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ATOMIC THEORY
HISTORY OF THE ATOM
The original idea (400 B.C.) came from ______________________, a Greek philosopher.
He expressed the belief that all matter is composed of very small, indivisible particles, which he named
atomos. John Dalton (1766-1844), an English school teacher and chemist, proposed his atomic theory
of matter in 1803. Dalton’s Atomic Theory states that:
1. All matter is made of tiny __________________________ particles called atoms.
2. Atoms of the ____________ element are identical; those of different atoms are different.
3. Atoms of different elements combine in whole number ________________ to form compounds
4. Chemical reactions involve the rearrangement of atoms. No _______ atoms are created or destroyed.
PARTS OF THE ATOM
Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny
solid ball that could not be broken up into parts. In 1897, a British physicist, J.J. Thomson, discovered
that this solid-ball model was not accurate. Thomson’s experiments used a __________________ ray
tube. It is a vacuum tube - all the air has been pumped out. Because these rays originate at the
____________________, they are called cathode rays. Thomson concluded that cathode rays are
made up of invisible, _________________________ charged particles referred to as electrons. From
Thomson’s experiments, scientists had to conclude that atoms were not just neutral
_________________, but somehow were composed of electrically charged particles. Matter is not
negatively charged, so atoms can’t be negatively charged either. If atoms contained extremely light,
negatively charged particles, then they must also contain positively charged particles — probably with
a much greater _____________ than electrons. J.J. Thomson said the atom was like ______________
pudding, a popular English dessert. R.A. ____________________ found the charge of an electron to
be -1.60 x 10-19 Coulombs in his famous _____________ _______________ experiment. In 1886,
scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the
positively charged anode. Years later, scientists determined that the rays were composed of positively
charged subatomic particles called _____________________. At this point, it seemed that atoms were
made up of __________________ numbers of electrons and protons. In 1909, a team of scientists led
by Ernest Rutherford in England carried out the first of several important experiments that revealed an
arrangement far different from the plum pudding model of the atom. The experimenters set up a leadshielded box containing radioactive polonium, which emitted a beam of positively charged subatomic
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particles through a small hole. The sheet of ________________ foil was surrounded by a screen
coated with zinc sulfide, which glows when struck by the positively charged particles of the beam.
The ________________ particles were expected to pass through without changing direction very much
because Rutherford thought the mass was evenly distributed in the atom. Because most of the particles
passed through the foil, they concluded that the atom is nearly all _______________ ______________.
Because so few particles were deflected, they proposed that the atom has a small, dense, positively
charged central core, called a ____________________. Alpha particles are deflected by it if they get
close enough to the nucleus. In 1910, J.J. Thomson discovered that neon consisted of atoms of two
different masses. Atoms of an element that are chemically alike but differ in mass are called
______________________ of the element. Because of the discovery of isotopes, scientists
hypothesized that atoms contained still a third type of particle that explained these differences in mass.
Calculations showed that such a particle should have a mass ____________________ to that of a
proton but no electrical _______________. The existence of this neutral particle, called a neutron, was
confirmed in the early 1930s. James _________________ is given credit for discovering the neutron.
NAME
SYMBOL
CHARGE
RELATIVE MASS
1/2000
proton
no
MODERN VIEW OF THE ATOM
The atom has two regions and is ___-dimensional. The nucleus is at the ___________________ and
contains the protons and _____________________. The electron cloud is the region where you might
find an electron and most of the volume of an atom. The atomic _________________ of an element is
the number of protons in the nucleus of an atom of that element. The number of protons determines
____________________ of an element, as well as many of its chemical and physical properties.
Because atoms have no overall electrical charge, an atom must have as many ____________________
as there are protons in its nucleus. Therefore, the atomic number of an element also tells the number of
electrons in a neutral atom of that element. The mass of a neutron is almost the same as the mass of a
________________. The sum of the protons and neutrons in the nucleus is the ________________
number of that particular atom. _____________________ of an element have different mass numbers
because they have different numbers of _______________, but they all have the same atomic number.
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AVERAGE ATOMIC MASS
The atomic mass is the weighted average mass of all the naturally occurring isotopes of that element.
To determine the average atomic mass, first calculate the contribution of each isotope to the average
atomic mass, being sure to convert each ___ a fractional abundance. The average atomic mass of the
element is the sum of the mass contributions of each isotope.
Elements can be represented by using the symbol of the element, the mass number and the atomic
number. The mass number is the __________________ mass rounded to a whole number.
1. Determine the following for the fluorine-19 atom.
a) number of protons
b) number of neutrons
d) atomic number
e) mass number
c) number of electrons
2. Repeat #1 for bromine-80.
3. If an element has an atomic number of 34 and a mass number of 78, what is the
a) number of protons
b) number of neutrons
c) number of electrons
d) complete symbol
4. If an element has 91 protons and 140 neutrons, what is the
a) atomic number
b) mass number
c) number of electrons
d) complete symbol
5. If an element has 78 electrons and 117 neutrons what is the
a) atomic number
b) mass number
c) number of protons
d) complete symbol
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