Transcript Chapter 4 Atomic Structure

How do atoms differ?
Section 4.3
REVIEW: THE ATOM
The nucleus
REVIEW
Particle
Location
Electron
In the space
surrounding
nucleus
Charge
Relative
Mass
(amu)
Actual
Mass
(g)
1-
0
(1/1840)
9.11 x 10-28
Proton
Nucleus
1+
1
1.673 x 10-
Neutron
Nucleus
0
1
1.675 x 10-
24
24
If all atoms are composed of
protons, neutrons and electrons,
how do elements differ?
–What makes an atom of oxygen an
oxygen atom?
ATOMIC NUMBER (Z)
• An element’s atomic number (Z)
represents the number of protons in its
nucleus
• The number of protons in an atom’s
nucleus determines the identity of the
atom
– If an atom has 9 protons it’s fluorine, if it has
20 it’s calcium
• In a neutral atom:
– number of protons = number of electrons, so:
atomic number = # protons = #
THE MODERN PERIODIC TABLE
Hydrogen
Atomic number (Z):
number of protons
Atomic mass:
weighted average
of element’s
isotopes masses
Element name
1
H
Symbol:
one or two letters
1.008
*Elements are arranged according to atomic number
– An element’s atomic number determines its
position on the periodic table
• The periodic table is organized left-to-right, topto-bottom by increasing atomic number
TAKE A LOOK AT A PERIODIC TABLE:
• Complete the following table:
Element
Number of
protons
Number of
electrons
sodium (Na)
Copper (Cu)
Boron (B)
66
14
MASS NUMBER (A)
• An atom’s mass number (A) represents the total
number of neutrons and protons in the nucleus
of an atom
Mass number = # protons + # neutrons
or
Mass number = atomic number + # neutrons
• How do you determine the number of neutrons
in an atom?
# neutrons = mass number (A) - atomic
number (Z)
• It is always true that:
– A carbon atom has 6 protons in its nucleus
– A neutral carbon atom has 6 electrons
• A carbon atom also contains neutrons, but unlike
electrons and protons the number of neutrons can
change from carbon atom to carbon atom
• Some carbon atoms have 6 neutrons, others have 7
neutrons and still others have 8 neutrons
ISOTOPES
• Dalton’s atomic theory states that all atoms
of a given element are identical. This is
mostly true
• Atoms of the same element can differ in the
number of neutrons
• most elements have two or more isotopes
• Isotopes are atoms of the same element with
different numbers of neutrons (and therefore
different masses)
– atoms have the same atomic number but
different mass numbers
IDENTIFYING ISOTOPES
• To distinguish one isotope from another an atom’s
mass number is added after the element’s name:
– carbon-12; carbon-14; uranium-235
• Remember the mass number of an isotope represents
the sum of the neutrons and protons in the nucleus
Potassium-39
Potassium-40 Potassium-41
protons
19
19
19
neutrons
20
21
22
electrons
19
19
19
isotope
Hydrogen–1
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium)
protons
electrons
neutrons
1
1
0
1
1
1
1
1
2
WRITING SYMBOLS FOR
ATOMS
MASS NUMBER = # PROTONS + # NEUTRONS
A
X
SYMBOL
ATOMIC NUMBER = # PROTONS
Z
PRACTICE
26 protons
56
Fe
26
26 electrons
30 neutrons
PRACTICE: FILL IN THE
BLANKS
symbol
atomic
number
Fe
mass
number
56
60
144
102
59
Al
number number of number of
of protons electrons
neutrons
27
45
45
31
235
92
U
uranium-235
used for nuclear reactors
and atomic bombs
238
92
U
uranium-238
TURN TO THE PERSON NEXT
TO YOU:
• You have an atom each of two isotopes
of carbon:
– What is the same about the two atoms?
– What is different about the two atoms?
– How can we tell the two atoms apart?
IMPORTANT TO NOTE:
• Isotopes with more neutrons have
greater mass, but are chemically the
same (under normal conditions)
• The number of protons determines the
identity of the element and electrons are
responsible for an element’s chemical
properties
THE MODERN PERIODIC TABLE
Hydrogen
Atomic number (Z):
number of protons
Average atomic
mass: weighted
average
of element’s
isotopes masses
Element name
1
H
Symbol:
one or two letters
1.008
*Elements are arranged according to atomic number
ATOMIC MASS
• The mass of an atom depends on the
number of electrons, protons and
neutrons it contains
• because atoms are extremely small
particles (even the smallest speck of
dust can contain
10,000,000,000,000,000 atoms) it’s
impractical to measure the mass of
atoms in grams
Particle
Actual Mass (g)
Electron
9.11 x 10-28
Proton
1.673 x 10-24
Neutron
1.675 x 10-24
MEASURING ATOMIC MASS
• Because the actual masses of protons and neutrons are
very small chemists have developed a way to measure
atoms based on the Carbon-12 atom as the standard
• Instead of grams, the unit we use is the Atomic Mass Unit
(amu)
– Carbon-12 is exactly 12 amu
– 1 amu is exactly 1/12 of the mass of a carbon-12 atom
• Protons and neutrons have a mass slightly greater than 1
Particle
Mass (amu)
amu
Electron
0.000549
Proton
1.007276
Neutron
1.008665
AVERAGE ATOMIC MASS
• When one consults the periodic table,
the atomic mass of carbon is not 12.00
amu but 12.011 amu
• This is because most elements have
more than one naturally occurring
isotope and the atomic mass on the
periodic table is the weighted average
of the mass of each of the element’s
isotopes
AVERAGE ATOMIC MASS
• In nature elements exist as a mixture of
their isotopes, for example chlorine:
• 75% chlorine-35
• 25% chlorine-37
• Atomic mass of chlorine is 35.453 amu
• So the average atomic mass is
calculated by taking the weighted
average of the isotopes’ masses
– This is why an element’s atomic mass is
not a whole number
AVERAGE ATOMIC MASS OF
CARBON
Isotope
Mass of
isotope
carbon12
carbon13
12.00000
amu
13.00335
amu
carbon-12:
carbon-13:
Abundance
of isotope
(%)
98.90%
1.10%
mass x abundance
12.000 x 0.9890 = 11.868 amu
13.00335 x 0.0110 =
+ 0.143
amu
12.011 amu
PRACTICE: CALCULATE ATOMIC MASS
FOR ELEMENT X. IDENTIFY THE
ELEMENT
Isotope
Mass (amu)
% abundance
X-6
6.015
7.5%
X-7
7.016
92.5%
Isotope
Mass (amu)
% abundance
6X
6.015
7.5%
7X
7.016
92.5%
1. Mass contribution = (mass)(% abundance)
6X
Mass contribution = (6.015)(.075) = 0.451 amu
7X
Mass contribution = (7.016)(.925) = 6.490 amu
2. Sum of the mass contributions:
0.451 amu + 6.490 amu = 6.941 amu
Which element is this?