Download Answer = 1.81 x 10 24 molecules

Atoms: The Building Blocks of Matter
Chapter 3
Jessica Baird, Zhenhao Li, Brianna Mays, Joey Powell
Chapter 3 Section 1
The Atom: From Philosophical Idea
to Scientific Theory
Democritus vs. Aristotle
• Democritus called nature’s basic particle, the atom (Greek for
“indivisible”)
• Aristotle did not believe in atoms, but thought that all matter was
continuous (could keep being divided)
Foundations of Atomic Theory
• Almost all chemists by the late 1700s agreed that an element was a
substance that could not be broken down further chemically
• Chemists also agreed that elements could combine to form compounds that
have different physical and chemical properties than those of the elements
used to form them
 Ex. NaCl has different physical and chemical properties than chlorine
(Cl) and Sodium (Na)
• There was controversy over whether elements always combined in the same
ratios when forming a particular compound
Law of Conservation of Mass
• Mass is neither created or destroyed during normal chemical
reactions or physical changes
Law of Definite Proportions
• A chemical compound contains the same elements in exactly the same
proportions by mass regardless of the size of sample or source
Law of Multiple Proportions
• If two or more different compounds are composed of the same
two elements, 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
Dalton's Atomic Theory
• All matter is composed of atoms
• Atoms of a given element are identical in size, mass, and
other properties
• Atoms cannot be subdivided, created, or destroyed
• Atoms of different elements combine in simple whole –
number ratios to form chemical compounds
• In chemical reactions, atoms are combined, separated, or
rearranged
Modern Atomic Theory
• Some parts of Dalton's theory was incorrect
 Atoms can be divided into smaller particles
 Atoms of the same element can have different
masses (isotopes)
• All matter is composed of atoms
• Atoms of any one element differ in properties from
atoms of another element
Chapter 3 Section 2
The Structure of the Atom
Atoms
• The smallest particle of an element that retains the
chemical properties of that element·
• An atom consists of two regions the nucleus and the
electron cloud·
• The nucleus has at least one positively charged proton
and one neutrally charged neutron·
• The electron cloud surrounds the nucleus and has
negatively charged electrons
• Protons, neutrons and electrons are called subatomic
particles
Discovery of the Electron
• In the late 1800s many experiments were conducted that
used an electric current that passed through various
gases at low pressure
• These experiments, in glass tubes called cathode-ray
tubes, would lead to the discoveries of the subatomic
particles
Cathode Rays and Electrons
• When a current was passed through a cathode-ray tube
the opposite side of the cathode glowed, this was caused
by a stream of particles called cathode rays
• Other experiments showed that when an object was
placed between the cathode and the other end of the tube
a shadow was cast and that when a paddle wheel was
placed inside the tube it moved
• These experiments and others proved that a cathode ray
had sufficient mass and was negatively charged
• JJ Thomson experimented further with the cathode rays
and renamed them electrons
Charge and Mass of the Electron
• Robert A. Millikan found that the mass of the electrons is
about one-thousandth the mass of an atom
• These experiments showed that two other inferences
could be made about atoms:
1. Because electrons are negative and atoms are
neutral, there must be something with a positive charge
2. Atoms must contain other particles that account for
most of their mass
Discovery of the Atomic Nucleus
• Ernest Rutherford, Hans Geiger and Ernest Marsden
bombarded a thin piece of gold foil with positively charged
particles
• They expected the particles to go through but some of
them deflected back
• Later they found that this was because atoms have a
very small, very dense area with
a
positive charge called the
nucleus
Composition of the Atomic Nucleus
• The nucleus contains protons and neutrons and there is
an equal number of protons and electrons in an atom
• Most of the mass is made up of neutrons and protons
• The number of protons in an atom determine its identity
Forces in the Nucleus
•
When two protons are really close to each other they
attract
• This is the same for two neutrons and a proton and a
neutron
• These forces are called nuclear forces
The Sizes of Atoms
• The radius of an atom is the distance from the center of
the nucleus to the outer part of the electron cloud
• Atomic radii are about 40 to 270 picometers while the
radii of the nucleus is about 0.001 picometer.
Chapter 3 Section 3
Counting Atoms
Number of Atoms
• The atomic number of an element is the number of
protons in the nucleus of each atom that element
• The atomic number is found on the periodic table on the
top, above the name of the element
• Mass number is the total number of protons and neutrons
in the nucleus of an isotope, it is found under the name of
the element
Isotopes
• Isotopes are atoms of the same element that have
different masses
• Mass Number-Atomic Number= Neutrons
• Nuclide- General term for any isotope of any element
Atomic Mass
• The Atomic Mass Unit (AMU) was based of off the
Carbon-12 Isotope, which was 12 amu
• 1 amu is equal to 1/12 the mass of an oxygen-12 atom
Average Atomic Mass
• Average atomic mass is the weighted average of the
atomic masses of the naturally occurring isotopes of an
element
• Average atomic mass is calculated by multiplying the
mass of the isotope by its relative abundance in decimal
form
Moles and Atoms
• A mole is the amount of a substance equal to the number
of particles in 12 grams Carbon-12
• Avogadro's Number:
 6.02 x 1023
• Avogadro's number is named after Amedeo Avogadro, a
chemist
• The number is used as a conversion between a number of
things and moles
• The mass of one mole of a pure substance is called the
molar mass
Conversions!!!!!
Grams
Use: Molar
Mass
Moles
Use:
Avogadro's #
# of
Atoms
Problems
1) How many molecules of carbon dioxide are found in 2.50
moles of carbon dioxide?
2) How many moles of O2 are represented by 7.45 x
1024 molecules of O2?
3) What would be the mass of 3.75 x 1021 atoms of iron?
Answers
1) 2.50 mol x 6.02 x 1023 molecules/mol
Answer = 1.51 x 1024 molecules
2) 7.45 x 1024 molecules/6.02 x 1023 molecules/mol
Answer = 12.4 moles
3) 3.75 x 1021 molecules/6.02 x 1023 molecules/mol =
0.00623 moles
0.00623 mol x 55.8 g/mol
Answer = 0.348 g
More Problems
1) How many molecules of water would be found in 54.0g of
water?
2) A certain laboratory procedure requires the use of .100
moles of magnesium. How many grams of magnesium would
you mass out on the balance?
More Answers
1) Molar mass of H2O = 18 g
54.0 g/ 18.0 g/mol=3 mol
(3.00 mol)(6.02x1023 molecules/mol)
Answer = 1.81 x 1024 molecules
2) 0.100 mol x 24.3 g/mol
Answer = 2.43 g of magnesium