Week 2 - Structure Of The Atom Flashcards
A negatively charged particle that has a very small mass compared to the mass of other subatomic
particles and surrounds the atom.
Electron
: An experiment, led by J.J. Thomson, which proposed that the atom was comprised of
negatively charged particles in a disperse field of positively charged particles.
plum pudding model
When certain substances emit charged particles
Radioactivity
An experiment, led by Ernest Rutherford, which proposed that atoms consist of a small
positively charged nucleus surrounded by negatively charged electrons.
gold foil experiment
A positively charged particle that has a relatively large mass compared to electrons. This can be
found in the nucleus of the atom.
Proton
An uncharged particle with a mass nearly equal to that of the proton. This can be found in the
nucleus of the atom.
Neutron
After the development of Dalton’s atomic theory, several important discoveries were made that led to a new understanding of the atom.
-read
In 1877, _____ (1832-1919) was studying how electrical current behaves in a vacuum tube. In one
experiment, he passed an electric current through an evacuated phosphorous-coated glass cylinder with an object in
the center, as shown in Figure 1.6.
Upon passing a current through the tube, _____ noticed that a “shadow” was cast by the object in the tube. The
phosphorus on the terminal end of the tube became brightly fluorescent, except for the region directly behind the
central object. He interpreted this to mean that the electrical current was blocked by the object. He reasoned that
the electrical current, which he later called cathode rays, was composed of streams of particles.
William crookes
Thomson’s interpretation of this effect was that cathode rays must consist of charged particles that have mass.
Thomson presented his work in 1897, where he referred to these negatively charged particles as corpuscles. Later
on, this name was changed and negatively charged particles became known as electrons. Thomson revised the model
of the atom into what became known as the plum pudding model. He hypothesized that the atom was comprised of
negatively charged particles in a field of positive charge (positively charged particles had not yet been discovered).
This proposed arrangement was compared to the arrangement of plums in plum pudding, as illustrated in Figure
1.7.
-read
In 1909, ______ devised what is known as the oil drop experiment to determine the
charge of a single electron.
Robert milikan and harvey fletcher
Using this information, Millikan calculated the charge of an electron to be 1.5924 × 10−19 coulombs. A coulomb (C)
is the SI unit for electric charge, where 1 coulomb = 1 ampere × 1 second. (Remember, an ampere is an SI base unit
for electric current.) Today, the accepted value for the charge of an electron is 1.602176487 × 10−19 C. Despite the relatively simple apparatus with which it was determined, Millikan’s value was within 1% of the currently accepted
value. Combining this value with information from J. J. Thomson’s experiments, Millikan was also able to calculate
the mass of an electron. The currently accepted value is 9.10938215 × 10−31 kg.
-read
Right around the time that Thomson was formulating his model of the atom, a scientist named Wilhelm Conrad
Roentgen was studying the behavior of electricity in discharge tubes. These were partially evacuated gas-filled
tubes which would conduct an electric current, similar to the Crookes tube used in the cathode ray experiments. .
Discovey of the x ray
This was the first “roentgenogram” ever taken. He interpreted this to mean that another ray, other than the cathode
rays, was being produced that could penetrate and travel through objects at a distance. He called these X-rays, and
he received the Nobel Prize in Physics in 1901 for his brilliant work.
-read
By 1900, it was known that the electron carried a negative charge. It was also known that the electron makes up
an extremely small fraction of the mass of an atom. Ernest Rutherford set out to determine how the remainder of
the mass and charge was distributed in the atom.
-rear
One of Rutherford’s famous experiments was called the ______ (illustrated in the Figure 1.11). In
this experiment, Rutherford used a radioactive source to direct alpha particles toward a very thin sheet of gold foil.
Surrounding the foil was a screen that fluoresced when struck by the alpha particles
Gold foil experiment
As shown in the video, most alpha particles easily passed through the gold foil and struck the fluorescent screen
behind the foil. However, there were some instances in which the alpha particles were deflected very strongly,
often back toward the emission source. If the ____ were correct, all of the alpha particles would
be expected to pass through the gold foil with little or no deflection. The strong deflection experienced by a small
portion of the alpha particles could be better explained by an atom that contained a very small, dense nucleus.
