Download Atomic mass

Distinguishing
Among Atoms
Objectives
• Define isotope and nuclide
• Use atomic number, mass number, and
charge to determine the number of
protons, neutrons, and electrons in an
isotope or ion
• Identify carbon-12 as the standard atom
for measuring relative atomic mass
• Determine the process to calculate
atomic mass of an element
Discovery of the Nucleus
Results of foil
experiment if
Plum Pudding
model had
been correct.
Actual results
The Nucleus
• Positively charged
– Contains protons and neutrons
• Very dense
• Central portion of the atom
– Contains most of the atom’s mass,
but . . .
– Occupies only a tiny fraction of its
volume
Particles in the Atom
Particle
Mass
Charge
Proton
1 amu
+1
Neutron
1 amu
0
Electron
1/1800 amu
–1
Rutherford’s Model of the Atom
• The nucleus is positively
charged and very dense
• Most of the atom is empty
space
–Electrons move in the empty
space
Models of the Atom
THOMSON
“Plum Pudding”
DALTON
Indivisible
-
-
Models of the Atom
++++
RUTHERFORD
Nucleus
Distinguishing Among Atoms
• The atomic number (Z) identifies the
atom
– protons in the nucleus
• The mass number (A) gives number of
protons and neutrons in the nucleus
Neutrons = A  Z
• In a neutral atom, electrons equal
protons
Important Point!!
• The number of protons
identifies the atom, but . . .
• The electrons have the most
influence on chemical and
physical properties
The Hydrogen Atom
• Hydrogen exists in three forms
– Protium
• One p+, one e–
• Most abundant form
– Deuterium
• One p+, one n0, one e–
– Tritium
• One p+, two n0, one e–
• Exists only in “trace” quantities
Isotopes
• Atoms of the same element
with different masses
–Same number of protons
–Different number of neutrons
Atomic Numbers and Mass
Numbers
Mass Number (A)
65
29
Cu
Atomic Number (Z)
29
Electrons
Protons + Neutrons
36 Neutrons
65
29
Cu
Protons
Remember!! Atoms are neutral!!
27
Electrons
Protons + Neutrons
36 Neutrons
65
29
2+
Cu
Protons
Remember!! Ions are charged!!
Calculating Electrons
electrons = atomic number  charge
e
–
e
= Z – charge
Neutral Atoms
–
e
= Z – charge
–
e
e = Z – (0)
–
e =Z
electrons equal protons
Positive Ions
–
e
= Z – charge
–
e
e = Z – (+)
–
e <Z
more protons than electrons
Negative Ions
–
e
= Z – charge
–
e
e = Z – ()
–
e >Z
more electrons than protons
NUCLIDE
• A general term for any isotope
of any element
Chlorine-37
Uranium-235
Carbon-14
Identifying nuclides
copper-65
nuclear notation
hyphen notation
Atomic Mass
Relative Atomic Mass
• The carbon-12 atom is the
standard
• Arbitrarily assigned mass of
12.000 amu
–All other atoms are compared to
the carbon-12 atom
Atomic Mass and Atomic Mass Units
•One atomic mass unit
(amu) equals exactly
1/12 the mass of a
carbon-12 atom
Atomic Mass and Atomic Mass Units
•Atomic mass is the
mass of an atom
expressed in atomic
mass units (amu)
Atomic Mass of an Isotope
–Found by comparing it with
the carbon-12 atom
–A relative atomic mass
–Relative to carbon-12
X
12
6
C
X = 1.5 times the mass of carbon-12
Relative Mass of X = 18.000
??? m
Y
12
6
C
Y = 0.5 times the mass of carbon-12
Relative Mass of Y = 6.000
??? m
Atomic Mass
• Most elements are
mixtures of isotopes
–The % composition of the
mixture is constant
Accepted Atomic Mass
• A weighted average of the
relative masses of the naturallyoccurring isotopes
–This is the mass shown on the
Periodic Table
Weighted Averages
Tests
Labs
Quizzes
Homework
40%
30%
20%
10%
Weighted Averages
Total Points = Score x Weight (Pct.)
Tests 85
Labs 90
Quiz 80
HW 100
Mean = 88.8%
x .40 =
x .30 =
x .20 =
x .10 =
34 points
27 points
16 points
10 points
Total = 87 points
Weighted Averages
Grades:
Grade = (Category x weight factor) +
(Category x weight factor) + …
Atomic Mass (AM):
AM = (isotope relative mass x weight factor) +
(isotope relative mass x weight factor) + . . .
An isotope’s “weight factor” is called its
percent abundance in nature
Atomic Mass
% abundance
AM   isotope relative mass 
100%
%A  
%B 

  Rel Mass A 
   Rel Mass B 
 
100%  
100% 

Hydrogen-1
1.007 8 m x 99.985
.999 85% = 1.007 6 m
Hydrogen-2
0.01515% = .000 30 m
2.014 0 m x .000

Average Atomic Mass = 1.007 9 m
Quick Check
• What important information does
the atomic number tell us?
• What is the difference between
relative mass and atomic mass?
Practice
• Write the nuclear symbol for bromine-80
– How many protons, electrons, and neutrons
are in this atom?
• Write the hyphen notation for the
element that has 6 protons and 7 neutrons
– How many electrons does this atom have?
– Write the nuclear symbol for this atom