THE STRUCTURE OF THE ATOMSC
IEN
CE
PH
OT
O^ L
IBR
AR
Y
Five key scientific
terms that will help
you understand
atomsALPHA PARTICLE
A positively charged object
emitted in a form of
radioactivity. Originally thought
to be a simple particle, hence
the name, today we know that
it consis t s of t wo protons and
two neutrons tightly bound
together. Some heavy nuclei are
unstable and spontaneously
emit these clumps in what is
known as alpha radioactivity.SCINTILLATOR
When some materials are
s truck by an incoming par ticle,
the energy that is absorbed is
then emitted as light. A screen
coated with zinc sulphide emits
faint flashes, visible in a
darkened room, when hit by
alpha particles. Early in the
20th century, Ernest
Rutherford detected alpha
particles this way, revealing
the atomic nucleus.ELEMENT
All substances are made from
combinations of chemical
elements, which consist of
atoms. Examples of elements
are hydrogen, carbon
and oxygen.PHOTON
In quantum theory, light waves
ac t a s if composed of a series of
individual particles, called
photons. A photon is therefore
a par ticle of light with no ma ss.RADIOACTIVITY
Atoms of one atomic element
may transform spontaneously
into another by emitting
particles, a process known
as transmutation.energy to one that is lower down, the
excess energy is carried away by a
photon of light. Conversely, if a n atom
is hit by a photon whose energy
matches the gap between two rungs,
the atom absorbs that photon, lifting
the electron up the ladder.Light fantastic
This absorption effect beca me obvious
when sunlight was examined. Like all
stars, the Sun emits electromagnetic
radiation across the entire spectrum.
It also has a lot of gas in its outer
atmosphere, containing a smorgasbord
of elements. In sunlight, the photons
wit h energies t hat happen to match
the gaps between rungs in the atomic
ladders are absorbed by the atoms of
these elements and never reach Earth.
By viewing starlight through a
diffraction grating (a piece of glass
scratched with close-packed grooves),
you split light into its component
colours. These ‘missing’ photons
show up as da rk lines.Quantum theory goes further in
explaining where electrons can be
around a nucleus. Any particle can
take on a wave-like character. What is
familiar for electromagnetic waves
occurs for electrons too. Imagine the
waves for electrons in atoms as if they
were wobbles on a length of rope.
When coiled like a lasso, t he number
of wavelengths in the circuit has to
fit perfectly into its circumference.
Imagine this circle like a clock face.
If t he wave pea ks at 12 o’clock, wit h
a dip at 6 o’clock, the next peak will
occur perfectly at 12: the wave ‘fits’
into the circle. However, a peak at 12
followed by a dip at 5 o’clock would
have its next peak at 10 and be out of
time with the beat of the wave: the
wave will not f it. So elect rons
circulating in atoms can only go on
paths where their waves fit perfectly
on the lasso. A single wave
corresponds to the lowest rung of the
energy ladder; two waves puts the
electron on the second rung and so on.
The energies of the
various waves are unique
to atoms of a given element.
The spectral lines that
result when electrons jump
f rom one r ung to a not her
are thus like a barcode,
identifying the elements
present in the Sun and
other stars. It also explains
the periodic regularity in
chemical behaviour
noticed by Mendeleev. So
although we can’t directly
‘see’ the electron waves
within atoms, this
hypothesis describes a
host of historical
phenomena and has led to
a wealth of technological
applications. We can
therefore claim to ‘know’
a great deal about the
inner structure of the
atom, even t hough it is a
world beyond Lilliput.NEED TO KNOW
“An elemental atom can occur with
different numbers of neutrons. Such
alternatives are known as isotopes”
by F R A N K C L OS E
Frank is Emeritus Professor of
physics at the University of Oxford.