Modern Chemistry
Chapter 3
Atoms: The Building Blocks of Matter
Chapter 3 Sec 1
The Atom:
From Philosophical Idea to Scientific Theory
• In Ancient Greece (400B.C.) Democritus was the first on record
ever to say matter was composed of atoms
• Atom is based on the Greek for indivisible
More Ch 3-1
• Aristotle who was next generation form Greece did not
believe in atoms…….
• It was not until 18th Century that experimentation began to
gather evidence to prove the existence of atoms
Foundations of Atomic Theory
• By the late 1700’s the definition of an element became widely
accepted
• An Element is something that cannot be broken down by
ordinary chemical means.
• It was also commonly believed that elements combined to
form compounds.
More Fundamentals of Atomic
Theory
• Chemical reactions were defined as the transformation of a
substance or substances into one or more new substances.
• By the 1790’s the analysis of matter became more quantitative
because reliable balances were becoming available.
Law of Conservation of Mass
• This law states that mass is neither created or destroyed
during ordinary chemical reactions or physical changed.
• Back in the day people had funny ideas…..Spontaneous
generation, magic, potions, alchemy
Law of Definite Proportion
• This law states that a chemical compound contains the same
elements in exactly the same proportion by mass regardless of
the size of the sample or the source of the compound.
• Example: water one drop vs gallons
• Example: water lake, pond, ice
Law of Multiple Proportions
• This law states that 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
• Example: hydrogen peroxide and water
• H2O2
H2O
Dalton’s Atomic Theory
• 1. All matter is composed of extremely small particles
called atoms.
• 2. Atoms of a given element are identical in size, mass, and
other properties.
• 3. Atoms cannot be subdivided, created or destroyed.
• 4. Atoms of different elements combine in simple wholenumber ratios to form compounds
• 5. In chemical reactions, atoms are combined, separated,
or rearranged.
Modern Atomic Theory
• Much of John Dalton’s Theory is still true today…..
• All matter is composed of atoms
• Atoms of any one element differ in properties from atoms of
another atom.
• Some of Dalton’s theory has been discarded:
isotopes, nuclear reactions, etc…..
Chapter 3 Sec 2
Structure of the Atom
• Atom is defined as the smallest particle of an
element that retains the properties of that
element.
• Today it is known that the atom has
two basic
parts: the nucleus and the electron cloud. The
three basic subatomic particles are electrons,
protons and neutrons
Discovery of the Electron
• Scientists of the late 1800’s
were trying to discover
the relationship between matter and electricity
by passing electric current through various
gasses at low pressures.
• These devices were called cathode ray tubes.
• It was noticed that when current passed through the
tube the cathode glowed. They hypothesized that this
was due to a stream of particles.
The Electron
Two main observations:
• 1. Cathode rays were deflected by a magnetic
field in the same manner as a wire carrying
electric current, which was known to have an
negative charge.
• 2. The rays were deflected away from a
negatively charged object.
The Electron
• J.J. Thomson 1897 said the ratio of charge to mass
was always the same regardless of the type of
metal used or type of gas used.
• He named this particle the “ELECTRON”
• In 1909 American physicist Robert Millikan
measured the charge of an electron.
• Today we know the mass 9.109*10-31 kg
Discovery of the Atomic Nucleus
• J.J. Thomson believed electrons were imbedded
throughout the atom ”plum pudding” model of the
atom.
• In 1911, Ernest Rutherford, Hans Geiger, Ernest
Marsden conducted the Gold Foil Experiment.
Bombard thin gold foil with alpha particles
• They are given credit for discovering the nucleus of the
atom. They concluded there was densely packed
matter with a positive charge present in the gold foil =
the nucleus
• See table on top of page 76.
The Nucleus
• Analogy if the nucleus is a marble the atom would be
the size of a football field
• Electron negative charged equal but opposite of the
proton positive charge
• Neutrons are not charged but have a mass slightly
larger than the proton.
Composition of the Atomic Nucleus
• Nuclear Forces are short range proton-neutron,
proton-proton, neutron-neutron forces holding
nuclear particles together.
• The size of the atom is extremely small! Atomic radii
range from 40 to 270 picometers ( reminder that is 10-12
of a meter)
• Nuclei are extremely dense about 2*108 metric tons/cm3
Chapter 3 Sec 3
Counting Atoms
• Atomic number is equal to the number of protons in an atoms
nucleus
• Isotopes different forms of the same element with differing
masses due to differing neutrons
• Mass Number is the total number of protons and neutrons
added together
• The Mole Avogadro’s number = 6.02x10 23
• Molar Mass
More Ch 3 sec 3
• The MOLE Avogadro’s number of molecules.
• Molar Mass (always read Periodic Table amu’s to a hundredth
of a gram)
• Atoms and molecules are soooooooo small that the scale we
use them on is huge!
Molar Mass
• With a Periodic Table and calculator by your side! 
• Molar mass of an element is just read off the Periodic table
and rounded correctly!
• Molar mass for a compound is added using masses from the
periodic Table and the subscripts in the formula.
Example: Molar Mass
•
•
•
•
H2O water
Molar mass
Hydrogen (1.01 grams) 2
Oxygen
(16.00 grams) 1
• 2.02g + 16.00g= 18.02grams
ANSWER!
Example: Molar Mass
•
•
•
•
C6H12O6 glucose
Carbon (12.01g)6= 72.06g
Hydrogen(1.01g)12= 12.12g
Oxygen (16.00g)6= 96.00g
• And the total is!!!!!!! 180.18
glucose
grams is the molar mass of
Molar Mass
•
•
•
•
Aluminum nitrate Al(NO3)3 Molar mass
Aluminum (26.98g) 1
= 26.98
Nitrogen (14.01g) 3
= 42.03
Oxygen (16.00g) 9
=144.00
• And the total is !!!!!
213.01 grams
MOLE Dimensional Analysis
• Amadeo Avogadro said one mole is equal to 6.02x1023
particles…….
• 6.02 EE 23
or 6.02 exp 23 don’t type x10……….. If
you have any questions about your calculator please see me!
• 2 mole is 12.04 x10 23 particles
• .5 mole is 3.01 x10 23 particles
• Mole calculations are FUN!