Chapter 4 Atomic Structure

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Chapter 4Atomic Structure
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The Atom
You cannot see the tiny fundamental particles
that make up matter. Yet, all matter is composed
of such particles, called atoms Atom the
smallest particles of an element that retains its
identity in a chemical reaction Several early
philosophers and scientists could not observe
individual atoms, but still were able to propose
ideas on the structure of atoms.
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Democrituss Atomic Philosophy

• Greek philospher Democritus (460B.C 370 B.C.)
  was among the first to suggest the existence of
  atoms.
• Democritus believed that matter consisted of
  tiny, indivisible and indestructible.
• Democrituss ideas did not explain chemical
  behavior.
• Lacked experimental support, because his approach
  was not based on scientific method.

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Daltons Atomic Theory
The modern process of discovery regarding atoms
began with John Dalton, an English chemist and
school teacher. Dalton used experimental
methods and transformed Democrituss ideas on
atoms into scientific theory. Dalton studied
the ratios in which elements combine in chemical
reactions. Based on the results of his
experiments, Dalton formulated hypotheses and
theories to explain his observations.
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Daltons Atomic Theory

• According to Daltons atomic theory, and element
  is composed of only one kind of atom, and a
  compound is composed of particles that are
  chemical combinations of different kinds of
  atoms.
• All elements are composed of tiny indivisible
  particles called atoms
• Atoms of the same element are identical. The
  atoms of any one element are different from those
  of any other element.

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Daltons Atomic Theory

• Atoms of different elements can physically mix
  together or can chemically combine in simple
  whole-number ratios to form compounds.
• 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.

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The Atom
The radii of most atoms fall within the range of
5 x 10-11 m to 2 x 10-10m. (very
small) Individual atoms are visible with
instruments such as scanning tunneling
microscopes.
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End of section 4.1
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Subatomic Particles

• Most of Daltons atomic theory is accepted today.
  Except, we now know atoms to be divisible.
• Atoms can be broken down into smaller particles,
  called subatomic particles.
• There are 3 kinds of subatomic particles.
• electrons
• Protons
• neutrons

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Electrons
In 1897, English physicist J.J. Thomson
discovered the electron. Electrons negatively
charged subatomic particles. Dalton performed
experiments that involved passing electric
current through gases at low pressure.
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Cathode-Ray Tube
Sealed gases in glass tubes fitted at both end
with metal disks called electrodes. Electrodes
were connected to a source of electricity. One
electrode (anode) became positively charged. The
other electrode (cathode) became negatively
charged. Result was a glowing beam (cathode ray)
that traveled from the cathode to the anode.
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Cathode-Ray Tube
Positively charged metal plate attracts the
cathode ray, while a negatively charged plate
repels it. Thomson knew that opposite charges
attract and like charges repel. He hypothesized
that a cathode ray is a stream of tiny negatively
charged particles (electrons) moving at high
speed.
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Cathode-Ray Tube
To test his hypothesis, Thomson set up an
experiment to measure the ratio of the charge of
an electron to its mass. He found this ratio to
be constant. In addition, the charge-to-mass
ratio of electrons did not depend on the kind of
gas in the tube or the type of metal used for the
electrodes. He concluded that electrons must be
parts of the atoms of all elements.
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The Electron
An electron carries exactly one unit of negative
charge The electrons mass is 1/1840 the mass
of a hydrogen atom. How do negatively charged
plates affect the path of cathode rays? The
negatively charged plate repels the cathode ray.
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Protons and Neutrons

• After a hydrogen atom loses an electron, what is
  left?
• Atoms have no net electric charge, they are
  electrically neutral
• Electric charges are carried by particles of
  matter
• Electric charges always exist in whole-number
  multiples of a single basic unit.
• When a given number of negatively charged
  particles combines with an equal number of
  positively charged particles, and electrically
  neutral particle is formed.

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Protons and Neutrons

• After a hydrogen atom loses an electron, what is
  left?
• A particle with one unit of positive charge
  should remain when a typical hydrogen atom loses
  an electron.
• In 1886, Eugene Goldstein observed a cathode-ray
  tube and found rays traveling in the direction
  opposite to that of the cathode rays.
• He concluded they were positive particles.
• Protons positively charged subatomic particles.

