Atoms and Elements
Chapter 2
Chapter 2
1
The Atomic Theory of Matter
Dalton’s Theory of Matter
Dalton’s theory describes the basic unit of matter,
the atom.
Chapter 2
2
The Atomic Theory of Matter
Dalton’s Theory of Matter
1. Each element is composed of extremely small
particles call atoms.
2. All atoms of a given element are identical; atoms
of different elements are different.
3. Atoms of an element are not changed into
different types of atoms by a chemical reaction.
4. Compounds are formed when atoms of more
than one element combine.
Chapter 2
3
The Atomic Theory of Matter
Dalton’s Theory of Matter
Law of Constant Composition
In a given compound the relative numbers and
kinds of atoms are constant.
Chapter 2
4
The Atomic Theory of Matter
Dalton’s Theory of Matter
Law of Multiple Proportions
When two elements form different compounds, the
mass ratio of the elements in one compound is
related to the mass ratio in the other by a small
whole number.
Example:
Carbon Monoxide (CO)  12g C and 16g O
Carbon Dioxide (CO2)  12g C and 32g O
There is a 1:2 ratio of oxygen between the two
compounds.
Chapter 2
5
The Discovery of Atomic Structure
Cathode Rays and Electrons
• A cathode ray tube (CRT) is a hollow vessel with an
electrode at either end.
• The tube is evacuated and then partially filled with a
gas.
• A high voltage is applied across the electrodes and the
gas glows.
• Since glowing gas originates from the cathode, it is
referred to as a “Cathode Ray”.
Chapter 2
6
The Discovery of Atomic Structure
Cathode Rays and Electrons
Chapter 2
7
The Discovery of Atomic Structure
Characteristics of Cathode Rays
• The rays are deflected by magnetic fields.
• Metal plates exposed to cathode rays acquire a
negative charge.
• The rays emitted are independent gas in the cathode
tube.
Chapter 2
8
The Discovery of Atomic Structure
Cathode Rays and Electrons
Based on this information, it was determined that
cathode rays must be some fundamental particle.
Cathode rays are Electrons.
Chapter 2
9
The Discovery of Atomic Structure
Cathode Rays and Electrons
In 1897, Thomson determined the charge to mass ratio
of an electron to be 1.76  108 C/g.
Chapter 2
10
The Discovery of Atomic Structure
Cathode Rays and Electrons
• Millikan determined the charge on an electron to be
1.60 x 10-19 couloumbs.
• This was done with the oil drop experiment.
Chapter 2
11
The Discovery of Atomic Structure
Cathode Rays and Electrons
Chapter 2
12
The Modern View of Atomic Structure
3 Major Components of the Atom
•Proton
positive (+) charge
•Neutron
no charge
•Electron
negative (-) charge
1.0073 amu
1.0087 amu
0.000548 amu
The proton and neutron reside in the nucleus
Chapter 2
13
The Discovery of Atomic Structure
Radioactivity
Radioactivity - The spontaneous emission of radiation.
There are three types of radiation
Chapter 2
14
The Discovery of Atomic Structure
Radioactivity
1. Alpha particles (a)
a) Positive charge.
b) “Bare” helium atom.
c) Most massive, least penetrating radiation.
2. Beta particles (b)
a) Electron ejected from the nucleus.
3. Gamma Ray(g)
a) This is a ray, not a particle.
b) Most penetrating.
Chapter 2
15
The Discovery of Atomic Structure
The Nuclear Atom
Rutherford’s a-particle experiment:
Chapter 2
16
The Discovery of Atomic Structure
The Nuclear Atom
• The a-particles were shot through a piece of gold foil.
• Most of the a-particles went straight through the foil
without deflection.
• A few of the particles were scattered.
• A very few of the particle were deflected back toward
the source.
Chapter 2
17
The Discovery of Atomic Structure
The Nuclear Atom
• Most of the volume of the atom must consist of a low
mass, diffuse negative charge (electron cloud).
• The center or nucleus of the atom must consist of a
dense positive charge.
Chapter 2
18
The Modern View of Atomic Structure
Chapter 2
19
The Discovery of Atomic Structure
Chapter 2
20
The Modern View of Atomic Structure
Isotopes, Atomic Numbers, and Mass Numbers
Atomic number (Z) = number of protons in the nucleus.
Mass number (A) = total number of protons and
neutrons in the nucleus.
A
Z
X
exam ple:
12
6
C
Chapter 2
21
The Modern View of Atomic Structure
Isotopes, Atomic Numbers, and Mass Numbers
• In this series of elements, the mass number (A)
varies but the atomic number (Z) is constant.
• This means that we are looking at a series of
isotopes.
Chapter 2
22
The Modern View of Atomic Structure
Isotopes, Atomic Numbers, and Mass Numbers
Isotope - Atom of an element with varying number of
neutrons.
Chapter 2
23
The Modern View of Atomic Structure
Atomic Weight
The atomic weight is the weighted average of the
mass of the various isotopes of an element in a
representative sample.
 99.985
  0.015
  0 

atom icweight H  
1
.
0078
amu

2
.
0141
amu

3
.
0161
amu

  100 
  100

100






 
 

1.00794amu
Chapter 2
24
The Modern View of Atomic Structure
The Mole
• The amount of substance that contains as many
elementary entities as there are atoms in exactly 12 g
of the carbon-12 isotope.
• The amount or number of carbon-12 atoms is:
6.02 x 1023.
• This value is also call Avogadro’s number.
Chapter 2
25
The Modern View of Atomic Structure
Molar Mass
• This is simply the mass (in grams) of one mole on
any substance.
• This number is commonly used as a conversion
factor in calculations.
Chapter 2
26
The Modern View of Atomic Structure
Molar Mass
How many moles of phosphorus (P) are contained in 15.51g?


1

m oles gram s
m
olar
m
ass




1

m oles15.51g 
 30.974g / m ol


1
  0.5007mol
moles15.51g 
 30.974g / mol
Chapter 2
27
The Modern View of Atomic Structure
Molar Mass
How many grams of boron (B) are contained in 1.289 mol?
grams molesmolarmass
grams1.289mol10.811g / mol
grams1.289mol10.811g / mol 13.94g
Chapter 2
28
The Periodic Table
Chapter 2
29
The Periodic Table
• Metals are located on the left hand side of the
periodic table (most of the elements are metals).
• Non-metals are located in the top right hand side of
the periodic table.
• Elements with properties similar to both metals and
non-metals are called metalloids and are located at
the interface between the metals and non-metals.
Chapter 2
30
The Periodic Table
• The elements in the periodic table are arranged in
such a way that substances that are chemically
similar are in a column (called groups).
• Columns are numbered from 1A to 8A or 1 to 18.
Chapter 2
31
The Periodic Table
Chapter 2
32
The Periodic Table
Some of the groups in the periodic table are given
special names.
Group 1A: Alkali metals (Li….).
Group 2A: Alkaline earth metals (Be…).
Group 6A: Chalcogens (O…..).
Group 7A: Halogens (F….).
Group 8A: Noble gases (He…..).
Chapter 2
33
The Periodic Table
Rows of elements are referred to as periods.
Chapter 2
34
End of Chapter Problems
10, 14, 18, 24, 28, 30, 40, 42, 58, 68
Chapter 2
35