Chapter 2: The Components of Matter
The topics in this chapter should be review from a previous
course. It is expected that you are able to review and master this
material quickly and somewhat independently.
From this Chapter you should:
• understand elements, atoms, compounds and molecules.
• know the fundamental differences between mixtures and compounds.
• understand and be able to apply the Law of Conservation of Mass and the Law of
Definite Composition.
• understand and be able to discuss/describe the atomic theory of matter.
• understand and be able to discuss/describe the history of the discovery of subatomic
particles.
• understand and be able to discuss/describe the structure of the atom, atomic number
and mass number.
• understand atomic mass, isotopes, mass spectra and be able to calculate average
atomic mass.
• become more familiar with the Periodic table: Families (columns) and Periods.
• understand different ways to represent molecules with formulas and models.
• be able to calculated the molecular or formula masses of compounds.
• understand the difference between molecular and ionic compounds.
• become proficient in inorganic nomenclature: names and formulas of compounds (LAB).
• be able to name and write formulas for a few types of simple organic compounds.
Larson-Foothill College
1
Solutions are
Homogeneous
mixtures.
Matter
Classifying Matter
Matter
Occupies Space
Heterogeneous Matter
Not uniform throughout
Homogeneous Matter
Uniform throughout
Solutions
Uniform but variable composition
Mixtures:
two or more pure substances
physically combined,
composition can vary.
Pure Substance
Fixed composition
Cannot be further purified
Elements
Cannot be divided into simpler substances
Halite (table salt, NaCl, ionic): Microscopic and macroscopic
Sand and dirt
is a heterogeneous
mixture.
Compounds
Elements united in fixed ratios
Ionic
(salts)
Covalent
(molecules)
Molecules of antifreeze (covalent):
Microscopic structure: Purple is
sodium ion, green is chlorine
ion. There is cubic symmetry in
the atoms' arrangement.
Larson-Foothill College
Macroscopic (~16cm) halite
crystal. The right-angles
between crystal faces are due
to the cubic symmetry of the
atoms' arrangement.
2
Mass Laws
•
What’s the Law of Conservation of Mass?
•
What’s the Law of Definite (or constant) Composition?
•
Fluorite, a mineral of calcium, is composed of the metal and fluorine. Analysis
shows that a 2.76 g sample of fluorite contains 1.42 g of calcium.
Determine the mass percent of fluorine in fluorite.
How many grams of calcium are in 125 g of fluorite?
Larson-Foothill College
3
Dalton’s Atomic Theory (1803-1806) Postulates
1. Each element is composed of extremely small, indivisible particles
called atoms.
2. All atoms of a given element are identical to one another in mass and
other properties, but the atoms of one element are different from the
atoms of all other elements.
3. Atoms of an element are cannot be changed into atoms of a different
element; atoms are neither created nor destroyed in chemical
reactions.
4. Compounds are formed when atoms of more than one element
combine; a given compound always has the same specific ratio of
different elements.
•
Which of these postulates is consistent with the Law of Conservation
of Mass?
•
Which of these postulates is consistent with the Law of Definite
Composition?
•
Which of these postulates have since been modified?
Larson-Foothill College
4
History of Modern Atomic Theory
In the late 19th and early 20th century, a series of experimentation allowed
scientists to establish a model of the atom that remains today the
foundation of modern atomic theory.
The Discovery of the Electron-J.J. Thomson
Cathode ray tubes (CRT) - mid 1800’s - electron beam
http://www.youtube.com/watch?v=O9Goyscbazk
What are the conclusions?
Thompson concluded that cathode rays are streams of negatively charged particles based
upon their behavior. He also measured the charge/mass ratio of the electron to be
1.76 × 108 C/g in 1897. The exact mass of the electron was still unknown. (C is the symbol
for coulomb, the SI unit for electric charge.)
Larson-Foothill College
5
History of Modern Atomic Theory
Millikan Oil Drop Experiment (1909)
Once the charge/mass
ratio of the electron was
known, determination of
either the charge or the
mass of an electron
would enable you to
determine the other.
Robert Millikan
(University of Chicago)
determined the charge on
the electron in 1909.
