Chapter 4
Atomic Structure
Defining the Atom
4.1
Early Models of the Atom
An atom is the smallest particle of an element that
retains its identity in a chemical reaction
Democritus’s Atomic Theory
Greek philosopher
Among the first to suggest the existence of atoms
Key Question, Part 1
How did the concept of the atom change from time of Democritus to
the time of John Dalton?
Democritus reasoned that atoms were indivisible and
indestructible. His ideas agreed with later scientific theory;
however, they did not explain chemical behavior and they lacked
experimental support because Democritus’s finding’s were not
based on the scientific method.
Dalton’s Atomic Theory
An English chemist and schoolteacher
Key Question, Part 2
How did the concept of the atom change from time of
Democritus to the time of John Dalton?
Dalton used experimental methods to transform
Democritus’s ideas on atoms into a scientific theory.
He studied the ratios in which elements combine in
chemical reactions.
Dalton’s Atomic Theory
Dalton formulated hypotheses and theories to explain his
observations, resulting in what is called Dalton’s Atomic
Theory:
All elements are composed of tiny indivisible particles called
atoms.
Atoms of the same element are identical. The atoms of one
element are different from those of any other element.
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 from
each other, joined, or rearranged in a different combination.
Atoms of one element, however, are never changed into
atoms of another element as a result of a chemical reaction
Sizing Up the Atom
Key Question
What instruments are used to observe individual atoms?
Individual atoms are observable with instruments such as
scanning electron microscopes. Electron microscopes
allow for individual atoms to be moved around and
arranged in patterns.
Structure of the Nuclear Atom
4.2
Subatomic Particles
Key Question
What are the three kinds of subatomic particles?
protons, neutrons, and electrons
•
Protons are positively charged subatomic particles
•
Electrons are negatively charged subatomic particles
•
Neutrons are subatomic particles with no charge
Electrons
English Physicist
J.J Thomson
Discovered the electron
Experiment
Passed electric currents through gases at low pressures
The gases were sealed in glass tubes fitted at both ends with metal disks called electrodes (connected to
electricity)
One electrode (anode) became positive, and the other (cathode) became negative
Observations
A glowing beam, or cathode ray traveled from the cathode to the anode
The cathode ray is deflected by electrically charged metal plates
A positively charged plate attracts the cathode ray, while a negatively charged plate repels it (opposites
attract)
Hypothesis
Hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed, he
called them corpuscles, later named electrons
Experiment 2 (testing his hypothesis)
This experiment was to measure the ratio of an electron’s charge to its mass, which was found to be
constant, this ratio did not depend on the type of gas in the cathode tube or the type of metal used for the
electrodes
Conclusion
That electrons are a component of the atoms of all elements
Robert A. Milikan
U.S. Physicist
Experimented to find the quantity of an electron’s charge
Experiment
Oil-drop experiment
He suspended negatively charged oil droplets between two charged plates
He changed the voltage on the plates to see how it would affect the droplet’s rate of fall
Conclusions
He found that the charge on each oil droplet was a multiple of 1.60 x 10-19 coulomb, meaning he
had determined the charge of an electron
Other Determinations
He used the charge of the electron and Thomson’s charge-to-mass ratio of an electron to
calculate the mass of an electron
The mass determined by Milikan is similar to the mass accepted today
1/1840 the mass of a hydrogen atom equals the mass of an electron
Protons and Neutrons
What remains of the atoms
that have lost the electrons?
#1 Atoms have no net electric charge; therefore, they are
electrically neutral
#2 Electric charges are carried by particles of matter
#3 Electric charges always exist in whole-number multiples of a
single basic unit, meaning there are no fractions of charges
#4 When a given number of negatively charged particles
combines with an equal number of positively charged
particles, an electrically neutral particle is formed
Knowing This It is Determined…
A particle with one unit of positive charge should
remain when a typical hydrogen atom loses an electron
Evidence for this charged particle was discovered in
1886 by Eugen Goldstein
Eugen Goldstein
Identified the positively charged subatomic particle (proton) in 1886
Observations
Observed a cathode-ray tube and found rays traveling in the opposite
direction of the cathode rays
He called these rays canal rays
Conclusion
He concluded that these rays were composed of positive particles, now
called protons
Each proton has a mass of about 1840 times that of an electron
James Chadwick
English Physicist
Confirmed the existence of the neutron
The mass of the neutron is nearly equal to that of the proton
*It is believed that protons and neutrons are composed of even smaller
subnuclear particles called quarks!
