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Transcript
Chapter 3 Atoms and Elements
3.4
The Atom
1
The Law of Multiple Proportions
and Dalton’s Atomic Theory
•
Elements are made up of tiny particles called atoms.
•
Each element is characterized by the mass of its atoms. Atoms of
the same element have the same mass, but atoms of different
elements have different masses.
•
The chemical combination of elements to make different chemical
compounds occurs when atoms join in small whole-number ratios.
•
Chemical reactions only rearrange how atoms are combined in
chemical compounds; the atoms themselves don’t change.
Subatomic Particles
Atoms contain subatomic particles.
•
•
•
•
Protons have a positive (+) charge.
Electrons have a negative (-) charge.
Neutrons are neutral.
Like charges repel and unlike charges
attract.
3
Atomic Structure: Electrons
Cathode-Ray Tubes: J. J. Thomson (1856-1940) proposed that cathode rays
must consist of tiny negatively charged particles. We now call them
electrons.
Rutherford’s Gold Foil Experiment
In Rutherford’s gold foil experiment, positively
charged particles
•
were aimed at atoms of gold.
•
mostly went straight through the atoms.
•
were deflected only occasionally.
Conclusion:
There must be a small, dense, positively charged
nucleus in the atom that deflects positive particles
that come close.
5
Rutherford’s Gold Foil Experiment
Copyright © 2009 by Pearson Education, Inc.
6
Structure of the Atom
An atom consists
• of a nucleus that
contains protons
and neutrons.
• of electrons in a
large, empty space
around the
nucleus.
7
Atomic Mass Scale
On the atomic mass scale,
• 1 atomic mass unit (amu) has a mass equal to 1/12 of the
mass of the carbon-12 atom.
• a proton has a mass of about 1 (1.007) amu.
• a neutron has a mass of about 1 (1.008) amu.
• an electron has a very small mass, 0.000 549 amu.
8
Particles in the Atom
9
Examples
Identify each statement as describing a
1) proton, 2) neutron, or 3) electron.
A. found outside the nucleus
B. has a positive charge
C. is neutral
D. found in the nucleus
10
Examples
 Is each of the following statements true or false?
 Protons are heavier than electrons
 Protons are attracted to neutrons
 Electrons are small that they have no electrical charge
 The nucleus contains all the protons and neutrons of an
atom
11
Chapter 3
Atoms and Elements
3.5
Atomic Number and Mass Number
12
Atomic Number
The atomic number
• is specific for each element.
• is the same for all atoms of an element.
• is equal to the number of protons in an atom.
• appears above the smbol of an element.
13
Atomic Number and Protons
Examples of atomic number and number of protons:
• Hydrogen has atomic number 1; every H atom has
one proton.
• Carbon has atomic number 6; every C atom has six
protons.
• Copper has atomic number 29; every Cu atom has 29
protons.
• Gold has atomic number 79; every Au atom has 79
protons.
14
Examples
State the number of protons in each.
A. A nitrogen atom
1) 5 protons
2) 7 protons
3) 14 protons
B. A sulfur atom
1) 32 protons
2) 16 protons
3) 6 protons
C. A barium atom
1) 137 protons
2) 81 protons
3) 56 protons
15
Electrons in An Atom
An atom
• of an element is electrically neutral; the net charge of
an atom is zero.
• has an equal number of protons and electrons.
number of protons = number of electrons
Aluminum has 13 protons and 13 electrons. The net
(overall) charge is zero.
13 protons (13+) + 13 electrons (13 -) = 0
16
Mass Number
The mass number
• represents the number of particles in the nucleus.
• is equal to the number of protons + the number of
neutrons.
17
Atomic Models
18
Examples
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30
2) 35
3) 65
B. How many neutrons are in the zinc atom?
1) 30
2) 35
3) 65
C. What is the mass number of a zinc atom that has
37 neutrons?
