Download Chapter 3

The Atom: From Philosophical Idea to Theory
Chapter 3
Atoms:
The Building
Blocks of Matter
3.1 The Atom:
From Philosophical Idea to Theory
Democritus (400 BC)
"The only existing things
are atoms and empty
space; all else is opinion”
Coined the term “atomos”
Law of Conservation of Mass
Mass is neither created
nor destroyed during
chemical or physical
reactions.
Total mass of reactants
=
Antoine Lavoisier
Total mass of products
Law of Definite Proportions
• A chemical compound contains the
same elements in exactly the same
proportions by mass regardless of the
size of the sample or the source of the
sample
Law of Multiple Proportions
• If two or more different compounds are
composed of the same two elements, then
the ratio of the masses of the second
element combined with a certain mass of
the first element is always a ratio of small
whole numbers
a. CO2 and CO
b. H2O and H2O2
Dalton’s Atomic Theory (1808)
 All matter is composed of extremely
small particles called atoms
 Atoms of a given element are identical
in size, mass, and other properties;
John Dalton
atoms of different elements differ in
size, mass, and other properties
Dalton’s Atomic Theory (1808)
 Atoms cannot be subdivided, created,
or destroyed
 Atoms of different elements combine in
simple whole-number ratios to form
John Dalton
chemical compounds
 In chemical reactions, atoms are
combined, separated, or rearranged
Modern Atomic Theory
Several changes have been made to Dalton’s theory.
Dalton said:
Atoms of a given element are identical in size,
mass, and other properties; atoms of different
elements differ in size, mass, and other
properties
Modern theory states:
Atoms of an element have a characteristic
average mass which is unique to that
element.
The Atom: From Philosophical Idea to Theory
Chapter 3
Atoms:
The Building
Blocks of Matter
3.2
The Structure of the Atom
Isotopes
Elements occur in
nature as
mixtures of
isotopes.
Isotopes are atoms
of the same
element that differ
in the number of
neutrons
Atomic Masses
Atomic mass is the average of all the naturally
occurring isotopes of that element.
Carbon = 12.011
Isotope
Symbol
Composition of
the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
6 protons
7 neutrons
1.11%
Carbon-14
14C
6 protons
8 neutrons
<0.01%
1 amu (atomic mass unit) = 1/12 the mass of a carbon-12
atom; also = 1.660540 x 10-27 kg
Modern Atomic Theory #2
Dalton said:
Atoms cannot be subdivided, created, or
destroyed
Modern theory states:
Atoms cannot be subdivided, created, or
destroyed in ordinary chemical reactions.
However, these changes CAN occur in
nuclear reactions!
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to
deduce the presence of a negatively charged
particle.
Cathode ray tubes pass electricity through a
gas that is contained at a very low pressure.
Some Modern
Cathode Ray Tubes
Thomson’s Atomic Model
Thomson believed that the electrons were like plums
embedded in a positively charged “pudding,” thus it
was called the “plum pudding” model.
Mass of the Electron
1909 – Robert
Millikan determines
the mass of the
electron.
Mass of the
electron is
9.109 x 10-31 kg
The oil drop apparatus
Conclusions from the Study
of the Electron
 Cathode rays have identical properties regardless of the
element used to produce them. All elements must contain
identically charged electrons.
Atoms are neutral, so there must be positive particles in
the atom to balance the negative charge of the electrons
 Electrons have so little mass that atoms must contain
other particles that account for most of the mass
Rutherford’s Gold Foil
Experiment
 Alpha particles are helium nuclei
 Particles were fired at a thin sheet of gold foil
 Particle hits on the detecting screen (film) are
recorded
Try it Yourself!
In the following pictures, there is a target hidden
by a cloud. To figure out the shape of the target,
we shot some beams into the cloud and
recorded where the beams came out. Can you
figure out the shape of the target?
The Answers
Target #2
Target #1
Rutherford’s Findings
 Most of the particles passed right through
 A few particles were deflected
 VERY FEW were greatly deflected
“Like howitzer shells bouncing off of
tissue paper!”
Cool Website!