Because some of the alpha particles emitted from the source were repelled by the nucleus, Rutherford concluded
that the nucleus must be made up of these positively charged alpha particles, which he named protons. He proposed
that atoms consist of a small, positively charged nucleus surrounded by negatively charged electrons, as shown in
the Figure 1.12.
Plum pudding model
A British chemist, ____ studied under Rutherford and brilliantly developed the application of X-ray
spectra to study atomic structure.
Henry moseley
In 1913, shortly after Rutherford’s work on the nucleus, he proposed what became known as a planetary
model of the atom.
Niels bohr
In 1932, he discovered the neutron. He was an English physicist who was mentored by
Rutherford. His experiment consisted of bombarding beryllium atoms with alpha particles through a paraffin wax
target and studying the effects. From his analysis, he concluded that the nucleus also contains a particle which has
equal mass to the proton, but unlike the proton, is electrically neutral - hence the name neutron. Here is a short
video clip describing Chadwick’s experiment: http://www.youtube.com/watch?v=HnmEI94URK8 (2:14).
James Chadwick
Experiments conducted during the early twentieth century revealed that the atom is comprised of subatomic
particles called electrons, neutrons, and protons.
-read
In 1877, he discovered cathode rays, which later became known as electrons. He discovered these rays using an apparatus he developed called the
William crookes
Crookes tube or cathode-ray tube.
______, which were initially called cathode rays, are negatively charged and have a very small mass
compared to the masses of other subatomic particles
Electrons
• In 1895, he discovered X-rays
Wilhelm Conrad Roentgen
In 1897, he showed that cathode rays are deflected in a magnetic field and proposed that cathode
rays are streams of negatively charged particles.
J.J thomson
In 1909, the magnitude of the charge carried by an electron was determined by _____ in an experiment known as the oil drop experiment. Information from this experiment was later used to also calculate the mass of an electron
Robert milikan
• In 1919, he discovered the presence of a positively charged nucleus with his famous gold foil experiment
Rutherford
He proposed a new atomic model that described the atom as comprised of a positively charged nucleus
surrounded by negatively charged electrons. In this model, most of the atom was thought to be empty
space.*Protons are positively charged and have a relatively large mass compared to electrons. Protons can
be found in the nucleus of the atom
Rutherford
In 1932, he discovered the neutron, a particle with a mass similar to that of the proton but without any
electrical charge
Chadwick
These are particles with a mass similar to that of the proton, but they have no electrical charge.this
also reside in the nucleus.
Neutron
A number of experiments revealed that the atom is comprised of subatomic particles called
electrons, neutrons, and protons.
Most of the atom’s mass is concentrated in a central ____, which contains
protons and neutrons. The ____ is surrounded by much less massive electrons, which account for most of the
volume occupied by the atom.
Nucleus
This was the first subatomic particle to be discovered, followed by discoveries
of the nucleus, the proton, and the neutron. In this lesson, we are going to study these important discoveries and how
they led to our current understanding of the atom
Electrons
His work was
later expanded upon by several other scientists. One scientist in particular, J. J. Thomson, was able to show that
cathode rays could be deflected by a magnetic field, as shown in the following video: http://www.youtube.com/w
atch?v=M1REuKMeI34 (0:58).
Crookes
Crookes noticed that a “shadow” was cast by the object in the tube. The
phosphorus on the terminal end of the tube became brightly fluorescent, except for the region directly behind the
central object. He interpreted this to mean that the electrical current was blocked by the object. He reasoned that
the electrical current, which he later called ____, was composed of streams of particles.
