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Chapter 3
Atoms: The Building Blocks
of Matter
1
Section 3.1 and 3.2
The Atom: From Philosophical Idea to Scientific
Theory and The Structure of the Atom
Objectives:
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Summarize the 5 essential points of Dalton’s atomic
theory.
Discuss scientists and experiments that led to discovery
of subatomic particles, their charges and their masses.
Explain the relationship between Dalton’s atomic theory
and the law of conservation of mass, the law of definite
proportions, and the law of multiple proportions.
Identify the structure of the atom.
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Atoms
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An atom of Hydrogen is so small that 1 g of H
contains 6.022 x 1023 (6.0221367) six hundred
thousand billion-billion atoms
This is the number of popcorn kernels
needed to cover U.S. 9 miles deep
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Atoms cannot be observed but existence can be
inferred by experiments
First atomic beliefs date back to Greek
philosophers
Democritus (430 – 362 BC) approx. 2500 years ago
developed the model of an indivisible particle he
called an atom
Democritus
(460 - 370 B.C.)
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 Democritus had no way of proving
his idea and no evidence, so many
philosophers did not agree with
him
 Aristotle believed matter
was continuous and atoms
did not exist.
 Aristotle’s thinking
prevailed for the next
2000 years
 Late 1700 and early 1800
scientists brought back
atom concept
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One scientist that believed in atoms
was John Dalton.
Some of Dalton's symbols for the elements
with his estimates of molecular weight
John Dalton –
Early 1800’s
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Dalton’s atomic theory
summarized in 5 statements:
1.
2.
3.
4.
5.
All matter is composed of extremely small
particles called atoms.
Atoms of a given element are identical in size,
mass, and other properties; atoms of different
elements differ 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.
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Memorize and Understand the
statements made in Dalton’s
Atomic Theory.
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Full acceptance of the atom came in the early
1900’s
The first part of the atom to be discovered was the
electron
In 1897, English physicist J. J. Thomson
discovered electron
He used a cathode ray tube – a tube filled with
gas at a low pressure
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Observed a greenish beam flowing from negative to
positive
He placed charged metal plates on each side of the
beam; beam deflected away from the negative plate
Thompson found this occurred with any gas in the tube
He deduced that atoms contained negative particles –
we know them as electrons.
Came up with the “Plum pudding”
model of atom
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1909 American physicist Robert Millikan confirmed the
existence of the electron and found the mass of an
electron was very small compared to its charge
He did this with his “oil drop-can” experiment.
Robert Millikan and Albert Einstein,
Caltech, 1931
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Scientists continued to develop their new knowledge
of electrons and came up with other inferences:
--An atom is neutral so there must be an equal positive charge to
balance negative charge
--Mass of electron is small compared to overall mass of an atom so an
atom must contain something else with large mass
--1911 British (New Zealand) physicist Ernest Rutherford was able to
prove this area of large mass. He determined that an atom is
mostly empty space with a nucleus having a positive charge
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Diagram of Rutherford’s
gold foil experiment
--Rutherford bombarded gold foil with alpha particles (+ charge); a small
amount of particles were deflected back, a few were
deflected to the side. He knew only + charge would cause this so
the atom had SOMETHING with a positive charge.
-- MOST went straight through meaning “lots of empty space”
--He had proven a dense, positively-charged nucleus.
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-- Rutherford speculated the existence of another type of particle
contributing to the density of the nucleus.
--Around 1932 James Chadwick identified this particle, the neutron
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Assignment

Finish your research worksheet to create a timeline of
these events. Turn this in next class.
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Assignment – On Canvas
Homework:
Choose one of these scientists: Dalton, Thomson, Rutherford,
Millikan, or Chadwick and write a narrative essay of at least 4
paragraphs describing “your” experiment and discoveries.
Include an introductory paragraph that provides information about
the time period in which you live and known information about atomic
structure.
Describe your experiment and tell what you discovered and how you
"figured" it out. (I suggest two paragraphs).
Conclude the essay by discussing how “your” discovery will further
advance the understanding of atomic structure.
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Modern Atomic Theory
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Today we know not all of Dalton’s ideas are true. Atoms of
same element can have different masses (isotopes) and
atoms can be divided into smaller parts.
atom = smallest particle of an element that retains the
chemical properties of that element
nucleus= small area in center of atom
Electron cloud = area outside of nucleus
proton = particle with positive charge in nucleus
neutron = particle with neutral charge in nucleus
electron = particle with negative charge in cloud
surrounding nucleus
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Today we know:

