Atoms: Building Blocks of Matter - Grass Range School · •Democritus devised a model with the...
Transcript of Atoms: Building Blocks of Matter - Grass Range School · •Democritus devised a model with the...
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Atoms: Building Blocks of Matter
Physical Science
Chapter 4
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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
oModel
• A model is a representation of evidence & ideas that explains facts or a concept
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The Greek Model • Democritus devised a model with the
following ideas:
oMatter could not forever be divided into smaller pieces.
o The smallest piece possible was an atom
oAtoms were all made of the same material but different sizes and shapes
oAtoms are infinite in number, constantly moving and joining together
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Dalton’s Model
• John Dalton, an English chemist, conducted many experiments and came up with the following theory:
oAll elements are composed of atoms.
oAtoms are indivisible & indestructible
oAtoms of the same element are alike
oAtoms of different elements are different
oCmpnds are formed by the joining of elemental atoms
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Thomson’s Model • J.J. Thomson conducted experiments (passing
electric current thru gas, causing negatively
charged particles to escape).
• …..led him to believe atoms were made of
smaller particles.
• Thomson’s Model:
o The atom is made up of pudding like
positively charged material in which
negatively charged electrons are
scattered like plums in a pudding (or
chips in a cookie).
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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 stated: o The atom has a
small, dense positively charged center.
o Negative electrons scattered around nucleus on edges of atom.
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The Bohr Model • Niels Bohr, a Danish
scientist, proposed & tested that negatively
charged e- were held by attraction to the + nucleus.
• Bohr’s Model stated:
oElectrons moved in orbits around the nucleus.
o These orbits were at certain distances from the nucleus depending upon the energy of the electrons.
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The Wave Model • Today’s atomic model is
based on wave mechanics.
• 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.
oAn atom has a small positively charged
nucleus surrounded by a large region in
which enough electrons are located to
make the atom neutral.
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Structure of an Atom An atom is made up of 3 subatomic particles: protons, neutrons, & electrons which have
consistent structure in all atoms. • Protons: positively charged particles in the nucleus
• Neutrons: neutrally charged particles in the nucleus
• Electrons: negatively charged particles surrounding the nucleus
• Neutral atoms have equal #s of protons & electrons, so that positive and negative charges balance.
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Subatomic particle masses • The mass of both protons & neutrons is
one amu (atomic mass unit)
• Electrons have almost no mass.
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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.
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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.
• An element with a different number of
neutrons (but same proton #) is called an
isotope.
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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.
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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.
• Levels farther from the nucleus have
more energy
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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 & then put them 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.
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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.
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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).
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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.”
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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.
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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 (valence).
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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.
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Element Key • Important information about the
elements is given in each square
of the periodic table including: oAtomic number
oChemical Symbol
oName
oAtomic Mass
• Practice using the Periodic Table
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Forces within the Atom not in ch 4, but impt
• The atom is held together by four forces.
• The four forces account for the behavior of the subatomic particles.
• The four forces are:
oElectromagnetic Force
oStrong Force
oWeak Force
oGravity
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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 cause they
are attracted to positively charged protons.
• Protons in nucleus, however, are repelled by
each other.
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Strong Force • The strong force glues protons (and
neutrons) 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.
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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.
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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.