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Greeks 400 BC
Aristotle
4 elements: earth(s), air(g),
fire(energy), and water(l)
Democritus
-matter was composed of
small indivisible particles called
"atomos"
-tom = to divide
atom = can't be divided
AlchemistNext 2000 years –tried to turn
cheap materials into gold.
Some elements and compounds
were discovered, and some
procedures were developed
Robert Boyle -1600's
-First person to perform quantitative
experiments
-wrote a book called "The Skeptical
Chymist"
-his research destroyed the notion of
only 4 elements
Antoine Lavoisier –
-named oxygen
-He determined the
nature of combustion
with careful
quantitative procedures.
-He developed the law of conservation
of mass (mass is neither created nor
destroyed)
-wrote first chemistry
textbook
-executed by guillotine
Law of Conservation of Mass- Mass
is neither created nor destroyed in a
chemical reaction. The same number
and types of atoms will be found in the
products as in the reactants. They are
simply rearranged.
H2
O2
H2O
Dalton’s Atomic Theory
John Dalton (1766-1844) developed the first useful atomic
theory of matter around 1803. Dalton's exceptional gift for
recognizing and interpreting patterns in experimental data
lead him from a problem in meteorology to the idea of
atoms as fundamental constituents of matter. He realized
the vital theoretical connection between atomic weights
and weight relations in chemical reactions. He was the first
to associate the ancient idea of atoms with stoichiometry.
Dalton's atomic theory rests on the following postulates.
DALTON’S ATOMIC THEORY
•All matter consists of tiny particles
called atoms.
•Atoms are indestructible and
unchangeable.
•All atoms of the same element are
identical.
•Atoms combine in new ways during
a chemical reaction.
Could anything at first sight seem
more impractical than a body which is
so small that its mass is an
insignificant fraction of the mass of an
atom of hydrogen? --which itself is so
small that a crowd of these atoms
equal in number to the population of
the whole world would be too small to
have been detected by any means then
known to science."
Recording made in 1934. From the soundtrack of the
film, Atomic Physics
What would J. J. Thomson’s atom look like?
He called his model of the atom plum pudding.
Ernest Rutherford
Rutherford began his study of the structure of
the atom as a colleague of J. J. Thomson. He
naturally viewed the atom as Thomson did.
To examine the structure of the atom Rutherford designed an
experiment where he aimed alpha particles so they would
strike a thin sheet of gold foil. He expected the dense high
speed alpha particles to go straight through the gold foil.
Rutherford and his coworkers repeated this same experiment using foils
made of many other elements besides gold. After finding similar results with
these experiments, Rutherford and his coworkers were able to show that the
centralized nucleus of any type of atom has several fundamental properties.
•A nucleus has a positive charge which is equal in magnitude the the
overall negative charge of the electrons surrounding the nucleus.
•A nucleus makes up over 99.9% of the total mass of the atom. (Just how
dense is the nucleus? Consider this: a piece of nuclear material the size
of a pea would have a mass of 250 million tons, which is about the mass
of 170 million cars!
•A nucleus has a diameter that is merely 0.01% the length of the atom
itself. ( If an atom were enlarged to the size of a football stadium, its
nucleus would have the size and position of a bee on the 50-yard line,
and the electrons would occupy the stadium seats!
• Describe Thomson’s model of the atom and
how did it differ from Dalton’s model of the
atom
• Describe Rutherford’s model of the atom.
In the Rutherford gold foil experiment, the fact that
most of the alpha particles were not
deflected as they passed through the gold foil
indicates that
(a) the nucleus is positively charged.
(b) the atom is mostly empty space.
(c) atoms are solid spheres touching each other in
the solid state.
(d) gold is very dense.
(e) none of the above is correct.
The Neutron
Scientists were confused as they realized that the total mass
of the atom was more than just the sum of the masses of the
protons and electrons. They suspected that the atom contained
a third type of subatomic particle that contributed to the
atomic mass.
However, as this particle did not have an electrical charge,
and therefore was neutral, it was more difficult to detect than
the electron and proton. In 1932, an English scientist named
James Chadwick showed that an atom contains a third kind of
subatomic particle which he called a neutron. A neutron has
about the same mass as a proton and is also found in the
nucleus of the atom. However, unlike the proton, the neutron
does not have an electric charge.
The Atom
An atom is the smallest particle of an element that displays all
the properties of that element.
All atoms of an element have the same number of protons.
Atoms have equal numbers of protons and electrons.
Isotopes are atoms of the same element (same number of
protons) but differ from each other in the number of neutrons in
their nucleus.
Nucleons are components of the nucleus = protons and neutrons.
Electrons are outside the nucleus.
The nucleus has most of the mass of the atom but occupies only
about 1/10,000 the total volume of the atom. The atom is
mostly space.
Which of the following particles has the
smallest mass?
(a) an electron
(b) a proton
(c) a neutron
(d) a hydrogen atom
(e) a hydrogen nucleus
Which statement is false?
(a) Ordinary chemical reactions do not involve
changes in nuclei.
(b) Atomic nuclei are very dense.
(c) Nuclei are positively charged.
(d) Electrons contribute only little to the mass of
an atom.
(e) The nucleus occupies nearly all the volume of
an atom.
• The atomic number of a certain element is
19, and its mass number is 39. An atom of
the
element contains _____ protons,
_____ neutrons, and the chemical symbol
for the element is _____.
• Give the number of protons, neutrons, and
electrons in the 31 46S 2– ion.
• What is the symbol for a species composed
of 35 protons, 44 neutrons, and 36
electrons?
Which response includes all the following statements that are
true, and no false statements?
I. Isotopes of an element differ only in the number of
protons.
II. The number of protons in an atom is its atomic
number.
III. The mass number of an atom is the sum of the number
of protons plus electrons in the atom.
IV. The volume occupied by the nucleus of an atom
represents a large percentage of the total volume of the
atom.
(a) I and III
(b) II, III, and IV
(c) II and IV
(d) II
(e) I and IV
Mass Number VS. Atomic Mass
• The mass number is the sum of the protons
and neutrons for an isotope. Since each
element has more than one naturally
occurring isotope, and the periodic table is a
table of elements not isotopes, the mass
number is not on the periodic table.
• The atomic mass of an element is on the
periodic table. It is a weighted average of
the masses of all the isotopes of an element.
• It is weighted based on the relative
abundance (decimal %) of each isotope.
Formula for finding Atomic Mass
• (mass x r.a.) + (mass x r.a.) + (mass x r.a.)...
isotope 1
isotope 2
isotope 3
Why is the atomic weight of chlorine
35.4527 instead of exactly 35?
• Copper has 2 naturally occurring isotopes.
Copper- 63 is 69% abundant the remaining
isotopes are copper-65. What is the average
atomic mass of copper?
A hypothetical element consists of the following naturally
occurring isotopes. What is the atomic mass of the
element?
Isotopes
Mass
Abundance
1
46.041 amu
26.00%
2
47.038 amu
58.00%
3
49.034 amu
16.00%