Download Chem Ch. 4.3

Section 4.3
How Atoms Differ
Objectives
• Explain the role of atomic number in
determining the identity of an atom.
• Define an isotope and explain why atomic
masses are not whole numbers.
• Calculate the number of electrons, protons,
and neutrons in an atom given its mass
number and atomic number.
Atomic Number
• Moseley (1887-1915) discovered that atoms
of each element contain a unique number of
positive charges in their nuclei.
• Therefore, it was concluded that the number
of protons identifies an atom as an atom of a
particular element.
• The number of protons in an atom is
referred to as an element’s atomic number.
Atomic Number
Carbon
6
C
12.011
• Atomic number
determines an
Element name
element’s position in
ATOMIC NUMBER
the periodic table Chemical symbol
from left to right and
top to bottom,
Average
atomic mass
elements are arranged
by increasing atomic
number.
Atomic Number
• Recall that atoms are neutral.
• The number of protons, therefore, MUST be
equal to the number of electrons.
• Atomic number = number of
protons = number of electrons
• Carbon, for example, has an atomic number
of 6 and 6 protons and 6 electrons.
Practice Problems
Complete the following table.
Element
Atomic number Protons
Electrons
10
Pb
50
25
Rb
Isotopes
• Recall that Dalton was wrong about atoms being
indivisible. He was also wrong when he stated
that all atoms of an element are identical.
• All atoms of an element DO have the same
number of protons and electrons. The number of
neutrons, however, MAY DIFFER from atom to
atom.
• Atoms that have the same number of protons but
different numbers of neutrons are called isotopes.
3 Isotopes of Hydrogen
• Each has 1 proton.
• They differ in the
number of neutrons
they contain.
• Because of this, each
atom has a different
mass.
• Note that the isotopes
of hydrogen have
special names.
Isotopes
• Elements are found in nature as a mixture of
isotopes.
• The percent abundance of the isotopes (percent
of each isotope in a sample) is constant no
matter where the sample is found.
• Isotopes have the same chemical behavior
because chemical behavior is determined by
the number of ELECTRONS an element has.
Isotopes and Mass Number
• To make it easy to identify the various isotopes of an
element, scientists add a number after the element’s name.
• This number is the mass number - the sum of the number
of protons and neutrons in the nucleus of an atom of the
isotope.
• Mass # = # protons + # neutrons
Example
• Oxygen-17
– oxygen has an atomic number of 8, so
oxygen-17 has 8 protons and electrons.
– Since mass # = # protons + # neutrons, then
# neutrons = mass # - #protons. So, in oxygen17, # neutrons = 17 - 8 or 9.
• Oxygen-17 has 8 protons, 8 electrons, and 9
neutrons.
Mass
number
Atomic number
Isotopes
• Chemists use another
shortened notation to
identify isotopes.
• The chemical symbol
for the element is
used. Written to the
left of the symbol, a
superscript indicates
the mass number and a
subscript indicates the
atomic number.
Practice Problems
Element
Atomic
number
Mass
number
53
128
# of
protons
Shortened
# of
# of
neutrons electrons notation
Neon-22
41
19
64
30
K
Atomic Mass Units
• Recall that the masses of protons, neutrons,
and electrons are VERY small numbers.
These numbers are difficult to work with.
• Scientists therefore developed a way to
measure their mass relative to a standard
atom (the carbon-12 atom).
• One atomic mass unit (amu) is defined as
1/12 the mass of a carbon-12 atom.
Atomic Mass Units
• The masses of the particles in terms of amu
are:
– electron = 0.000549 amu (We round to 0 amu)
– proton = 1.007276 amu (We round to 1 amu)
– neutron = 1.008665 amu (We round to 1 amu)
• Note that the mass of a proton & neutron is
slightly different; also, correct masses of
atoms will not be whole numbers but
decimals.
Atomic Mass
• The atomic mass of an element is the
weighted average mass of the isotopes of
that element.
• Atomic mass, therefore, takes into account
the percent abundance of each isotope as
well as the mass of an atom of that isotope.
• The isotope in greater abundance will have
a greater effect on determining the atomic
mass.
Analyzing Atomic Mass
• Analyzing an element’s atomic mass can
SOMETIMES give you a clue as to what
the most abundant isotope for that element
is.
• The atomic mass of fluorine is 18.998 amu.
From this you can conclude that most
fluorine occurs in the form of fluorine-19.
Calculating Atomic Mass
1 Multiply the % abundance of
each isotope by its mass.
Chlorine-35
34.969(75.77%) = 26.496
amu
36.966(24.23%) = 8.9569
amu
2 Add the mass contributions
Atomic mass: 34.969 amu
of all the isotopes together. % abundance: 75.77%
35.453 amu is the atomic
mass of chlorine.
Chlorine-37
Atomic mass: 36.966 amu
% abundance: 24.23%
Practice Problems
• Use the following information to calculate the
atomic mass of magnesium:
magnesium-24: atomic mass 23.985, % abundance 78.99%
magnesium-25: atomic mass 24.986, % abundance 10.00%
magnesium-26: atomic mass 25.982, % abundance 11.01%
• Use the following information to calculate the
atomic mass of X; then identify X.
6X
= 6.015 amu; 7.5% abundance
7X = 7.016 amu; 92.5% abundance