can be measured
• As e- drop orbitals, energy released in
form of light/heat
• Like in fireworks (2:34)
Electron Cloud Model
• Electrons travel around
nucleus in random orbits.
• cannot predict location at any
given moment.
• Electrons travel so fast, they
appear to form a “cloud”
around nucleus.
• Ex. - Airplane propeller
Atomic Orbitals
• Rooms in “Electron Hotel”
• Region of space where e- likely located
• Each orbital can have 2 e- max
• Denser region = higher probability
Energy Levels, Orbitals, and Electrons
Energy
Level
(floors in
hotel)
Number of
Orbitals
(hotel rooms)
Maximum number of
electrons
(occupants)
1
1
2
2
4
8
3
9
18
4
16
32
Electron Configuration
• Arrangement of e-’s (occupants) in orbitals
(rooms)
• Each orbitals holds 2 e-’s max (1 double
bed)
• Stable when e-’s in orbitals w/ least energy
• Ground state
• i.e. Lithium (atomic # = 3) has 1st 2 e-’s in the
1st energy level (fills up 1 room w/ double bed)
• 3rd e- goes to 2nd energy level
Electron Configuration
• If Lithium absorbs
enough energy, 3rd ejumps energy levels
• Excited state
• Less stable (like gymnast
on beam)
• Eventually releases
energy (often as light)
• Returns to ground state
How exactly are the particles
arranged?
• Bohr Model of the atom:
Electrons move in orbits at fixed distances from the nucleus (planetary
model)
The 3rd ring
can hold up
to 18 eThe 4th ring
and any after
can hold up to
32 e-
All of the
protons and
the neutrons
The 1st ring can
hold up to 2 eThe 2nd ring can
hold up to 8 e-
What does carbon look like?
Mass # = 12
atomic # = 6
6 p and 6 n live
in the nucleus
p+ = 6
no = 6
e- = 6
Drawing Atoms
• Draw the following atoms in your notes:
• 1. Beryllium has an atomic # of 4 and a mass # of 9
Beryllium Atom
Drawing Atoms
• 2. Sodium has an atomic # of 11 and a mass # of 23
Sodium Atom