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Topic 2
The Microscopic World
Unit 5 Atomic structure
Teaching Notes
5.2
Symbols for elements
J. J. Berzelius, a Swedish chemist, is generally given credit for creating modern symbols for elements.
Berzelius proposed that all elements should be given a symbol corresponding to the first letter of their
names. In case that two elements begin with the same letter, the second letter or an important letter in
the name is added. In some cases, the Latin name of the element is used.
5.3
States of elements
The three states of matter
Everything in the world is made up of matter. The Particle Theory of Matter states that all matter is made
up of very small, moving particles.
Everything in the world is either a solid, a liquid or a gas. Solid, liquid and gas are known as the
three states of matter. Solids are hard and difficult to break. Liquids are runny and take the shape of their
containers. Gases can be compressed easily. Solids, liquids and gases have different properties. These
differences can be explained by the Particle Theory of Matter.
The differences between the three states are due to the ways their particles are arranged.
Solid state The particles in a solid are packed tightly in a fixed pattern. There are strong forces
holding them together. They can only vibrate about fixed positions.
Liquid state The particles in a liquid have more freedom of movement than in a solid. They are
still close together but are not in a fixed pattern. The forces that hold them together are weaker than in a
solid.
Gaseous state The particles in a gas are widely spread. They move about very quickly. There are
almost no forces holding them together.
5.4
How to classify elements?
About three fourths of the elements are metals. All the metals except mercury are solids at room
temperature and pressure. Mercury is a shiny, silver-coloured liquid at room temperature and pressure.
Note that if a high voltage is used, substances which are normally classified as non-conductors (for
example, air) may conduct electricity.
5.5
Basic structure of an atom
The atomic theory
In the early 1800s, an English schoolmaster, John Dalton, put forward an atomic theory to explain the
structure of matter and certain aspects of chemical reactions. John Dalton’s atomic theory is summarized
as follows:

All matter consists of tiny particles called atoms.

Atoms cannot be created, destroyed, subdivided or changed from one type to another.

All the atoms of a particular element have the same properties.

Atoms of different elements have different properties.

Atoms combine in simple whole number ratio to form a compound.

A chemical change involves the joining, separation or rearrangement of atoms.
Discovery of electrons and protons
Dalton’s atomic theory was later proved wrong by an English scientist J.J. Thomson in 1897.
Thomson passed high voltage electricity through a gas in a tube at low pressure. He found that a
stream of rays, called cathode rays, moved from the negative electrode to the positive electrode. The
cathode rays produced a green glow on the fluorescent screen. Besides, the cathode rays were also
deflected strongly towards the positive plate.
Thomson then drew the following conclusions:

These rays are composed of particles.

The particles carry negative charges since they are deflected towards the positive plate.

The particles are relatively light since they show a large deflection.

