Yo 2017 JC1 student Lockheed2000.
This is an interesting question that was asked by Cambridge in a
TYS qn, yet a proper understanding of how the lamp works actually
goes beyond the H2 syllabus.
For the simplified version that will suffice for the H2 syllabus
(which being oversimplified for A level purposes, is unfortunately
inaccurate and misleading in several ways) :
Neon is already gaseous, while sodium is solid, under standard
conditions. Hence upon switching on the lamp, the neon gas is
ionized first, producing a red glow, while it takes some time for
the sodium metal to be vaporized then ionized, then producing the
yellow-orange flow of sodium. Na(g) having a less endothermic 1st
ionization energy compared to Ne(g), ionizes more readily, and
hence being present in a larger quantity, its yellow-orange glow
thus predominates.
(Beyond H2 syllabus, for those interested)
But the above simplified A level H2 version is misleading in
several ways. Firstly, the ionization itself doesn't produce the
glow. The glow occurs when the atoms (whether neon or sodium)
transit from a higher energy state, to a lower energy state, in the
process emitting electromagnetic radiation or photons of a specific
frequency corresponding on that element's emission spectrum (that
sodium has a brighter glow than neon, is actually due to their
different emission spectrums, rather than the misleading A level
explanation given above). This actually happens after, and is
separate from, the ionization process.
When the lamp is turned on, the electricity applied ionizes and
oxidizes the Ne(g) at the anode, and the Ne+(g) cations are
accelerated by the applied electric field toward the cathode, in
the process colliding with Ne(g) atoms, which transfer their
electrons to the Ne+(g) cations. The Ne(g) atoms (having lost an
electron during the collisions), ionize into Ne+(g) and accelerate
toward the cathode, while the Ne+(g) cations (having gained an
electron during the collisions), become reduced to Ne(g) *and*
return to a lower energy state, emitting a photon of light of
neon's characteristic frequency (ie. red glow) corresponding to
neon's emission spectrum.
The above process generates the heat required to first vaporize
the Na(s) to Na(g), and then ionize the Na(g) to Na+(g) at the
anode, and thereafter, the same collision process described above
with Ne(g) and Ne+(g), then takes place with the Na(g) and Na+(g),
resulting in the emitting of photons of light of sodium's
characteristic frequency (ie. bright yellow-orange glow)
corresponding to sodium's emission spectrum. In practice,
inter-elemental collisions between the neon and sodium atoms and
ions do occur as well.
Advice to JC students : always first understand the simplified A
level concepts first (A levels is a transition bridge between O
level child's play, and the real Uni level science), and then only
if you're interested and able to handle it, delve into the more
correct Uni level details.
Dear UltimaOnline,
I am a J1 this year, and I've just started my first Chem
tutorial on Atomic Structure. However I'm stuck on this question.
Can you help me with it?
Give a brief explanation of the following observation in terms
of ionisation energy.
Orange street-lamps contain sodium with a small amount of neon,
and light is produced when gaseous atoms are ionised in an electric
field. It is observed that when the street-lamps are turned on, a
red glow characteristic of neon is first emitted and the orange
glow of sodium predominates after a time.