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Protons and Neutrons
English physicist James Chadwick confirmed the
existence of another subatomic particle.
Neutron subatomic particles with no charge
but with a mass nearly equal to that of a proton.

Particle Symbol Relative Charge Relative Mass Actual mass (g)
electron e- 1- 1/1840 9.11 x 10-28
proton p 1 1 1.67 x 10-24
neutron n0 0 1 1.67 x 10-24
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Atomic Nucleus
J.J. Thomson though that electrons were evenly
distributed throughout an atom filled uniformly
with positively charged material. Electrons
stuck into a lump of positive charge. This
model of the atom was short-lived due to work of
Ernest Rutherford, a former student of Thomson.
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Rutherfords Gold-Foil Experiment
In 1911, Rutherford used alpha particles (Helium
atoms that have lost their two electrons and have
a double positive charge because of the two
remaining protons) to test the current theory of
atomic structure. The experiment used a narrow
beam of alpha particles directed at a very thin
sheet of fold foil. The alpha particles
should have passed easily through the gold with
only slight deflection.
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Rutherfords Gold-foil Experiment
However, the great majority of alpha particles
passed straight through the gold atoms, without
deflection. Also, a small fraction of the
alpha particles bounced off the gold foil at
very large angles.
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Rutherfords Gold-foil Experiment
Based on his experimental results, Rutherford
suggested a new theory of the atom. He proposed
that the atom is mostly empty space, thus
explaining the lack of deflections of most of the
alpha particles. He concluded that all the
positive charge and almost all the mass are
concentrated in a small region that has enough
positive charge to account for the great
deflection . Nucleus the tiny central core of
an atom and is composed of protons and neutrons.
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Rutherfords Atomic Model
Rutherford atomic model is know as the nuclear
atom In the nuclear atom, the protons and
neutrons are located in the nucleus. The
electrons are distributed around the nucleus and
occupy almost all the volume of the atom. The
nucleus is tiny compared with the atom as a
whole. Although an improvement over Thomsons
model of the atom, Rutherfords model turned out
to be incomplete and had to be modified (chapter
5)
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Questions
What are 3 types of subatomic particles? Proton,
neutron, electrons. How does the Rutherford
model describe the structure of atoms? A
positively charged nucleus surrounded by
electrons, which occupy most of the volume.
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Questions
Describe Thomsons and Millikans contributions
to atomic theory. Thomson Cathode ray
experiments which concluded that electrons must
be parts of the atoms of all elements. Millikan
determined the charge and mass of the
electron. What experimental evidence led
Rutherford to conclude that an atom is mostly
empty space? The great majority of the alpha
particles passed straight through the gold foil
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Questions
Compare Rutherfords expected outcome of the
gold-foil experiment with the actual outcome.
Expected all alpha particles to pass straight
through with little deflection. Found that most
passed straight through, but some particles were
deflected at large angles and some bounced back.
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End of Section 4.2
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Distinguishing Among Atoms
How are atoms of hydrogen different from atoms of
oxygen? Elements are different because they
contain different number of protons. Atomic
number of an element is the number of protons
in the nucleus of an atom of that element.
Example all hydrogen atoms have 1 proton and
the atomic number of hydrogen is 1. The atomic
number identifies an element.
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Distinguishing Among Atoms
Most of the mass of an atom is concentrated in
its nucleus and depends on the number of protons
and neutrons. Mass number the total number of
protons and neutrons in an atom Example Helium
atom contains 2 protons and two neutrons, so its
mass number is 4 If you know the atomic number
and mass number of an atom of any element, you
can determine the atoms composition.
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Distinguishing Among Atoms
Example Oxygen Atomic number is 8 number of
p e- (So oxygen has 8 electron s and 8
protons.) Mass number is 16 number of p plus
the number of n0. (So oxygen has 8 neutrons)
Number of neutron mass number atomic
number 197 Au 79
Mass number
Atomic number
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Isotopes
There are some elements that have different kinds