Approx. 117 different kinds of elements have been identified –
each has a different number of protons = atomic number

Nucleus is very dense contains neutrons and protons

Protons and neutrons have approx. the same mass

Why do the protons not repel each other in such a tightly packed
space? nuclear forces= short range forces that hold protons to
protons, protons to neutrons, and neutrons to neutrons

Number of protons = number of electrons so an atom is
neutral
Number of neutrons does not always equal protons!!!!

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19
Diagram showing locations of p+, no, and e- in an atom.
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Evidence for Modern Theory:

law of conservation of mass – mass is neither
created nor destroyed during ordinary chemical
reactions or physical changes

mass before = mass after
law of definite proportions – chemical compounds
contain the same elements in exactly the same
mass proportions


CO (carbon dioxide) has a ratio of 12 g of C to 16 g of O regardless if
the sample weighs 28 g or 56 g.
law of multiple proportions – if 2 or more different
compounds are composed of the same 2 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


in other words: different elements can combine to form more than 1
compound – but they do so in whole number ratios.
CO (carbon monoxide) = 1 Carbon : 1 Oxygen
CO2(carbon dioxide) = 1 Carbon : 2 Oxygen
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Section 3.3
Counting Atomic Particles
Objectives:
 Explain isotopes.
 Define atomic number and mass number, and
describe how they apply to isotopes.
 Given the identity of a nuclide, determine its
number of protons, neutrons, and electrons.
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Atomic number for each element = number of protons
Proton number is always the same for an element
Protons = electrons
Atomic Mass Number of neutrons may vary and must be calculated
using Atomic Mass Number
Atomic Mass Number = protons + neutrons
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Masses of atoms in grams are very small
Ex. Atom of oxygen-16 has mass of 2.657 x 10 -23 g
Standard was arbitrarily assigned as carbon-12
Atomic mass unit = 1 amu = 1/12 the mass of a
carbon-12 atom
Isotopes have different masses (this only slightly
alters their behavior)
Subatomic particles also have mass; electron =
0.0005486 amu; proton = 1.007267 amu; neutron =
1.008665 amu
Proton and neutron are close to one, but not equal
to one
Electron amu is so small it is not counted
Mass number = no. of protons + neutrons
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Example: All atoms of the element K
 Have
the same number of protons
 Have the same number of electrons (= to p+)
 May have different numbers of neutrons
Mass number = atomic mass rounded = 40
39.990
K
p+ = ?
e-
= ?
no = ?
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Atomic number = # of protons
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
Isotope = atoms of an element that have
different numbers of neutrons – so they
have different atomic masses
Nuclide – general term for isotopes of an
element.
Example H has 3 nuclides:

Protium = 1proton 0 neutrons 1 electron
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Deuterium = 1 proton 1 neutron 1 electron
Tritium = 1 proton 2 neutrons 1 electron
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2 Methods of Designating Isotopes
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Hyphen notation:
Hydrogen – 3
or
H-3 with 3 being the mass number
Nuclear symbol
H
3
1
or
3
1
H
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Nuclear Symbol
superscript = mass number
subscript = atomic number
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How would you write the nuclear symbols for
the 3 nuclides of Hydrogen? Hyphen method?
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How many protons, neutrons, and
electrons are in chlorine-37?
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How many protons, neutrons, and
electrons are in bromine-80?
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How many protons, neutrons, and
240
electrons are in U 92 ?
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Write the hyphen notation for the
element that contains 15 electrons
and 15 neutrons.
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
Most elements occur naturally in mixtures of their
isotopes
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Average atomic mass = weighted average of the atomic
masses of the naturally occurring isotopes of an element

Hydrogen has 2 naturally occurring isotopes, the other is
radioactive.

