Scientific level
CHEMISTRY | UNIVERSE ATOMS
THE CLIP
Atoms and molecules found in celestial bodies, such as stars, are sources of light and can be picked up and
analysed with the help of a spectrograph. The light emitted from a particular atom or molecule is divided into a
unique sequence of thin lines. Scientists taking two samples of a molecule, such as water, one from space and
the other from Earth, will find out that they have exactly the same sequence and therefore are exactly the same
thing. Hence we can conclude that everything found in the Universe, including Earth and humans, is made up of
the same elements.
THIS DEMONSTRATES ....
That all matter found in the Universe is made up from the same elements.
ATOMS AND ATOMIC STRUCTURE
An atom is the smallest possible particle of an element that retains its chemical properties. Most atoms are
composed of three types of subatomic particles: –

Electrons – which have a negative charge,

Protons – which have a positive charge, and,

Neutrons – which are neutral.
A Helium Atom Model
These particles effect an atom’s external properties. All atoms differ in the number of each of the subatomic
particles they contain. The number of protons in an atom (called the atomic number) determines the element of
the atom. Within a single element, the number of neutrons may also vary, determining the isotope of an element.
Atoms are electrically neutral if they have an equal number of protons and electrons. Electrons that are furthest
from the nucleus may be transferred to other nearby atoms or even shared between atoms. Atoms which have
either a deficit or a surplus of electrons are called ions.
Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. Atoms are
able to bond into molecules and other types of chemical compounds. Molecules are made up of multiple atoms;
for example, a molecule of water is a combination of two hydrogen atoms and one oxygen atom.
The periodic table is a tabular method of displaying the chemical elements.
ATOMIC SPECTRUM
Since each element in the periodic table consists of an atom in a unique configuration with different numbers of
protons and electrons, each element can also be uniquely described by the energies of its atomic orbitals and
the number of electrons within them. Normally, an atom is found in its lowest-energy ground state; states with
higher energy are called excited states. An electron may move from a lower-energy orbital to a higher-energy
orbital by absorbing a photon with energy equal to the difference between the energies of the two levels. An
electron in a higher-energy orbital may drop to a lower-energy orbital by emitting a photon. Since each element
has a unique set of energy levels, each creates its own light pattern unique to itself: its own spectral signature.
ATOMIC SPECTROSCOPY
Atomic spectroscopy is the determination of elemental composition by its electromagnetic or mass spectrum.
Atomic spectroscopy is closely related to other forms of spectroscopy. It can be divided by atomization source or
by the type of spectroscopy used. In the latter case, the main division is between optical and mass spectrometry.
Mass spectrometry generally gives significantly better analytical performance, but is also significantly more
complex. This complexity translates into higher purchase costs, higher operational costs, more operator training,
and a greater number of components that can potentially fail. Because optical spectroscopy is generally less
expensive and has a performance adequate for many tasks, it is far more common.
OPTICAL SPECTROMETRY
Electrons exist in energy levels within an atom. These levels have well defined energies and electrons moving
between them must absorb or emit an energy equal to the difference between them. In optical spectroscopy, the
energy absorbed to move an electron to a more energetic level and/or the energy emitted as the electron moves
to a less energetic energy level is in the form of a photon (a particle of light). Because this energy is well-defined,
an atom's identity (i.e. what element it is) can be determined by the energy of this transition. The wavelength of
light can be related to its energy. It is usually easier to measure the wavelength of light than to directly measure
its energy.
MASS SPECTROMETER – HOW IT WORKS
The Basic Principle
If something is moving and you subject it to a sideways force, instead of moving in a straight line, it will move in a
curve - deflected out of its original path by the sideways force.
Suppose you had a cannonball traveling past you and you wanted to deflect it as it went by you. All you've got is
a jet of water from a hose-pipe that you can squirt at it. Frankly, it is not going to make a lot of difference!
Because the cannonball is so heavy, it will hardly be deflected at all from its original course.
But suppose instead, you tried to deflect a table tennis ball travelling at the same speed as the cannonball using
the same jet of water. Because this ball is so light, you will get a huge deflection.
The amount of deflection you will get for a given sideways force depends on the mass of the ball. If you knew the
speed of the ball and the size of the force, you could calculate the mass of the ball if you knew what sort of
curved path it was deflected through. The less the deflection, the heavier the ball. This principle can be applied
to atomic sized particles.
An outline of what happens in a mass spectrometer
Atoms can be deflected by magnetic fields - provided the atom is first turned into an ion. Electrically charged
particles are affected by a magnetic field although electrically neutral ones aren't. The sequence is:
Stage 1: Ionisation
The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things
which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon,
for example). Mass spectrometers always work with positive ions.
Stage 2: Acceleration
The ions are accelerated so that they all have the same kinetic energy.
Stage 3: Deflection
The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they
are deflected.
The amount of deflection also depends on the number of positive charges on the ion - in other words, on how
many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.
Stage 4: Detection
The beam of ions passing through the machine is detected electrically.
A Full Diagram of a Mass Spectrometer
ATOMS AND THE BIG BANG
In spectroscopic analysis, scientists can use a spectrometer to study the atoms in stars and other distant
objects. Due to the distinctive spectral lines that each element produces, they are able to tell the chemical
composition of distant planets, stars and nebulae.
In models of the Big Bang, one theory predicts that within one to three minutes of the Big Bang almost all atomic
material in the universe was created. During this process, nuclei of hydrogen and helium formed abundantly, but
almost no elements heavier than lithium. Hydrogen makes up approximately 75% of the atoms in the universe;
helium makes up 24%; and all other elements make up just 1%. Since all the atoms and therefore all elements
were created at the same time that the Universe was created during the Big Bang, all matter, including the Earth,
other planets and us, is made up of the same chemical composition.
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References:
Wikimedia Foundation 2006, Wikipedia - The Free Encyclopedia [Online],
wikipedia.org/wiki/Main_Page, 2 Mar, 2006.
The Mass Spectrometer – How it Works [Online],
http://www.chemguide.co.uk/, 2 Mar, 2006.
Periodic Table of the Elements [Online],
http://www.gettysburg.edu/academics/physics/, 2 Mar, 2006.