of atoms of the same element Example there are
three different kinds of Neon atoms Isotopes
are atoms that have the same number of protons,
but different numbers of neutrons. Because
isotopes of an element have different numbers of
neutrons, they also have different mass numbers.
Isotopes are chemically alike because they have
identical numbers of protons and electrons, which
are the subatomic particles responsible for
chemical behavior.
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Hydrogen Isotopes
0 neutrons Mass - 1
1 neutron Mass - 2
2 neutrons Mass - 3
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Chemical Symbols of Isotopes
Write the chemical symbols for three isotopes of
oxygen. Oxygen 16, oxygen 17, and oxygen 18.
Mass Number (
protons neutrons) 16 17
18 O O
O 8 8
8
Atomic number
( proton electrons)
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Atomic Mass
Actual masses of individual atoms are small and
impractical to work with. It is more useful to
compare the relative masses of atoms using a
reference isotope as a standard The carbon-12
atom was assigned a mass of exactly 12 atomic
mass units. Atomic mass unit (amu) one twelfth
of the mass of carbon-12 atom.
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Atomic Mass
In nature, most elements occur as a mixture of
two or more isotopes. Each isotope of an
element has a fixed mass and a natural percent
abundance. Example almost all naturally
occurring hydrogen (99.9985) is hydrogen-1.
The other two isotopes are present in trace
amounts. The atomic mass of hydrogen is 1.0079
amu, and Is very close to the mass of hydrogen-1
(1.0078 amu)
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Atomic Mass
The slight difference takes into account the
larger masses, but smaller amounts of the other
two isotopes of hydrogen. Atomic mass of an
element is a weighted average mass of the atoms
in a naturally occurring sample of the element.
The atomic mass of copper is 63.546 amu. Which
of coppers two isotopes is more abundant copper
-63 or copper-65? Atomic mass of 63.546 is
closer to 63 than 65, thus copper-63 must be more
abundant.
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Atomic Mass
Atomic mass multiply the mass of each isotope
by its natural abundance, expresses as a decimal,
and then add the products. Element X has two
natural isotopes. The isotope with a mass of
10.012 amu has a relative abundance of 19.91.
The isotope with a mass of 11.009 amu has a
relative abundance of 80.09. Calculate the
atomic mass of this element. (10.012 amu x
0.1991) (11.009 amu x 0.8009)
(1.993 amu) (8.817 amu) Atomic
mass 10.810
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Question
Copper 63 has a mass of 62.93 amu and 69.2
abundance. Copper-65 has a mass of 64.93 amu and
30.8 abundance. What is coppers average atomic
mass? (62.93 amu x 0.692) (64.93 amu x
0.308) (43.548 amu)
(19.998 amu) Atomic mass 63.55
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Periodic Table
Periodic Table an arrangement of elements in
which the elements are separated into groups
based on a set of repeating properties.
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Periodic Table
Each element is identified by its symbol place
in a square. The atomic number of the element
is shown centered above the symbol. Elements are
listed in order of increasing atomic number, from
left to right and from top to bottom. Period -
each horizontal row of the periodic table. Within
a given period, the properties of the elements
vary as you move across it from element to
element. Group each vertical column of the
periodic table. Elements within a group have
similar chemical and physical properties. Each
group is identified by a number and the letter A
or B.
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Periodic Table
What distinguishes the atoms from one element
from the atoms of another? The number of
protons What equation tells you how to
calculate the number of neutrons in an
atom? Mass number atomic number of
neutrons. How do the isotopes of a given element
differ from one another? Different mass number
and different numbers of neutrons.
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Periodic Table
What makes the periodic table such a useful
tool? It allows you to compare the properties
of the elements What does the number represent
in the isotope platinum-194? Write the symbol
from this atom using superscripts and
subscripts. It represents the mass number 194
Pt 78
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Periodic Table
Name the elements that have properties similar
to those of the element calcium (Ca). Beryllium
(Be), magnesium (Mg), strontium (Sr), Barium
(Ba), radium (Ra) 194 Consider Pt
how would changing the value of the
78 subscript change the chemical properties of
the atom? The subscript is the number of protons
in atoms of the isotope. Changing the number of
protons would change the chemical identity of the
isotope to that of another element.
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End of Chapter 4