Chapter 2
Atoms, Molecules and Ions
Pentacene on a copper surface (AFM image)
Science, Volume 325, Issue 5944, pp. 1110 – 1114 (28 August 2009).
(Dalton’s) Atomic Theory
• Each element is made up of atoms.
• Atoms of a given element are identical while atoms of
different elements differ.
• Chemical compounds are made up of specific whole
number ratios of atoms.
• Chemical Reactions involve the reorganization of atoms
– atoms (and masses) do not change.
John Dalton, 1766 -1844
Lithograph
Properties of Charged Particles
Sir Joseph John Thompson,
1856 - 1940
Thomson was awarded the 1906 Nobel Prize.
An evacuated tube, containing a small
amount of a gas was attached to a
power supply:
Electron mass/charge = -5.6857 x 10-9 g coulomb-1
• The same cathode rays (electrons) were
seen no matter what gas the tube was filled
with.
• But the positively charged portion left over
had a different mass/charge dependent on
the type of gas used.
Thompson’s Conclusion:
Electrons are a fundamental unit of
all materials, and atoms look like a
“sea” of positive charge with
imbedded electrons:
Millikan’s Oil Drop Experiment
Determined the charge on an electron =
1.592 X 10-19 coulombs
Types of Radiation
(Marie Curie/Rutherford)
•-rays are high-energy light.
•  -particles are helium
nuclei.
• -particles are high energy
electrons
Marie Curie
1867-1934
•Met Pierre Curie, Physics Professor at the Sorbonne in 1894
•Succeeded her husband as Head of the Physics Laboratory at the Sorbonne
•Following the death of Pierre Curie in 1906, she took his place as Professor of
General Physics in the Faculty of Sciences, the first time a woman had held this
position.
•Received the Nobel Prize for Physics in 1903
•Received a second Nobel Prize in Chemistry in 1911
•Discovered two elements (Polonium and Radium)
Marie Curie
(Maria Sklodowska)
1867-1934
•Met Pierre Curie, Physics Professor at the Sorbonne in 1894
•Succeeded her husband as Head of the Physics Laboratory at the Sorbonne
•Following the death of Pierre Curie in 1906, she took his place as Professor of
General Physics in the Faculty of Sciences, the first time a woman had held this
position.
•Received the Nobel Prize for Physics in 1903
•Received a second Nobel Prize in Chemistry in 1911
•Discovered two elements (Polonium and Radium)
Rutherford’s Gold Foil Experiment
The Composition of the Atom
Rutherford
protons 1919
James Chadwick
neutrons 1932
The nucleus is made up of protons and
neutrons
The “Modern” Atom
The Fundamental Particles
Particle
Mass
g
Electron
Proton
Neutron
9.109 x 10-28
1.673 x 10-24
1.675 x 10-24
Charge
amu
(0)
1
1
Coulombs
(e)
–1.602 x 10-19
+1.602 x 10-19
0
–1
+1
0
Atoms are uncharged species – thus they contain the same
number of protons and electrons (charged species are
known as ions)
The Concept of Atomic Number
12
6
C
The Concept of Atomic Number
12
The atomic number (Z) = # of protons
6
C
The Concept of Atomic Number
The atomic mass (A) = protons + neutrons
The atomic number (Z) = # of protons
12
6
C
Atomic Mass Units
The atomic mass (A) = protons + neutrons
12
6
-23
• Mass of carbon-12 = 1.9926 X 10
C
g
• Mass of carbon-12 = 12 amu (atomic mass units - amu or u)
Isotopic Distributions
Why does the periodic table say that the mass of carbon is 12.011 amu?
Isotopic Distributions
Why does the periodic table say that the mass of carbon is 12.011 amu?
98.9% of all carbon atoms have A = 12 (6 neutrons)
1.1% of all carbon atoms have A = 13 (7 neutrons)
12.011 is the average mass of a large sample of carbon atoms. When
we measure “large” amounts of atoms, we can always use the
average atomic mass.
Mass Spectromeric Isotope Indentification
Mass Spectrometric Isotope Identification
202Hg
200Hg
199Hg
201Hg
198Hg
204Hg
196Hg
How many atoms are in 1 g of copper?
How many atoms are in 1 g of copper?
If Cu has an atomic mass of 63.546 amu, how does this easily
convert to grams?
Avogadro’s Number (the mole) relates atomic mass to sample mass:
NA = 6.022 X 1023 = 1 mole
1 mole of atoms = mass in grams equal to the atomic mass
How many atoms are in 1 g of copper?
If Cu has an atomic mass of 63.546 amu, how does this easily
convert to grams?
Avogadro’s Number (the mole) relates atomic mass to sample mass:
NA = 6.022 X 1023 = 1 mole
1 mole of atoms = mass in grams equal to the atomic mass
The molar mass, M, is the mass of one mole of a substance.
Ions and Ionic Compounds
A charged species is called an ion.
Metals tend to lose electrons to form positively charged ions –
cations.
Nonmetals tend to gain electrons to form negatively charged ions –
anions.
When forming compounds, the metal and nonmetal ions combine
so that the net charge on a formula unit is zero.
23
11
Na
+
The atomic charge = protons – electrons
Ions and Ionic Compounds
Ionic compounds are held together by the electrostatic attraction
of opposite charges - this is also known as coulombic attraction.
This attraction is an ionic bond.
Ionic compounds behave as an aggregate, not individual molecules
Predicting Ionic Charges
Main group elements tend to lose or gain electrons in order to equal the electron
count of the nearest noble gas
Predicting Ionic Charges
Main group elements tend to lose or gain electrons in order to equal the electron
count of the nearest noble gas
Transition Metals Are Able
To Form Multiple Cations:
+1, +2, +3 and/or +4