Cathode rays
This experiment consisted of an apparatus in which small, charged oil drops were passed through an electric field. The electric field was created by two oppositely charged parallel metal plates. The
rate at which the oil drops fell through the field was used to determine the magnitude of the charge of an electron
Oil drop experiment
He devised an experiment in which he covered a discharge tube with black cardboard, and several feet away, he placed a
sheet of paper that had been chemically treated with a compound called barium-platinum cyanide. When he applied
voltage to the discharge tube, he noticed the screen several feet away became fluorescent, emitting light. This was
peculiar, because the tube had been completely covered by material that would block the escape of any cathode rays.he continued to explore this phenomenon. He moved the screen further away, he turned the screen around,
and he placed objects between the screen and the discharge tube. In all cases, the screen still fluoresced when the
discharge tube was turned on. He had his wife place her hand atop a photographic plate, and the rays
were shone towards the plate. After developing the plate, he observed an image of his wife’s hand that “showed
the shadows thrown by the bones of her hand and that of a ring she was wearing”
Wilhelm Conrad Roentgen
Modern day X-rays that one might receive
for a medical exam operate on the same principles that Roentgen discovered. Several major discoveries followed
shortly after Roentgen’s discovery of X-rays. Just two months later, in 1896, radioactivity was discovered by a
Frenchman named _____. He discovered that certain substances, like uranium salts, emit charged
particles.
Henri becquerel
Following this work, _____ began to study the behavior of various radioactive substances
in 1897. In fact, _______ coined the word “radioactivity.” Their work resulted in the discovery of mass
changes in radioactive elements, which later became known as radioactive decay. They also identified two new
radioactive elements, which later became known as polonium and radium. They were awarded the Nobel Prize
in Physics in 1903 for their work.
Following this work, Marie and Pierre Curie began to study the behavior of various radioactive substances
in 1897. In fact, Madame Curie coined the word “radioactivity.” Their work resulted in the discovery of mass
changes in radioactive elements, which later became known as radioactive decay. They also identified two new
radioactive elements, which later became known as polonium and radium. The Curies were awarded the Nobel Prize
in Physics in 1903 for their work. Marie Curie later won a Nobel Prize in Chemistry (1911) for her contributions to
our understanding of radioactivity. We will study radioactivity further in the chapter Nuclear Chemistry.
She later won a Nobel Prize in Chemistry (1911) for her contributions to
our understanding of radioactivity. We will study radioactivity further in the chapter Nuclear Chemistry.
Marie curie
He was a physicist from New Zealand who was working
under the direction of J. J. Thomson. He conducted several experiments on the radioactive properties of uranium. He
discovered that uranium released two different types of particles, which he referred to as alpha (α) particles, which
were positively charged, and beta (β) particles, which were negatively charged. It was later shown that beta particles
were simply free electrons.
Ernest rutherford
His discoveries resulted in a more accurate positioning of elements in the
periodic table by closer determination of atomic numbers.
In 1913, almost fifty years after Mendeleev , he published the results of his measurements of the
wavelengths of the X-ray spectral lines of a number of elements which showed that the ordering of the wavelengths
of the X-ray emissions of the elements coincided with the ordering of the elements by atomic number. With the
discovery of isotopes of the elements, it became apparent that atomic weight was not the significant player in
the periodic law as Mendeleev, Meyers and others had proposed, but rather, the properties of the elements varied
periodically with atomic number.
When atoms were arranged according to increasing atomic number, the few problems with Mendeleev’s periodic
table had disappeared. Because of his work, the modern periodic table is based on the atomic numbers of the
elements.
Henrey moseley
His model was based upon the work done by Max Planck and Albert Einstein, who at the time
were studying quantum theory which looks at the energy associated with matter. The planetary model was useful
for relating atomic structure to the wavelengths of light that an element emits when heated. His model, as well as
the work of Planck and Einstein, will be discussed in the chapter Electrons in Atoms.
Niels bohr
His work resulted in a new understanding of the nucleus of the atom; it is comprised of both protons and
neutrons. Because the masses of subatomic particles are so small, a new unit, called an atomic mass unit (amu),
was defined. Protons and neutrons each have a mass of approximately one amu. The Table 1.1 describes the
characteristics of the three subatomic particles we have discussed.
James chadwick
He proposed the plum-pudding model of the atom. This
model described the atom as a disperse field of positive charge containing small negatively charged particles.
J.J thomson