Since all electrode materials (e.g. iron, silver, etc.) produce rays with identical characteristics, all of
these materials must have this kind of particles within them.
Thomson called these particles electrons. He also concluded that all matter was composed of
atoms and had electrons. All atoms consist of electrons.
In his experiments with cathode rays, Thomson had noticed a red glow near the negative electrode.
To investigate this red glow, Thomson used a discharge tube with a central negative electrode which had
a hole in it.
This time he saw a red fluorescence in the tube as well as a green fluorescence. The green fluorescence
was caused by electrons. The red glow was caused by rays which were deflected by magnetic and
electric fields in the direction opposite to electrons. Thomson called these rays positive rays.
The mass of the particles in positive rays depended on the nature of the gas in the tube. For
example, when the gas in the tube was hydrogen, the mass of the positive particles was almost identical
to that of a hydrogen atom. If the gas in the tube was oxygen, the positive rays had a mass almost
identical to that of oxygen atoms. Thomson concluded that positive rays were produced when
fast-moving electrons dislodged electrons from atoms of the gas.
The lightest positive ions are produced when the gas in the tube is hydrogen. This unit of positive
charge is called a proton.
Discovery of neutrons
An interesting complication was revealed by Frederick Soddy when he demonstrated that the lead that
resulted from the radioactive decay of uranium-238 had an atomic mass of 206, while that from decay of
thorium-232 had a mass of 208. He differentiated between these two types of lead by calling them
different isotopes. This discovery meant that not all the subatomic particles had been discovered, since
there had to be a reason for the existence of different types of lead.
In 1932, James Chadwick bombarded a sample of beryllium with alpha particles (nuclei from helium
atoms), and observed a beam of rays being given off. Experiments with electric and magnetic fields
showed that the particle was electrically neutral. Its mass was shown to be similar to that of a proton.
The different masses of the isotopes could therefore be explained by proposing the presence of different
numbers of neutrons in the nuclei.
Early atomic models
Dalton saw atoms as being like billiard balls. When Thomson discovered electrons, he simply
incorporated them into the Dalton’s model, proposing that atoms were spheres that had these small,
negatively charged particles distributed throughout them, rather like plums in a pudding. Since he knew
that atoms were electrically neutral, he proposed that the rest of the sphere was positively charged.
Thomson’s model of the atom:
Between 1906 and 1908, an English scientist Ernest Rutherford devised experiments to test the
‘plum pudding’ model of the atom. He bombarded a thin sheet of gold foil with positively charged
particles called alpha particles. When an alpha particle hit the fluorescent screen, light could be seen
through the microscope. Most of the particles went straight through the foil. Some of the particles were
deflected by the foil. Others were bounced back as if they had struck something positively charged with a
large mass.
Rutherford suggested that the positively charged particles, called protons, should not be distributed
throughout the atom. They were concentrated in a small area at the centre of the atom called the
nucleus.
Thus, according to Rutherford, an atom consists of:
1
a positively charged nucleus containing positively charged protons located at the centre of the atom;
2
an equal number of negatively charged electrons moving around the nucleus at high speed.
Rutherford’s model of the atom:
The Bohr’s model of the atom
Rutherford’s nuclear model of the atom was a major advance but it had several flaws. The application of
the laws of classical, Newtonian physics would require that electrons moving around a nucleus should
give off energy (as radiation). This should lead to the electrons slowly down and being drawn towards the
nucleus by electrostatic attraction. This would result in the collapse of all matter, which was clearly not
the case. It had been known for some time that each element would absorb and emit particular energies
of light.
The Danish physicist, Niels Bohr, deduced that classical physics could not be used to explain the
motion of electrons in atoms. He said further that the electrons circled the nucleus without loss of energy,
and that they could only move in circular orbits of certain energies. This past point was called
quantisation and could be used to explain the characteristic emission lines from elements.
According to the Bohr’s model of the atom, the number of electrons in the nth orbit is equal to 2n2.
However, the presence of extra electrons in larger atoms made it too complicated for such a simple
approach.
Bohr’s model of the atom:
The Schrödinger’s model of the atom
Albert Einstein proposed that light could be considered both as waves and a stream of particles. In 1926,
Erwin Schrödinger’s proposed that electrons behave as waves around the nucleus. This means that the
electron is smudged out as a complex three-dimensional wave all round the nucleus, rather than being at
any one point.
This ’wave-particle duality’ means that we can only talk of the probability of finding an electron
within a certain region of space. The greater the probability of finding an electron at a particular point, the
denser is the negative charge cloud.
Schrödinger’s model of the atom:
The region of space to which the electrons are restricted are called atomic orbitals, each of which
can hold up to two electrons. Orbitals that have the same energy combine to make subshells and
subshells of roughly similar radii combine further to give shells of electrons. These orbitals are labelled
using quantum numbers, which define the shell and subshell that the orbital belongs to.
5.6
Atomic number
Remind students that they need to memorize the number of protons and electrons, but not the number of
neutrons, in atoms of the 20 simplest elements.
5.8
Isotopes
Chemistry and Society
Medical uses of radioisotopes
In 1896, a French scientist named Henri Becquerel discovered that uranium compounds spontaneously
give off radiation that can pass through paper wrapping and produce an image on film. Uranium is said to
be radioactive.
Elements are radioactive if they have unstable nuclei. As the nucleus breaks apart or decays,
particles (such as alpha and beta particles) or rays (such as gamma rays) are emitted. There are both
natural and synthetic radioactive elements.
For some elements, all isotopes are radioactive. For other elements, only one or some isotopes are
radioactive. Therefore we talk about radioisotopes rather than radioactive elements. The release of
radiation by radioisotopes is called decay.
Radioisotopes have become extremely useful tools in medical diagnosis and treatment. Using them
allows doctors to detect many diseases early and to treat them more successfully than ever before.
Technetium-99m
Technetium?9m is a radioisotope commonly used in medicine for diagnosis. The letter m stands for
metastable, meaning that the isotope gives off radiation and becomes a more stable form of the element.
Technetium-99m container
Technetium-99m is commonly used for imaging bones or blood-flow patterns in the heart. Since
technetium-99m has a half-life of 6.02 hours, it allows a physician to inject it into the body of a patient
and to follow its path for a substantial period of time. Moreover, its half-life is short enough that it does
not remain in the body for a long time.
Technetium-99m used for
imaging bones or heart
Question
The following table shows some other radioisotopes used in medicine.
Radioisotope
Use(s)
Cobalt-60
Iodine-131
Sodium-24
Thallium-201
Consult a nurse or physician about the uses of these radioisotopes. Some chemistry reference
books in the library may help you. You may also go to the following websites to search for the information
you need.
http://www.epa.gov/radiation/radionuclides/cobalt.htm
http://www.epa.gov/radiation/radionuclides/iodine.htm
http://www.ausetute.com.au/nuclesum.html
http://www.nsc.org/issues/rad/isotopes.htm
Notes for question
Radioisotope
Use(s)
Cobalt-60
Cobalt-60 decays with the emission of beta particles and gamma rays. The
gamma rays can destroy cancerous tissues.
Iodine-131
Iodine-131 concentrates in the thyroid gland, the liver and certain parts of the
brain. It is used to monitor goitre and other thyroid problems, as well as liver
and brain tumours. It is an effective tracer in locating tumours because these
abnormal growths use up large amounts of albumin, which can be iodinated
and followed with a radiation detector.
5.9
Sodium-24
Salt solutions containing Na-24 can be injected into the bloodstream to follow
the blood and locate obstructions in the circulatory system.
Thallium-201
Thallium-201 tends to concentrate in healthy heart tissue while technetium-99
concentrates in abnormal heart tissue. The two can be used in a
complementary fashion in surveying the extent of damage from heart
disease.
Relative masses of atoms and the carbon-12 scale
Remind students that they do not need to memorize the relative atomic masses of elements. These will
be given if necessary.
5.10
The arrangement of electrons in atoms
How to draw the electron diagram?
Notice that the electrons (except those in the first shell) are in pairs for easy drawing and counting.
Remind students that they need to memorize the electronic arrangements of the atoms of the first 20
simplest elements.