Download Atomic Structure - Mr. Cervantes Science Classes

Atomic Structure
Scientific Modeling
A. Scientists develop models to explain
things they cannot directly observe
B. Models of Atoms are based on scientific
discoveries
1. Models can be wrong and still use useful
2. Scientists construct models based on the
data or evidence that is available at the time
3. Early history regarding atoms
4. The greek philosopher Democritus of
Abdera lived in the 4th centure B.C
5. He was the first person to suggest the idea
of atoms ( he called them atomos)
6. His ideas were not useful because they
were not based on experimental evidence
C. John Dalton’s Atomic therory (early 1800’s)
1. Dalton was the first person to develop a
model of the atom
2. His model was based on experimental
evidence which showed that elements
combine in the same percent by mass each
time the same compound in formed
3. Dalton thought the atom was indivisible
uniformly dense, solid sphere that entered
into chemical reactions, but was left
unchanged by the reaction
II. Electrons, Protons, and Neutrons
A. Dalton’s theory that the atom is indivisible
has been revised
1. There are dozens of subatomic particles
2. Electrons, protons, and neutrons are the
particles important in chemistry
C. Electrons
1. Negatively charged subatomic particles
a. Sir Joseph J Thomson (1856 – 1940),
English physicist, experimented with
cathode ray tubes
1. A cathode ray tube is a closed glass tube
with metal disks called electrodes at each
end.
2. The tube contains a gas at low pressure
3. When connected to a high – voltage
source, the tube glows.
a. The anode ( one electrode) becomes
positively charged while the cathode ( the
other electrode) becomes negatively
charged
b. The glowing beam travels from the cathode
the anode and thus is called the cathode ray
b. Thomson discovered that the cathode rays could
be deflected (bent) by magnets or electrically
charged plates.
c. Thomson showed that the cathode ray is a
collection of very small negatively charged
particles that are all alike. He named them
electrons.
D. Protons
1. Thomson and Goldstein further investigated
and found that when the CRT was subjected
to high-voltage electricity, the negative beam
of electrons (cathode rays) moved to the
anode
2. A careful observation revealed another beam
of particles traveling in the opposite direction
toward the cathode
3. The new beam called (anode rays or canal
rays) contains positive particles, because
they move toward the cathode ( negative
electrode)
4. The positively charged particles were
called protons
5. Protons are 1840 times heavier than an
electron
E. Thomson revised Dalton’s model of 1903.
1. Thomson’s model of the atom showed the
bulk of the atom was made of the positive
protons, with the negative electrons
dispersed throughout to give an overall
neutral charge.
F. Discovery of the nucleus
1. Ernest Rutherford in 1911 used an
experiment called the Gold Foil Experiment
D. Neutrons
1. Sir James Chadwick – confirmed the
presence of a third subatomic particle, the
neutron
2. Neutrons have no charge
3. The mass of a neutron equals that of the
proton
4. Neutrons are found in the nucleus
E. New discoveries of subatomic particles
1. These particles are unstable and do not exist
in ordinary matter
2. There are 2 families of particles
a. Leptons
• Electrons – best known of the leptons
• Mu – mesons
• Tau-mesons
• Neutrinos (3 types)
b. Hadrons
1. Hadrons consist of smaller particles called
quarks
2. Hundreds of hadrons are known. The most
familiar hadrons are the following
• Protons
• Neutrons
• pion
Atomic Number and atomic Mass
A. Atomic number – the atomic number of an
element is the number of protons in the
nucleus of an atom of that element
1. Elements differ from one another only by
the number of the protons the atoms
contain in the nuclei
2. This difference ( in the number of protons)
is the key to the following
• Arrangement of the elements in the periodic
table
• Chemical properties of the elements
Because no two elements have the same atomic
number, the atomic number (number of
protons) defines the element
3. In a neutral atom, the number of protons
must equal the number of electrons
• Protons and electrons have equal but
opposite charges
• Hydrogen has one positive charge
(one proton) and one negative charge
(one electron). Together they balance the
electrical charges to produce a neutral atom
B. Mass Number – the mass number of an
element is the total number of protons and
neutrons of an atom of that element
Mass number – atomic number = # of neutrons
Isotopes
• Isotopes are atoms of the same element that
have the same number of protons but different
numbers of neutrons.
• Isotopes of an element are chemically alike
Atomic Mass
A. Since both protons and neutrons have a mass
of 1 atomic mass unit (AMU) each it is
reasonable to think that the mass of an atom
should be expressed as a whole number
B. The atomic mass of an element is the
weighted average of the masses of all the
isotopes of that element
1. When calculating the average atomic mass you
must take into account the relative abundance
of each isotope
Chlorine has two isotopes both with 17 protons in
their nuclei. One isotope has 18 neutrons and
thus its atomic mass is 354 amu. This isotope is
referred to as chlorine – 35. The other isotope
is chlorine – 37. It has 20 neutrons. In nature
the isotopes are found in a ratio of almost 3:1.
Use (0.75) for the 3 and 25% (0.25) for the 1
Chlorine – 35
Chlorine – 37
35 amu ∙ 0.75 = 26.25
37 amu ∙ 0.25 = 9.25 +
Total = 35.5 amu
The mass spectrometer
• The mass spectrometer separates atoms of
slightly different masses, thus it can separate
isotopes of the same element