Subatomic Particles in an atom!
• 3 main particles in the atom:
• Neutrons
• Protons
• Electrons
LOCATION
e-
In physics, we will learn that even
these particles are made of smaller
particles!
SIZE
p+
n
Remember, most of an atom is
empty space!
• An atomic model the size of
Busch Stadium and parking
would contain a pea sized
nucleus containing 95.95% of
the atoms mass.
• The pea at the pitcher’s mound
would be the nucleus, and an
ant crawling on the parking lot
outside would be an electron!
Electrons
• JJ Thompson, RA Millikan
• Located around nucleus
• Negatively (-) charged particles
• Mass = essentially zero - only 1/2000 of a
proton or neutron
• Charge = -1
• Occupies the majority of space in an atom
e-
Protons
• Eugen Goldstein (1886)
•
•
•
•
•
Located in….
The tiny, dense nucleus!
Mass of 1.67 x 10-24 grams
We call that 1 amu, or a mass of 1
Charge = +1
p+
Neutrons
• James Chadwick (1932)
•
•
•
•
•
Located in….
The tiny, dense nucleus!
Mass of 1.67 x 10-24 grams
We call that 1 amu, or a mass of 1
Charge = 0
n
Nuclear Symbols
X = element symbol
Z = mass number
= # protons + #
neutrons
A = atomic number
= # protons
(gives an element its
identity!!)
So… # neutrons = Z-A
Z
X
A
charge
Atomic Number
•
•
•
•
•
•
•
Number of protons
Number on bottom of symbol
Gives an atom its identity!
If 6 p+ in nucleus, the atom is….
Carbon!
It 7 p+ in nucleus, the atom is….
Nitrogen!
14
7
12
6
N
C
How was the atomic number of an element determined?
•
•
•
•
•
•
•
•
Henry Moseley, and English scientist, bombarded
atoms of elements with high speed cathode rays
(electrons!)
He found that the atoms gave off X-rays
He found that there was a pattern between the
amount of X-rays given off and the element that gave
them off!
He theorized that the nucleus of an atom was
causing these X-rays to be given off
The more protons in the nucleus, the more x-rays
that were given off!
He determined from this exactly how many protons
were in the nucleus of each element!
He called the number of protons the atomic number,
and he said that this gave an element its identity!
He rearranged the periodic table according to atomic
number, and not according to atomic mass!
This cleared up any irregular patterns that were
found in the periodic table from before!
Moseley was a great scientist!
• He was a physics professor at Oxford
University!
• Henry Moseley did most of his
experimentation when he was only 27
years old!
• Unfortunately, he died a year later, in
1915, during World War I. He was killed
by a sniper in Turkey!
• His short career lasted less than four
years…..
Mass Number
•
•
•
•
•
•
•
•
•
•
•
•
Mass Number = number of protons + number of neutrons
Number on the top of the symbol
It tells you the mass of atom!
Not all atoms of the same element weigh the same!
Two atoms of Carbon are shown below. One weighs 12, one weighs 14.
If they are both Carbon, the must both have the same number of…..
Protons! Each has…..
6 protons!
So different numbers of….
Neutrons!
How many neutrons
does each have? How would
we figure that out?
Mass Number - Atomic Number =
Number of neutrons
12
C
6
6 neutrons!
14
C
6
8 neutrons!
ISOTOPES
•
•
•
•
•
•
•
•
•
•
•
Atoms with same # Protons, but different # neutrons
Example– 35Cl and 37Cl are isotopes of chlorine
They only differ in their…..
Mass!
They both have how many protons….?
17!
Look at the periodic table, though - the periodic table
lists the AVERAGE ATOMIC MASS
For chlorine, it is 35.453 amu. This doesn’t look like
an average between 35 and 37!
That is because they don’t exist in nature 50-50!
Chlorine-35 makes up 75.77% of all Chlorine atoms
in the universe
Chlorine-37 makes up 24.23% of all Chlorine atoms
in the universe
ISOTOPES - again….
• How do we calculate the average atomic mass?
• Take each atom’s mass number, and multiply by the RELATIVE
ABUNDANCE (That is what percentage it is in the universe)
• You must convert the relative abundance to a decimal
Example:
(35 amu)(.7577) + (37 amu)(.2423) =
35.4846 amu
• This is called a weighted average - because you don’t just add and
divide by two!
Electrons and atom charge
• If # p+ = # e• Then atom is NOT
CHARGED
• Charge = #p+ - #e-
• If #p > #e• Then + charged
(cation)
• If # p < # e• Then – charged
(anion)
•ALL ELEMENTS ARE NEUTRAL, AND NOT
CHARGED - AN ELEMENT HAS TO REACT IN
ORDER TO BE CHARGED!
•A CHARGED ATOM IS CALLED AN ION
How many p+, n, e- ??
108
Ag
47
41
20
+2
Ca
47
p+ = 20
= 47
e- = 18
n = 61
n = 21
Charge on atom…. 0
Charge on atom…. +2
p+ =
e-
Write the nuclear symbol for…
• An atom containing 18 e-, 16 P,
and 17 N.
• What element is it??
• Atomic number = 16
• # p+ = 16
• Element is….
• Sulfur
• Number of electrons tells us…
• There are two more electrons
than protons
• The electrons are winning by
2!
• Charge is -2!
• Mass Number = #p+ + #n
33
S
16
-2
Which of the following represent
isotopes of the same element?
16 X
8
16 X
7
14 X
7
14 X
6
12 X
6
Look for same atomic #: (same # protons)
16 X and 14 X both have 7 protons, so are isotopes
7
7
of Nitrogen.
14 X and 12 X both have 6 protons, so are isotopes
6
6
of C.