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G fact-ite
G eo fac t s an d ac t i v i t i es f o r t h e c l ass room
Gold is a metallic
element
Elements are the building blocks
of our planet and gold is one of 94
naturally occurring elements. It is
a metallic material with a distinct
yellow colour and metallic lustre.
Its Atomic Number is 79, its
Atomic Mass is 196.967
and its symbol is Au.
Gold is not an abundant element.
It is estimated gold atoms comprise only
0.3 parts per billion [ppb] of the Earth’s
crust. This translates to 3.1 ppb by weight
due to its density. However, sometimes
geological processes concentrate gold
into ore deposits where it may be
present in amounts above 5 parts
per million [ppm] by weight and
sometimes as solid nuggets.
Properties of gold
Measuring gold
Gold is highly valued for many
reasons but its physical properties
are essential to the functioning of
modern society. Gold is very soft
[2.5 - 3.0 on the mohs hardness
scale] and is the most malleable and
ductile of metals. This means it can
be beaten into very thin sheets and
drawn out into very thin wires. Gold
does not readily form compounds
and does not oxidise or tarnish.
Only a mixture of hydrochloric acid
and nitric acid [called ‘aqua regia’],
some cyanide solutions and mercury
will dissolve gold under surface
temperatures and pressures.
Gold has a long history of use in
society so it is no surprise that
measurements of gold involve old
and somewhat obscure units. Gold
is always traded in Troy ounces.
Take note: a Troy ounce is not the
same as other ounces.
1 Troy Ounce = 31.1 grams
Because gold is so soft it is often alloyed
with other metals in order to improve
its durability. Commonly it is mixed with
copper, platinum or nickel. Gold alloys are
rated using the 24 point carat system.
24 carat gold is pure (100% Au w/w),
18-carat gold is 75% gold by weight and
12-carat gold is 50% gold by weight. The
carat system is also applied to Platinum.
All metals conduct heat and electricity;
gold is no exception. In fact it is a
better conductor than most metals.
Silver is a better electrical conductor
but gold is the metal of choice in missioncritical situations due to its ability to also
resist corrosion.
Just be careful though. This system
refers to gold content by weight.
Sometimes people assume a given purity
to mean the amount by volume. With gold
being such a dense material 50% gold
by weight does not mean 50% gold by
volume!
The melting point of gold is ~1064oC
and it boils to a vapour at 2856oC.
Bullion dealers sometimes quote gold
purity using ‘fineness’. 4 nines fineness
or 9999 fineness is simply another way
of saying 99.99% gold! 4 nines gold is
effectively 24 carat gold.
Gold is a very dense material. It has
a specific gravity or density of 19.3.
This means 1cm3 weighs 19.3 grams
while 1cm3 of water only weighs 1 gram!
Acknowledgement: Greg McNamara, Education and Outreach, Geological Society of Australia
http://www.gsa.org.au
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G eo fac t s an d ac t i v i t i es f o r t h e c l ass room
Golden numbers
1. The Atomic Number of gold is 79. The mass
number of the only stable isotope is 197.
How many protons, neutrons and electrons
does a stable gold atom have?
Explain your answer.
2. Gold has 4 synthetic isotopes. Au-195,
Au-196, Au-198 and Au-199.
How many protons, neutrons and electrons
does each synthetic gold isotope have?
Explain your answer.
3. Gold is a rare element but it is sometimes
concentrated into ore deposits by geological
processes. Modern mining and metallurgical
techniques have allowed ores with as little as
2 grams per tonne of gold to be mined profitably.
Is an ore body containing 5ppm gold potentially
able to be mined at a profit?
Explain your answer.
4. An exploration company discovers two ore bodies that
could both be mined by open-cut methods. OB1 has
an average gold concentration of 7 grams per tonne
and an estimated ore-rock reserve of 75 million tonnes.
OB2 has an average gold concentration of 2.5 grams
per tonne and estimated ore-rock reserve of 200 million
tonnes. OB1 has 100 million tonnes of overburden that
will cost 10% of the gold reserve to remove. OB2 has
45 million tonnes of overburden that will cost 5% of the
gold reserve to remove.
The company only has a permit to open one mine.
Which ore body would be the most profitable to mine?
Explain your reasoning.
5. Gold is very ductile. Some claim that a troy ounce
of gold can be drawn out into a wire 60km long.
Assuming the wire has a constant circular crosssection what will its diameter be?
Show your working.
6. Gold is very malleable. Gold can be beaten into a
thin sheet and used to guild the surface of objects,
even food. It is said that one troy ounce of gold can
be beaten into a sheet that covers 9m2.
Assuming the sheet is of even thickness and exactly
3m square what will the thickness be?
Show your working.
7. A gold prospector walks into a precious metal dealer
in Perth with a bag full of her gold pannings. On
average the pannings are 95% gold by weight. The
buying price for gold on the day is US$605 per
troy ounce. The conversion rate between US$ and
Australian$ is 0.78. The prospector left the dealers
office with a cheque for A$3560.00.
Assuming there were no other fees or charges how
many grams of gold did the prospector have in her bag?
Show your working.
8. A lot of jewellery is made of 9 carat gold.
What percentage of gold by weight is in 9 carat gold?
Nickel has a density of 8.9
What percentage of gold by volume is in a 100g 9 carat
gold bracelet when the gold is alloyed with nickel?
Show your working.
9. Will a 9 carat gold ring containing copper look any
different to a 9 carat gold ring containing nickel?
Explain your answer.
Acknowledgement: Greg McNamara, Education and Outreach, Geological Society of Australia
http://www.gsa.org.au