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Atomic Structure
What is an atom?
 Atom: the smallest unit
of matter that retains the
identity of the substance
 Made up of 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.
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
 In the electron cloud we find:
 Electrons: the subatomic particle with a negative charge
and relatively no mass

Mass of ~ 1/1836 amu
Electron
Cloud
Nucleus
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: Carbon’s atomic # is 6

carbon has 6 protons
How do we know the number of
neutrons in an atom?
 Atomic Mass: the # of protons plus neutrons in
the nucleus
 # of neutrons = mass # - atomic #
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
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
The 3rd ring
can hold up
to 8 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
Isotopes
 Dalton’s 1st 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.
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%