Transcript Atomic Structure

Warm Up
1.What did Rutherford’s scattering experiment
show? Need to list 2 important findings.
2. What did Thomson discover? Describe the
atomic model he constructed to explain this.
3. Describe Dalton’s water and electricity
experiment. How did this show that atoms
probably exist?
4. What is the current model being used today?
Describe the model?
Atomic Structure
What is an atom?
• Atom: the smallest unit of matter
that retains the identity of the
substance
Atomic Structure
• Atoms have 2 regions
1) Nucleus: the center of the atom that
contains most of the mass of the atom.
2) Electron cloud: surrounds the nucleus &
takes up most of the space of the atom.
Electron
Cloud
Nucleus
What’s in the Nucleus?
• In the nucleus we find:
• Protons: positively charged subatomic
particles
• Mass of 1 amu
• Neutrons: neutrally charged subatomic
particles
• Mass of 1 amu
What’s in the Electron Cloud?
• In the electron cloud we find:
• Electrons: the subatomic particle with a
negative charge and relatively no mass
• Mass of ~ 1/1836 amu
Subatomic Particles
Particle
Charge
Mass (g)
Location
Electron
(e-)
-1
9.11 x 10-28
Electron
cloud
Proton
(p+)
+1
1.67 x 10-24
Nucleus
Neutron
(no)
0
1.67 x 10-24
Nucleus
How do we know the number of
protons in an atom?
• Atomic number (#)= # of protons in
an atom
• Ex: Hydrogen’s atomic # is 1
• hydrogen has 1 proton
• Ex: Carbon’s atomic # is 6
• carbon has 6 protons
**The number of protons identifies the
atom-it’s an atom’s fingerprint.
How do we know the number of
neutrons in an atom?
• Mass #: the # of protons plus
neutrons in the nucleus
• # of neutrons = mass # - atomic #
Example
• Li has a mass # of 7 and an atomic # of
3
• Protons = 3 (same as atomic #)
• Neutrons= 7-3 = 4 (mass # - atomic #)
Mass # vs. Atomic Mass
Mass # ? = The Atomic mass on the
periodic table rounded either up or
down
How do we find the number of
electrons in an atom?
• Most atoms are neutral (have no overall
charge)
• Because the only charged subatomic
particles are the protons and electrons…
they must balance each other out in an
electrically neutral atom.
• Therefore..
• # Electrons = # Protons
* (in a neutral atom..)
*
Examples
• He has a mass # of 4 and an atomic #
of 2
•
p+
=2
no = 2
e- = 2
• Cl has a mass # of 35 and an atomic
# of 17
• p+ = 17,
no = 18,
e- = 17
How exactly are the particles
arranged?
• Bohr Model of the atom: electron
configurations
All of the
The 3rd ring
can hold up
to 8 e-
protons and
the neutrons
The 1st ring can
hold up to 2 e-
The 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
Isotopes
st
1
• Dalton’s
postulate was
wrong. Atoms of the same
element can be different (they
can have different # of neutrons)
• Thus, different mass numbers.
• These are called isotopes.
Isotopes are atoms of the same
element having different masses, due
to varying numbers of neutrons.
Isotope
Hydrogen–1
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium)
Protons Electrons
Neutrons
1
1
0
1
1
1
1
1
2
Nucleus
Naming Isotopes
• We name the isotope based
on its mass number
• carbon-12
• carbon-14
• uranium-235
Isotopes
• Elements
occur in
nature as
mixtures of
isotopes.
Atomic Mass
 How heavy is an atom of oxygen?
 It depends.. b/c there are different oxygen
isotopes.
 We are more concerned with the average
atomic mass.
 This is determined based on the abundance of
each isotope
 We don’t use grams for this mass because
the numbers would be too small.
Measuring Atomic Mass
• Instead we use the Atomic Mass
Unit (amu)
• defined as one-twelfth the mass of
a carbon-12 atom.
• Each isotope has its own atomic
mass, thus we determine the
average from percent abundance.
Atomic Mass
Atomic mass is the average of all the
naturally occurring isotopes of that element.
Isotope
Symbol
Carbon-12
12C
Carbon-13
13C
Carbon-14
14C
Composition of
the nucleus
6 protons
6 neutrons
6 protons
7 neutrons
6 protons
8 neutrons
Carbon = 12.011
% in nature
98.89%
1.11%
<0.01%