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Chapter 4
Atomic Structure
Ms. Riggins
Lawndale High School
Section 4.1 - Defining the Atom
The discussion of atoms began as a
philosophical concept in ancient Greece
and India over 2000 years ago
The discussion entered the scientific
community in the early 1800s
Democritus’ Philosophy
• The Greek philosopher Democritus was the
first to suggest the existence of atoms (400BC)
•He theorized that everything
is composed of “atoms”
•He said that atoms were
indivisible, always in motion, and
had a lot of empty space
between them
Democritus’ Philosophy
He also said that there are an infinite
number and kind of atoms with
different shapes and sizes
He believed that iron atoms were solid
and strong with hooks
He believed that water atoms were
smooth and slippery
He believed that salt atoms were sharp
and pointed because of their taste
Democritus Connecting Atoms
He believed that air atoms were light
and whirly
He also believed that atoms had hooks
to hold them together
About 2000 year later, John
Dalton used experiments to
transform Democritus’ ideas on
atoms into a scientific theory
Defining the Atom
We now know that ALL matter is
composed of particles called atoms (can’t
see it, but it’s there)
• Atoms – the smallest particle of an
element that retains it identity in a
chemical reaction
John Dalton
In 1766, Dalton ran a series of experiments
with many different types of elements to
formulate his atomic theory
Dalton’s Atomic Theory
(1.) All elements are composed of tiny indivisible
particles called atoms. (from Democritus)
(2.) Atoms of the same element are identical. The
atoms of any one element are different from those
of any other element.
(3.) Atoms of different elements can physically mix
together or can chemically combine to form compounds.
Dalton’s Atomic Theory
(4.) Chemical reactions occur when atoms are
separated, joined, or rearranged. Atoms of one
element, however, are never changed into atoms of
another element as a result of a chemical reaction.
Section 4.2 –
The Structure of the Atom
• There are three kinds of subatomic particles in an
atom: electrons, protons, and neutrons
In 1897, J. J. Thomson discovered the electron
(negatively charged subatomic particles) by using a
cathode ray and came up with the Plum Pudding Model
Sir Joseph John Thomson
Until the late 1900s, scientists believed
that atoms were indivisible
J.J. Thomson proved them wrong when
he discovered the electron
He also discovered the concept of
isotopes (we’ll talk about this in the
next section)
Plum Pudding Model
Thomson’s plum pudding model
stated that electrons were
embedded in a positive charge
(like fruit in a Jello cup)
Concept of the Proton
Atoms were known to be electrically
neutral, which meant that there had to
be some positively charged matter to
balance the negative charges
Millikan’s Oil Drop Experiment
In 1909, Robert Millikan did an experiment
to find the charge and mass of an electron by
comparing the gravity force versus the
buoyant force on a tiny drop of oil suspended
between two metal electrodes
In 1916, he reported that the mass of an
electron is 1/1840 the mass of a hydrogen
Ernest Rutherford (student of Thomson)
In 1911, Rutherford proposed that atoms
had a positive charge concentrated in the
Rutherford’s model
became known as the
planetary model of atoms
In 1917, he was accredited
for the first person to split
the atom
Rutherford’s Gold-Foil Experiment
Ernest Rutherford’s experiment disproved
the plum pudding model of the atom and
suggested that there was a positively
charged nucleus (central core of an atom)
Conclusion of Rutherford’s Experiment
• Atoms are mostly empty space, thus explaining
the lack of deflection of most of the alpha
• All the positive charge and almost all the mass of
an atom are concentrated in a small region (nucleus)
• Nucleus – tiny central core of an atom composed
of protons and neutrons
• Electrons are distributed around the nucleus and
occupy almost all the volume of the atom (marble
and football stadium)
Structure Of An Atom
So by this point, we know that protons and
neutrons are located in the nucleus and
electrons are around the outside of the nucleus
Properties of Subatomic Particles
Summary of Atomic Theory
Democritus proposes idea of atom
Dalton develops Atomic Theory
Thomson uses cathode ray to
discover electron
Millikan measures the mass of an e-
Rutherford uses gold foil experiment
to discover nucleus
Section 4.3 –
Distinguishing Among Atoms
Elements are different because they
contain different numbers of protons.
Atomic Number - the number of protons in
the nucleus of an atom
*Remember since atoms are electrically
neutral, the number of protons equals the
number of electrons (until we get to Chp. 5)
Quick Practice…
How many protons and electrons are in each atom?
1. Fluorine
2. Calcium
3. Aluminum
4. Boron
5. Neon
6. Magnesium
Mass Number
Mass Number – the total number of protons PLUS
neutrons in an atom
Therefore…the number of neutrons in an atom is the
difference between the mass number and the atomic
# of Neutrons = Mass # – Atomic #
Atomic Mass Units
Atomic Mass Unit (amu) is 1/12 the
mass of a carbon-12 atom
Atomic mass is the weighted average of
the atom in naturally occurring samples
of the element
Shorthand Notation
(You need to know this notation)
Practice Shorthand Notation…
How many protons, neutrons, and electrons are in each atom?
Atomic #
Mass #
1. Beryllium (Be)
2. Neon (Ne)
3. Sodium (Na)
How many neutrons are in each atom?
1. Carbon-12
2. Fluorine-19
3. Sulfur-32
Isotopes – atoms that have the same number of
protons, but different numbers of neutrons (also
different mass numbers)
Write the following isotopes of oxygen:
1. Oxygen-16
2. Oxygen-17
3. Oxygen-18
Atomic Mass
Atomic Mass – weighted average mass of the atoms
in a naturally occurring sample of the element
In order to calculate the atomic mass of an element:
(1.) Multiply the mass of each isotope by its
natural abundance
(2.) Add the products together
Let’s practice…
Calculate the atomic mass of the following element, X
The isotope 10X has a mass of 10.012amu and a
relative abundance of 19.91%. The isotope 11X has a
mass of 11.009amu and a relative abundance of
ANSWER = 10.810amu
More Practice…
1. The element copper has naturally occurring
isotopes with mass numbers of 63 and 65. The
relative abundance and atomic masses are 69.2%
for mass = 62.93amu, and 30.8% for mass =
64.93amu. Calculate the average atomic mass of
2. Calculate the atomic mass of bromine. The two
isotopes of bromine have atomic masses and
relative abundance of 78.92amu (50.69%) and
80.92amu (49.31%).
Preview of the Periodic Table
Periodic Table – an arrangement of elements in
which the elements are separated into groups based
on a set of properties
Period – horizontal rows of the periodic table
(there are 7)
Group/Family – vertical columns of the periodic table
• Elements within a group have similar
chemical and physical properties
Chapter 4 Assessment Page 122
#’s 34 – 55, 59, 61, 64, 65, 71, 78,
81, 85, 88