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Atoms: Building
Blocks of Matter
Physical Science
Chapter 4
An Atomic Model of
Matter
• Scientists have tried to determine the
composition of matter for 2400 years using
various methods
o Indirect evidence
• Evidence gathered without actually
seeing or touching the object
o Model
• A model is a collection of evidence
and ideas that explains the facts
The Greek Model
• Democritus devised a model with the
following ideas:
o Matter could not forever be divided into
smaller pieces.
o The smallest piece possible was an atom
o Atoms were all made of the same
material but in different sizes and shapes
o Atoms are infinite in number, constantly
moving and joining together
Dalton’s Model
• John Dalton, an English
chemist, conducted many experiments and
came up with the following theory:
o All elements are composed of atoms.
o Atoms are indivisible & indestructible
o Atoms of the same element are alike
o Atoms of different elements are different
o Compounds are formed by the joining of
elemental atoms
Thomson’s Model
• J.J. Thomson conducted experiments
that led him to believe the atom was
made up of even smaller particles.
• Thomson passed electric currents
through gas.
• The gas gave off negatively charged particles.
• Thomson’s Model
o The atom is made up of pudding like positively
charged material throughout which negatively
charged electrons are scattered like plums in a
pudding.
Rutherford’s Model
• Ernest Rutherford experimented with shooting
positively charged rays through gold foil.
• Some of the positive charges bounced off the
gold foil rather than passing through it.
• He developed a new theory on the structure of
the atom.
• Rutherford’s theory states:
o The atom has a
small, dense
positively charged
center.
o Negative electrons were
scattered around the
nucleus on the edge of the
atom.
The Bohr Model
• Niels Bohr, a Danish
scientist, proposed that
negatively charged
electrons were held
by their attraction to the positive nucleus.
• Bohr’s Model stated:
o Electrons moved in definite orbits around
the nucleus.
o These orbits are at certain distances from
the nucleus depending upon the energy
of the electrons.
The Wave Model
• Today’s atomic model is based
on wave mechanics. A full
timeline can be found here.
• The wave model states:
o Electrons do not travel around the nucleus in
a specific path, but can be found in an area
called the electron cloud.
o An atom has a small positively charged
nucleus surrounded by a large region in
which enough electrons are located to
make the atom neutral.
Structure of an Atom
• An atom is made up of three main
subatomic particles: protons, neutrons,
and electrons.
• Protons: positively charged particles in
the nucleus
• Neutrons: neutrally charged particles in
the nucleus
• Electrons: negatively charged particles
orbiting the nucleus
Protons
• Protons are small positively charged
particles that exist in the nucleus of the
atom.
• All protons are identical in structure
regardless of the element in which
they are found.
• The mass of the proton is one amu
(atomic mass unit)
Atomic Number
• The difference between elements is the
number of protons they have in their
nucleus.
• The number of protons in the nucleus is
called the atomic number.
• The number of protons in an element
always stays the same, however the
number of neutrons can vary.
• An element with a different number of
neutrons (but same proton #) is called an
isotope.
Neutrons
• Neutrons are the neutrally charged
particle that exist in the nucleus of the
atom.
• All neutrons are identical.
• Neutrons also have a mass of 1 amu.
Lets watch the good video on atomic number, mass
and mass number!!
Mass Number and Atomic
Mass
• All atoms have a mass number.
• The mass number is equal to the number of
protons + the number of neutrons.
• Any sample of an element that occurs in
nature contains a mixtures of isotopes.
• Therefore, the atomic mass is an average of
the atomic masses of all the existing isotopes
of a particular element.
Electrons
• Electrons are the negatively charged
particles orbiting the nucleus in the electron
cloud.
• Electrons have a mass of 1/2000 of an amu
and are not used to calculate atomic mass.
• An uncharged atom always has an equal
number of protons and electrons, so that the
positive and negative charges balance.
Electron Cloud
• The electron cloud is the area around the
nucleus in which the electrons can be
found.
• Electrons whirl around the nucleus billions of
time in a second, however their movement
is not random.
• Electrons are locked into an orbit a specific
distance from the nucleus depending on
how much energy they have.
• The higher the energy the farther from the
nucleus.
Energy Levels
• Distance an electron is
from the nucleus is called
it’s energy level.
• Each energy level can
hold a specific number of
electrons.
• The arrangement of the electrons in the
energy levels accounts for how an atom
bonds with other atoms.
Arranging the Elements
• Russian chemist Dmitri Mendeleev was the
first scientist to arrange the elements in a
logical manner.
• He classified the elements according to
chemical and physical properties.
• He then put the elements in order of
increasing atomic mass.
• This led to a table of elements that
increased by atomic mass and were
arranged in columns of similar
characteristics.
Predictions
• Mendeleev found that there were holes in his
periodic table of elements.
• Mendeleev predicted that elements would be
discovered that would fit into the holes in his
table of elements.
• Within his lifetime, three of the elements that
Mendeleev predicted were discovered.
The Modern Periodic Table
• Mendeleev’s table had some problems.
• Some elements did not fit the
characteristics of the columns where
they were located.
• Mendeleev’s table was refined by Henry
Moseley after the discovery of atomic
particles.
• Moseley arranged the elements according
to atomic number (# of protons) instead of
atomic mass (protons + neutrons).
Periodic Law
• Moseley’s table was based on periodic law, which
states, “the physical and chemical properties of
the elements are a function of their atomic
number.”
Periodic Table Design
• The periodic table is a
classification system that organizes
the elements in a logical, usable
and meaningful way.
• The periodic table is designed so
that it is easy to predict an
element’s physical and chemical
properties.
Columns of the Periodic
Table
• The columns in the periodic table
represent groups or families of
elements.
• Families of elements have similar
chemical and physical properties.
• BECAUSE each atom of these
elements have the same number of
electrons in the outer shell.
Rows in the Periodic Table
• Each horizontal row in the periodic table
represents a period.
• Elements in a period do not have similar
properties.
• Elements in a period always increase by one
proton or one electron from left to right.
• There are seven periods of elements.
• The rare earth elements are pulled out of
their rows, however and are listed below the
table.
Element Key
• Important information about the
elements is given in each square
of the periodic table including:
o Atomic number
o Chemical Symbol
o Name
o Atomic Mass
Forces within the Atom
• The atom is held together by four
forces.
• The four forces account for the
behavior of the subatomic particles.
• The four forces are:
o Electromagnetic Force
o Strong Force
o Weak Force
o Gravity
Electromagnetic Force
• The electromagnetic force can either
attract or repel particles.
• Particles with the same charge are repelled.
• Particles with different charges are
attracted.
• Electrons are kept orbiting the nucleus
because they are attracted to the positively
charged protons.
• Protons in the nucleus, however, are
repelled by each other.
Strong Force
• The strong force glues protons
together to form the nucleus.
• The strong force only works when
protons are close together.
• The strong force is the strongest of the
four forces.
Weak Force
• The weak force is responsible for the
process of radioactive decay.
• The weak force allows a neutron in the
nucleus to change into 1 proton and 1
electron.
• Radioactive decay is what powers the
sun.
Gravity
• Gravity is the force of attraction
between all objects that have mass.
• The effect of gravity on the nucleus is
not fully understood
and is the weakest
of the four forces.