Conclusions:
 The nucleus is small
 The nucleus is dense
 The nucleus is positively charged
Atomic Particles
Particle
Charge
Mass (kg)
Location
Electron
-1
9.109 x 10-31
Electron
cloud
Proton
+1
1.673 x 10-27
Nucleus
Neutron
0
1.675 x 10-27
Nucleus
The Atomic
Scale
 Most of the mass of the
atom is in the nucleus
(protons and neutrons)
 Electrons are found outside
of the nucleus (the electron
cloud)
 Most of the volume of the
atom is empty space
“q” is a particle called a “quark”
The Atom: From Philosophical Idea to Theory
Chapter 3
Atoms:
The Building
Blocks of Matter
3.3
Counting Atoms
Atomic Number
Atomic number (Z) of an element is the
number of protons in the nucleus of
each atom of that element.
Element
# of protons
Atomic # (Z)
Carbon
6
6
Phosphorus
15
15
Gold
79
79
Isotopes…Again (must be on the test)
Isotopes are atoms of the same element having different
masses due to varying numbers of neutrons.
Isotope
Proton
s
Electrons
Neutrons
Hydrogen–1
(protium)
1
1
0
Hydrogen-2
(deuterium)
1
1
1
Hydrogen-3
(tritium)
1
1
2
Nucleus
Mass Number
Mass number is the number of protons
and neutrons in the nucleus of an isotope.
Mass # = p+ + n0
p+
n0
e-
Oxygen - 18
8
10
8
18
Arsenic
75
33
42
33
75
- 31
Phosphorus
15
16
15
31
Nuclide
Mass #
Practice
1. How many protons, neutrons & electrons make
up an atom of carbon-13? 6 protons, 6 electrons & 7
neutrons
2. Write the nuclear symbol for oxygen-16.
3. Write the hyphen notation for the element
whose atoms have 7 electrons & 9 neutrons.
Nitrogen-16
The Mole
• In chemistry, we use a new unit to
quantify an amount of a substance.
• This unit is called the mole;
abbreviated mol.
(no, not an animal or a dermatological
spot)
• The same way the supermarket uses
a dozen to quantify 12 objects, we
use a mole to quantify
6.02 x 1023 objects.
• This unit helps chemists determine
the number of particles in a sample of
matter without counting them.
The Mole
1 dozen = 12
1 gross = 144
1 ream = 500
1 mole = 6.02 x 1023
There are exactly 12 grams of carbon12 in one mole of carbon-12.
Avogadro’s Number
6.02 x 1023 is called “Avogadro’s Number” in
honor of the Italian chemist Amadeo Avogadro
(1776-1855).
I didn’t discover it. Its just
named after me!
Amadeo Avogadro
Calculations with Moles:
Converting moles to grams
How many grams of lithium are in 3.50
moles of lithium?
3.50 mol Li
X
6.94 g Li
1 mol Li
= 24.3 g Li
Calculations with Moles:
Converting grams to moles
How many moles of lithium are in 18.2
grams of lithium?
18.2 g Li
X
1 mol Li
6.94 g Li
= 2.62 mol Li
Calculations with Moles:
Using Avogadro’s Number
How many atoms of lithium are in 3.50
moles of lithium?
3.50 mol Li
X
6.022 x 1023 atoms Li
1 mol Li
= 2.11 x 1024 atoms Li
Calculations with Moles:
Using Avogadro’s Number
How many atoms of lithium are in 18.2 g of
lithium?
18.2 g Li
X
1 mol Li
6.94 g Li
6.022 x 1023 atoms Li
X
1 mol Li
(18.2)(6.022 x 1023)/6.94 = 1.58 x 1024 atoms Li
More Practice
1. What is the mass is grams of 3.6 mol of carbon?
43 g
2. What is the mass in grams of 1.733 mol of sulfur?
23.5 g
3. How many moles of copper are in 3.22 g copper?
0.0507 mol
4. How many moles of lithium are in 2.72 x 10-4 g
lithium?
3.92 x 10-5 mol
5. How many moles of carbon are in 2.25 x 1022
atoms of carbon?
0.0374 mol
Even More Practice!
6. How many moles of oxygen, O, are in 2,000,000
atoms oxygen? 3 x 10-18 mol
7. How many atoms of sodium are in 3.80 mol
sodium? 2.29 x 1024 atoms
8. What is the mass in grams of 5.0 x 109 atoms of
neon?
1.7 x 10-13 g
9. How many atoms of carbon are in 0.0201.0
g carbon?
x 1021 atoms
10. What mass of silver contains the same number of
atoms as 10.0 g of boron?
99.8 g