Electron Charge: 1.602x10-19 C
Electron Mass = ? (calculate it)
Larson-Foothill College
6
History of Modern Atomic Theory
Radioactivity-further evidence that the atom is divisible!
•
•
•
•
The spontaneous emission of radiation by an atom.
First observed by Henri Becquerel (1896).
Also studied by Marie and Pierre Curie.
Three types of radiation were identified by Ernest Rutherford:
– α particles (He nucleus, + charge and heavy)
– β particles (electron, – charge and light)
– γ rays (high energy electromagnetic radiation: no charge, no mass)
Larson-Foothill College
7
Discovery of the Nucleus
In 1910, Ernest Rutherford directed a beam of α particles at a thin
sheet of gold foil and observed the pattern of scatter of the
particles.
Larson-Foothill College
8
Rutherford is credited with the “discovery” of the
nuclear atom.
• Based on the alpha particle scatter pattern, Rutherford
postulated in 1911 that the atom contains a very small,
dense nucleus with the electrons surrounding the
nucleus.
• Most of the volume of the atom is empty space.
• Protons were later “discovered” by Rutherford in 1919.
(They were first detected by Eugen Goldstein in 1886, emitted
in the opposite direction compared to electrons from a CRT.)
• Neutrons were finally discovered by James Chadwick in
1932.
Larson-Foothill College
9
Model of the Atom
•
•
•
•
Electrons (–), e–
Protons (+), p+
Neutrons (0), n0
Neutral atoms: number
of protons=number of
electrons
Table 2.2
Properties of the Three Key Subatomic Particles
Charge
Name
Relative Absolute (C)*
(Symbol)
Mass
Relative
(amu)†
Absolute (g) Location in
Atom
Proton
(p+)
1+
+1.60218x10-19 1.00727
1.67262x10-24 Nucleus
Neutron
(n0)
0
0
1.67493x10-24 Nucleus
Electron
(e-)
1-
-1.60218x10-19 0.00054858 9.10939x10-28 Outside
nucleus
* The
†
1.00866
coulomb (C) is the SI unit of charge.
The atomic mass unit (amu) equals 1.66054x10-24 g.
Larson-Foothill College
10
Mass Spectrometry: The Discovery of Isotopes!
• A mass spectrometer measures the relative mass and
abundance of an element’s isotopes.
Mass Spectrum of Cl
The two isotopes of Cl are clearly defined.
Determine their approximate percent abundance.
Larson-Foothill College
11
Some Examples of Isotopic Abundances and Masses
Determined Using Mass Spectrometry
Larson-Foothill College
12
Atomic Number, Mass Number and Isotopic
Notation
• Atomic Number, Z: The number of protons in the nucleus of
an element. Each specific element has a unique atomic
number.
• Mass number, A: The number of protons + neutrons in the
nucleus of an element. Mass number is NOT unique.
Isotopes
• Atoms of the same element with different masses.
• Isotopes have different numbers of neutrons.
11
6
C
12
6
C
13
6
C
14
6
C
Isotopic notation : AZ X
carbon -12 : 126 C
What is the isotopic notation for Uranium-235?
Larson-Foothill College
13
Atomic Mass
• Mass values on the periodic table are relative to carbon-12.
The mass values are given as amu (atomic mass units).
• 1 amu = 1/12 the mass of an atom of carbon-12 (defined).
• We convert to absolute units (g, kg, lbs. etc.) by using the
conversions:
1 amu = 1.661x10-24 g
1 g = 6.02214x1023 amu (look familiar?)
• The atomic masses listed in the periodic table are average
masses. These are weighted averages based upon the
naturally occurring isotopic abundances.
Average Atomic Mass =
Larson-Foothill College
14
Calculating Atomic Masses
Naturally occurring magnesium has the following isotopic
abundances.
(a)What is the average atomic mass of magnesium?
(b)Sketch the mass spectrum of magnesium.
Larson-Foothill College
15
The Periodic Table
• Mendeleev is given credit
for first proposal of the
Periodic Table, published in
1896.
• Groups or families; these
are the columns of the
periodic table and contain
elements with similar
properties.
• Rows; these are called
periods.
Larson-Foothill College
16