THE ATOM. MODELS OF THE ATOM.

Do you like mysteries? Are you curious? Humans are curious. Someone always
wants to know something that is not easy to detect or to see what can’t be seen.
For example, people began wondering about matter more than 2500 years ago.
Some of the early philosophers thought that matter was composed of tiny
particles. They reasoned that you could take a piece of matter, cut it in half, cut
the half piece in half again, and continue to cut again and again. Eventually, you
wouldn’t be able to cut any more. You would have only one particle left. They
named these particles atoms, a term that means “cannot be divided”.
Describing the unseen.
Early philosophers didn’t try to prove their theories by doing experiments as
scientists now do. Their theories were the result of reasoning, debating, and
discussion, not of evidence or proof. Today, scientists will not accept a theory
that is not supported by experimental evidence. But even if these philosophers
had experimented, they could not have proven the existence of atoms. The kind
of equipment needed to study matter was a long way from being invested. Even
as recently as 500 years ago, atoms were still a mystery.
In 1800, Dalton proposed a modern atomic model based on experimentation.
This theory can be summarized in:
 All matter is made of atoms. Atoms are indivisible and indestructible.
 All atoms of a given element are identical in mass and properties.
 Each element has different atoms.
 Compounds are formed by a combination of two or more different kinds
of atoms
 Atoms are rearranged in reactions.
Thomson’s atomic model.
Thomson revised Dalton’s model of the atom and did some experiments using
metals and different gases in a tube. He found that some charged particles
were produced no matter what
elements were used for the metal or
the gas in the tube. Thomson
concluded that the rays were
negatively charged particles of
matter. How did Thomson know the
particles were negatively charged?
He knew that opposite charges
attract to each other. He observed
that these particles were attracted
to the positively charged
electrode, so he reasoned that the

particles must be negatively charged.
These negatively charged particles are now called electrons. Thomson also
inferred that electrons are a part of every kind of atom because they are
produced by every kind of metal. Perhaps the biggest surprise that came
from Thomson’s experiments was the evidence that particles smaller than

1

the atom do exist. Based on this discovery, Thomson proposed his own model
of the atom in which he pictured the atom as a watermelon where the red part,
the fleshy part will be the positive charge. The negatively charged particles
were spread evenly among the positive charge, as are seeds inside the
watermelon. The positive charge is equal to the negative charge. Therefore,
the atom is neutral. It was later discovered that not all atoms are neutral
because the number of negative charges within an element can vary.
Rutherford’s experiments.
A model is not accepted in the scientific community until it has been tested and
the tests support previous observations. In 1906 Ernest Rutherford and his
coworkers began an experiment to find out if Thomson’s model of the atom was
correct. They wanted to see what would happen when they fired fast-moving,
positively charged bits of matter, called alpha particles, at a thin film of a metal
such as gold. Alpha particles, which come from unstable atoms, are positively
charged.

The figure shows how
the experiment was set
up. A source of alpha
particles was aimed at
a thin sheet of gold foil.
The foil was surrounded
by a fluorescent screen
that gave a flash of light
each time it was hit by a
charged particle.
Rutherford ́s prediction
was that most of the
speeding alpha particles would pass right through the foil and hit the screen on
the other side, just like a bullet fired through a pane of glass. Rutherford
reasoned that the thin, gold film did not contain enough matter to stop the
speeding alpha particle or change its path.
Rutherford was shocked when the student who was controlling the experiment
rushed in to tell him that some alpha particles sere veering off at large angles.
Rutherford expressed his amazement by saying, “It was about as believable as
if you had fired a 15-inch shell at a piece of tissue paper, and it came back and
hit you”. How could such an event be explained? The positively charged alpha
particles were moving with such high speed that it would take a large positive
charge to cause them to bounce back. Now Rutherford and his team had to
come up with an explanation for these unexpected results. Now and then, an
alpha particle might be affected slightly by a positive charge in the atom and
turn a bit off course. However, large changes in direction were not expected.
The actual results did not fit this model so Rutherford proposed a new one.
He hypothesized that almost all the mass of the atom and all of its positive
charge are crammed into an incredibly small region of space at the center
of the atom called the nucleus. Eventually, his prediction was proved true. In
1920 scientists identified the positive charges in the nucleus as protons. A
proton is a positively charged particle present in the nucleus of all atoms. The