The Atomic Nucleus
Key Question
How can you describe the structure of the nuclear atom?
The protons and neutrons are located in the positively
charged nucleus. The electrons are distributed around
the nucleus and occupy almost all the volume of the
atom.
When subatomic particles were
discovered, scientists wondered how
the particles were put together in
an atom.
J.J Thomson
Thought that the electrons were evenly distributed
throughout an atom filled uniformly with positively
charged material
Thomson’s Atomic Model
The Plum Pudding Model
Electrons were stuck into a lump of positive charge,
similar to raisins stuck in dough
This model was short lived
A former student of Thomson, Ernest Rutherford
proved Thomson’s model wrong
Rutherford
He tested the existing plum pudding model
Experiment
Gold-Foil Experiment
Used alpha particles (Helium atoms that have lost their two
electrons and have a double positive charge)
A narrow beam of alpha particles was directed at a very thin
sheet of gold foil
Hypothesis
They expected the alpha particles to pass through the gold
with only a slight deflection due to the positive charge
thought to be spread out in the gold atom
Observations
Most alpha particles went straight through the gold foil, or
were slightly deflected
A small fraction of the alpha particles bounced off the gold
foil at very large angles
Some even bounced straight back toward the source
Rutherford-Continued
Conclusion
He proposed that the atom is mostly empty space explaining
the lack of deflection 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 of some of
the alpha particles
He called this region the nucleus, which contains the protons
and neutrons and is positive
His atomic model is known as the NUCLEAR ATOM
This model was an improvement over Thomson’s model;
however, it did not account for the chemical properties of
elements!
Bohr Model and Quantum Mechanical Model
Distinguishing Among Atoms
4.3
Atomic Number and Mass Number
Key Question
What makes one element different from another?
Elements are different because they contain different
numbers of protons.
Personal Periodic Tables
You will need this periodic table for the
WHOLE YEAR!
We will be adding information to it and making
corrections throughout the entire year.
Do not lose this periodic table!
Atomic Number
Atomic number is the number of protons in the nucleus of
an atom of that element
Individual Practice
What is the atomic number of fluorine?
How many protons does fluorine have?
How many protons does beryllium have?
ANSWERS Located in notes BELOW!
Mass Number
Most of the mass of an atom is concentrated in its
nucleus and depends on the number of protons and
neutrons
The total number of protons and neutrons in an atom
is called the mass number
# neutrons = mass number – atomic number
Practice: Determining the
Number of Neutrons
IN CLASS ONLY!
Model
Shorthand, page 114 number 18
Individual Practice
Number 19
Isotopes
Key Question
How do isotopes of an element differ?
Isotopes of an element have different numbers of neutrons;
therefore, they 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.
Isotopes are atoms that have the same number of
protons but different numbers of neutrons.
Practice
Isotopes
Model
Number 20, page 115
IN CLASS ONLY!
Individual Practice
Number 21, page 115
Model
Number 22, page 118
Individual Practice
Number 23, page 118
Atomic Mass
Atomic mass unit (amu)is defined as one twelfth of
the mass of a carbon-12 atom
Atomic mass of an element is a weighted average mass
of the atoms in a naturally occurring sample of the
element (all isotopes; therefore, decimal form)
Atomic Mass
Key Question
How do you calculate the atomic mass of an element?
Multiple the mass of each isotope by its natural abundance,
expressed as a decimal, and then add the products.
*The isotope with the mass closest to the atomic mass is the
most abundant isotope.
Practice
Atomic Mass
Model
Number 24, page 119
Individual Practice
Number 25, page 119
IN CLASS ONLY!