1) 37
2) 65
3) 67
19
Examples
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
C. The element is
1) Si
2) Ca
3) Se
20
Chapter 3
Atoms and Elements
3.6
Isotopes and Atomic Mass
24Mg 25Mg 26Mg
12
12
12
21
Isotopes
Isotopes
• are atoms of the same element that have different
mass numbers.
• have the same number of protons, but different
numbers of neutrons.
22
Atomic Symbol
An atomic symbol
• represents a particular atom of an element.
• gives the mass number in the upper left corner and
the atomic number in the lower left corner.
23
Atomic Numbers
carbon-12
mass number
12
6
C
6 protons
6 electrons
6 neutrons
C
6 protons
6 electrons
8 neutrons
atomic number
carbon-14
mass number
14
6
atomic number
Information from Atomic Symbols
The atomic symbol for a specific atom of an element
gives the
• number of protons (p+),
• number of neutrons (n),
• and number of electrons (e-).
25
Information from Atomic Symbols
Examples of number of subatomic particles for atoms
Atomic symbol
16
8
31
O
8 p+
8n
8 e-
P
15
15 p+
16 n
15 e-
65
Zn
30
30 p+
35 n
30 e-
26
Examples
Naturally occurring carbon consists of three isotopes:
12C, 13C, and 14C. State the number of protons, neutrons,
and electrons in each of the following:
protons
12C
13C
14C
6
6
6
______
neutrons ______
electrons
______
______
______
______
______
______
______
27
Examples
Write the atomic symbols for atoms with the
following subatomic particles:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
28
Examples
1. Which of the pairs are isotopes of the same element?
2. In which of the pairs do both atoms have 8 neutrons?
A.
15X
15X
8
B. 12X
6
7
14X
6
15X
C.
7
16X
8
29
Isotopes of Magnesium
30
Isotopes of Magnesium
31
Isotopes of Sulfur
A sample of naturally
occurring sulfur contains
several isotopes with the
following abundances
Isotope % abundance
32S
95.02
33S
0.75
34S
4.21
36S
0.02
32S, 33S, 34S, 36S
16
16
16
16
Copyright © 2009 by Pearson Education, Inc.
32
Atomic Mass
The atomic mass of an element
• is listed below the symbol of each element
on the periodic table.
• gives the mass of an “average” atom of each
Na
22.99
element compared to 12C.
• is not the same as the mass number.
33
Isotopes of Some Elements and Their
Atomic Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.
34
Atomic Mass for Cl
The atomic mass of chlorine is
• due to all the Cl isotopes.
• not a whole number.
• the average of two isotopes:
35Cl
and 37Cl.
35
Learning Check
Using the periodic table, specify the atomic mass of
each element.
A.
calcium
__________
B.
aluminum
__________
C.
lead
__________
D.
barium
__________
E.
iron
__________
36
Calculating Atomic Mass
The calculation for atomic mass requires the
• percent(%) abundance of each isotope.
• atomic mass of each isotope of that element.
• sum of the weighted averages.
mass of isotope(1)x (%) + mass of isotope(2) x (%) +
100
100
37
Atomic Masses
Why is the atomic mass of the element carbon 12.01 amu?
carbon-12:
98.89 % natural abundance
12 amu
carbon-13:
1.11 % natural abundance
13.0034 amu
mass of carbon = (12 amu)(0.9889) + (13.0034 amu)(0.0111)
= 11.87 amu + 0.144 amu
= 12.01 amu
Calculating Atomic Mass for Cl
35Cl
has atomic mass 34.97 amu (75.76%) and 37C
has atomic mass 36.97 amu (24.24%).
• Use atomic mass and percent of each isotope to
calculate the contribution of each isotope to the
weighted average.
Atomic mass 35Cl x % abundance =
Atomic mass 37Cl x % abundance =
• Sum is atomic mass of Cl is
39
Learning Check
Gallium is an element found in lasers used in compact
disc players. In a sample of gallium, there is 60.10% of
69Ga (atomic mass 68.926) atoms and 39.90% of 71Ga
(atomic mass 70.925) atoms.
What is the atomic mass of gallium?
40