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Chapter 4
Atomic Structure
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.
Democritus’s Atomic Philosophy
Greek philosopher 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 particles.
• Democritus’s ideas did not explain chemical
behavior.
• Lacked experimental support, because his
approach was not based on scientific method.
Dalton’s 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
Democritus’s 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.
Dalton’s Atomic Theory
According to Dalton’s atomic theory, an 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.
Dalton’s 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.
The Atom
The radii of most atoms fall within the range of
5 x 10-11 m to 2 x 10-10m. (very, very small!)
Individual atoms are “visible” only with special
instruments such as scanning tunneling
microscopes.
End of section 4.1
Subatomic Particles
Most of Dalton’s atomic theory is accepted today.
Except, we now know atoms to be divisible.
Atoms can be broken down into smaller particles,
called subatomic particles.
particles
There are 3 kinds of subatomic particles.
1. Electrons
2. Protons
3. Neutrons
(There are many more, but we won't worry about them.)
Electrons
In 1897, English physicist J.J. Thomson discovered
the electron. (The negative charge was assigned
arbitrarily. A positive charge could have been given!)
Electrons – negatively charged subatomic particles.
Thomson performed experiments that involved
passing electric current through gases at low
pressure.
Cathode-Ray Tube
Sealed gases at low pressures 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, or a beam
of negatively charged particles) that traveled from the
cathode to the anode.
Cathode Ray Tube
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.
Cathode-Ray Tube
To test his hypothesis about the particle called an
electron, Thomson’s student, Millikan, set up an
experiment to measure the ratio of the charge of an
electron to its mass (aka the Millikan oil drop
experiment).
He found this ratio to be constant.
Also, 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.
The Electron
An electron carries exactly one unit of negative
charge.
The electron’s 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.
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, an electrically neutral
particle is formed.
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 neutral hydrogen atom loses
an electron.
After all,
-1 + +1 = 0
Electron
Proton
Protons and Neutrons
In 1886, Eugene Goldstein observed a cathoderay 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.
Protons and Neutrons
English physicist James Chadwick confirmed the
existence of another subatomic particle in 1932.
Neutron – subatomic particles with no charge but
with a mass nearly equal to that of a proton.
Particle Symbol Relative Relative
Charge
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
Atomic Nucleus
J.J. Thomson thought that electrons were evenly
distributed throughout an atom filled uniformly with
positively charged material.
Thomson’s model of the atom had electrons stuck
into a lump of positive charge, called the plumpudding model. We can also call it the chocolate
chip ice cream model.
This model of the atom was short-lived due to work
of Ernest Rutherford, a former student of Thomson.
Rutherford’s 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 gold foil.
He theorized that alpha particles should pass easily
through the gold with only slight deflections.
Rutherford’s 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.
Rutherford’s 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.
Rutherford’s 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 Thomson’s model of
the atom, Rutherford’s model turned out to be
incomplete and had to be modified (see Chapter 5).
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.
Questions
Describe Thomson’s and Millikan’s 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
Questions
Compare Rutherford’s expected outcome of the goldfoil 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.
End of Section 4.2
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.
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 2
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
atom’s composition.
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
Mass number
197
Au
Atomic number
79
Written as Au-197 or Gold-197
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.
Hydrogen Isotopes
0 neutrons
Mass # = 1
Hydrogen-1
1 neutron
Mass # = 2
Hydrogen-2
2 neutrons
Mass # = 3
Hydrogen-3
Neon Isotopes
Neon-20
10 protons
10 neutrons
10 electrons
Neon-21
10 protons
11 neutrons
10 electrons
Neon-22
10 protons
12 neutrons
10 electrons
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
O
8
18
O
8
O
8
Atomic number
(# proton = # electrons)
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.
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 average atomic mass of all hydrogen isotopes is
1.0079 amu, and is very close to the mass of
hydrogen-1 (1.0078 amu)
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
copper’s 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.
Atomic Mass
Atomic mass = multiply the mass of each isotope by
its natural abundance, expressed 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
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 copper’s 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
Periodic Table
Periodic Table – an arrangement of elements in
which the elements are separated into groups based
on a set of repeating properties.
Periodic Table
Each element is identified by its symbol, placed 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. (Think: a sentence, written
horizontally, ends in a period.)
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.
Periodic Table
What distinguishes the atoms of 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.
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 (#p+ + #n0)
194
Pt
78
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), because they are all in the same
column
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.
End of Chapter 4