2

rest of each atom is empty space occupied by the atoms almost mass less
electrons.
Rutherford’s new model of the atom fitted the experimental data. Most alpha
particles could move through the foil with little or no interference because of the
empty space that makes up most of the atom. However if an alpha particle
made a direct hit on the nucleus of a gold atom, which has 79 protons, the
alpha particle would be strongly repelled and bounce back.
Rutherford’s nuclear model was applauded as other scientists reviewed the
results of the experiments. However, some data didn’t fit. Once again, more
questions arose and the scientific process continued. For instance, atom’s
electrons have almost no mass. According to Rutherford’s model, the only other
particle in the atom was the proton. That meant that the mass of an atom should
have been approximately equal to the mass of its protons. However, it wasn’t.
The mass of most atoms is at least twice as great as the mass of its protons.
That left scientist with a dilemma and raised a new question. Where does the
extra mass come from if only protons and electrons made up the atom?
Furthermore, it seemed pretty stressed to queue up 70, 80 or more protons in
such a tiny space (the nucleus) and don ́t expect some kind of tightness. So
many positives charges in a very small placed didn ́t seem to be the perfect
place for the protons; but it was. Some idea came to mind: what if the protons
were not the only particles present in the atom ́s nucleus? Could it be that
another particle could accompany protons and stabilize them? For that purpose
this “magical” particle should be neutral (no repulsion, no attraction) and as said
before, have mass.
It was proposed that another particle must be in the nucleus to account
for the extra mass. The particle, which was later called the neutron, would
have the same mass as a proton and be
electrically neutral. Proving the existence of
neutrons was difficult though, because a
neutron has no charge. It took another 20
years before scientists were able to show by
more modern experiments that atoms contain
neutrons.
The model of the atom was revised again to
include the newly discovered neutrons in the
nucleus. The nuclear atom has a tiny
nucleus, tightly packed with positively
charged protons and neutral neutrons.
Negatively charged electrons occupy the space surrounding the nucleus.
The number of electrons in a neutral atom equals the number of protons
in the atom.
Bohr ́s atomic model.
Rutherford ́s model seemed to have some flaws, as said before. More
miscalculations were found later; the first one was related to the “eternal”
movement of the electron around the nucleus that he proposes. Scientists knew
that charged particles could not be moving for ever; eventually they would lose

3

all the energy available and collapse agains the nucleus. That wasn ́t the case,
so, the electron was not moving constantly as Rutherford proposed.
The movement of the electron was tricky to understand, but the development of
an equipment called spectrograph came quite handy. The spectrograph made a
drawing (spectrum) in which each movement of the electron could be shown as
a line. If the movement of the electron
was as Rutherford supposed, endless,
the spectrum of the electron ́s movement
would be completely black; each
movement supposes a line, infinite
movements will outcome a black
spectrum. But, as scientists proved later,
the spectrum was not black; it showed
independent lines at specific
distances.

Trying to explain this strange phenomenon, Niels Bohr, a Danish scientist,
theorized that electrons absorb energy and move to higher energy states.
Then, these excited electrons give off that energy as light waves when they
fall back to a lower energy state. But, why were only certain lines of light
given off and not others? To answer this question, Bohr suggested that
electrons could have only certain amounts of energy, due to the position
they have inside the atom. When they absorb energy, electrons absorb only
the amount needed to move to a specific higher energy state. Then, when the
electrons fall back to a lower energy state, they emit only certain amounts of
energy.
Look carefully to the following drawing to better understand Bohr ́s idea about
the electron ́s location inside the atom.
Because electrons can have only certain
amounts of energy, Bohr reasoned than they
can move around the nucleus only at distances
that correspond to those amounts of energy.
These regions of space in which electrons can
move about the nucleus of an atom are called
energy levels. Each energy level can hold a
limited number of electrons. The lowest energy
level is the smallest, the more comfortable for
the electrons and the closest to the nucleus.
Identifying characteristics.
Each chemical element is different and has unique properties. These
differences can be described in part by looking at the relationships between the
atomic particles in each element. The periodic table contains numbers that
describe these relationships.
Number of protons and neutrons.
Look up the element chlorine on the periodic table. Cl is the symbol for chlorine,
but what are the two numbers, above and below the symbol? The top number is
the element’s atomic number. It tells you the number of protons in the nucleus
of each atom of that element. Every atom of chlorine, for example, has 17
protons in its nucleus. This number is identified as Z.