H – 1 occurs 99.985% and H – 2 occurs 0.015%

Ave. atomic mass = sum of % occurance x atomic masses
0.99985 x 1.007825 amu =
1.007673826
0.00015 x 2.014102 amu = + 0.0003021153
1.007975941

Atomic mass should be rounded to 2 places past the
decimal before it is used in calculations
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Section 3.3
Counting Atomic Particles
Objectives:
 Define mole in terms of Avogadro’s number,
and define molar mass.
 Solve problems involving mass in grams,
amount in moles, and number of atoms in an
element.
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Mole (mol)= SI unit for amount of substance
It is a counting unit like “dozen”
1 mole = 6.022 x 10 23
With elements, 1 mole = atomic mass in grams
Ex. Oxygen atomic mass = 16.00 so 1 mol of
oxygen has a mass of 16.00 g.
Molar mass = mass of one mole of a pure
substance
Molar mass is stated as ratio between grams/mol
Ex. Molar mass of Hg = 200.59 g
1 mol
Molar mass of Li = 6.94 g
1 mol
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What is the mass in grams of
3.50 mol of the element Cu?
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A chemist produced 11.9 g of Al.
How many moles of Al were
produced?
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Solve these problems:
1.
2.
3.
4.
5.
How many moles are 76.3 g Ca?
How many grams are in 3.14 mol of
selenium?
How many grams are in 4.25 mol lead?
How many moles are 125.3 g Ag?
How many grams are in 116.8 mol of N?
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
1 mole = 6.022 x 1023 atoms, molecules,
or ions
1 mol
6.022 x 1023 atoms(molecules or ions)

6.022 x 1023 called Avogadro’s number
40
How many moles of Ag are 3.01
x 1023 atoms of Ag?
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How many atoms are in 8.5 mol
of K?
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Solve these problems
1) How many atoms are in 2.0 mol Na?
2)How many mol Ca are 12.044 x 1023 atoms?
3) 32.615 x 1023 atoms of P equals
of P.
mol
4) A student measures 4.51 mol of Fluorine.
How many atoms did she measure?
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Reminder:
Mole Conversion Factors
1 mole = atomic mass in grams for elements
ex: 40.08 g Ca
1 mol Ca
1 mol Ca
40.08 g Ca
1 mole = 6.022 x 1023 atoms (molecules, ions)
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However:
You cannot make a straight conversion
from atoms to grams without using molar
mass and Avogadro’s number!
 You must first convert to moles!

grams
Use Molar Mass
moles
atoms
Use Avogadro’s Number
45
What is the mass in grams of
1.20 x 1023 atoms of Cu?
Hint: you must change grams to moles and then changes moles to atoms!
46
If a beaker contains 4.38 x 1025
atoms of Li, how many grams are
present?
47
If a beaker contains 3.750 g of Zn,
how many atoms of Zn are
present?
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Review

Create a Venn diagram that compares and
contrasts Dalton’s atomic theory and the Modern
Atomic Theory.
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1.
How many protons, electrons, and neutrons
are in an atom of bromine-80?
2.
Write the nuclear symbol for carbon-13.
3.
Write the hyphen notation for the element that
contains 15 electrons and 15 neutrons.
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4. What is the mass in grams of 2.25 mol of the
element Fe?
5. What is the mass in grams of 0.0135 mol of the
element Na?
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6. How many moles of Ca are in 5.00 g ?
7. How many moles of Au are in
3.60 x 10-10 g?
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8. How many moles of Pb are in 1.50 x 1012
atoms?
9. How many moles of Sn are in 2500 atoms?
10.How many atoms of Al are in 2.75 mol of Al?
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Chapter
3
Test
Today!!!
John Dalton’s Atomic Theory
Be sure you have reviewed:
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•
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Experiments AND Discoveries of Thomson, Millikan,Rutherford and
Chadwick
3 laws: Multiple Proportions, Definition Composition, Conservation
of Mass
Differences in Average Atomic Mass, Atomic Number, Mass
Number
Protons, Neutrons and Electrons in Atoms of Elements
Isotopes and how to determine p+, e-, and no
How to work problems involving:
•
•
•
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Grams to Moles
Moles to Grams
Moles to Atoms
Atoms to Moles
Atoms to Grams (2 step conversion)
Grams to Atoms(2 step conversion)
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