4

Isotopes.
Although the number of protons changes from element to element, every atom
of the same element has the
same number of protons.
However, the number of
neutrons can vary even for
one element. For example,
some chlorine atoms have 18
neutrons in their nucleus while
others have 20. These two
types of chlorine atoms are
chlorine-35 and chlorine-37.
They are called isotopes,
which are atoms of the

same element that have different numbers of neutrons.
You can tell someone exactly which isotope you are referring to by using its
mass number. An atom’s mass number is the number of protons plus the
number of neutrons it contains. The numbers 35 and 37, which were used to
refer to chlorine, are mass numbers and they are identified by the letter A.
Hydrogen has three isotopes with mass numbers of 1, 2 and 3. Each hydrogen
atom always has one proton, but in each isotope the number of neutrons is
different.

So, Z= no of protons; A= no of protons + no of neutrons.

Ions.
An atom or molecule which has gained or lost one or more of its electrons is no
longer neutral. It becomes a
charged particle called an ion.
If an atom has lost electrons,
then that atom has more
protons than electrons and its
charge is positive. We call it a
cation. If an atom gains
negative charge, electrons,
then it has more electrons
than protons so its charge is negative. We call it an anion.
Radioactivity.

Unstable atomic nuclei will
spontaneously decompose to form
nuclei with higher stability. The
decomposition process is called
radioactivity. The energy and
particles which are released during
the decomposition process are
called radiation.
Radioactivity releases a great
amount of energy because part of nuclear mass becomes energy (E=mc2
).

5

There are three types of radiations:
 Alpha radiations (α): it consists of a stream of positively charged
particles which have an atomic mass of four and a charge of +2 (a
helium nucleus).
 Beta radiation (β): it is a stream of electrons.
 Gamma radiation (γ): it is a stream of high-energy photons. These
emissions results from an energy change within the atomic nucleus. This
radiation changes neither the atomic number nor the atomic mass.
A huge amount of energy can be obtained by transforming small amounts of
matter into energy. There are two methods:
 Nuclear fission. A heavy nucleus (uranium, plutonium, etc.) splits into
several smaller fragments. These fragments or fission products are about
equal to half the original mass. Two or three neutrons are also emitted.
This fact can made possible a chain reaction with an unprecedented
energy yield.
 Advantages: the fission process has high energy efficiency;
1kg of uranium produces as much energy as 2000 tons of oil.
 Disadvantages. It presents risk of radioactive contamination
and it is difficult to eliminate waste.

 Nuclear fusion: it joins two light elements (usually hydrogen) forming a
more massive element (helium) releasing a tremendous amount of
energy in the process. It has been used as bomb. Nowadays its peaceful
use is being researched.
 Advantages: a lot more energy than fission can be obtained.
There are large reserves of energy (hydrogen from ocean
water) and it almost does not produce waste or polluting
substances.
 Disadvantages: it is necessary an extremely high temperature
to start the nuclear reaction (lasers are used to reach this
temperatures). Magnetic fields are used to keep this matter in a
determine space. All these processes are being researched.

Applications of radioactivity and radioisotopes.
Radioactive isotopes, called radioisotopes, are not always harmful; they find
numerous uses in different areas such as medicine, chemistry, biology,
archaeology, agriculture, industry and engineering.
Determination of age of minerals and rocks and archaeological objects:
many minerals contain radioactive isotopes. The age of any of these minerals
can be determined by counting the isotopes in the mineral and using the known
decay rate to calculate the time required in that process. Example: radiocarbon
dating (14C).
Medicine: sterilization of surgical equipment, cancer treatment and study or
organs, forensic investigations, etc.
Industry: X rays to examine steel plates, welding and construction.

6

Scientific research and experiments: to study reaction mechanisms and
chemical manufacturing.
Other uses: auxiliary generator for satellites, precision clocks, sterilization of
agricultural pets, etc. resumelo