A gold nugget of 5 millimetres (0.20 in) in diameter (bottom) can be expanded through hammering into a
of about 0.5 square metres (5.4 sq ft).
Gold is the most
malleable of all metals; a single gram can be beaten into a sheet of 1 square meter, and an
avoirdupois ounce into 300 square feet. Gold leaf can be beaten thin enough to become semi-transparent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.
 Such semi-transparent sheets also strongly reflect
infrared light, making them useful as infrared (radiant heat) shields in visors of heat-resistant suits, and in sun-visors for
 Gold is a good
conductor of heat and
Gold has a density of 19.3 g/cm3, almost identical to that of
tungsten at 19.25 g/cm3; as such, tungsten has been used in
gold bars, such as by plating a tungsten bar with gold,
 or taking an existing gold bar, drilling holes, and replacing the removed gold with tungsten rods.
 By comparison, the density of
lead is 11.34 g/cm3, and that of the densest element,
osmium, is 22.588 ± 0.015 g/cm3.
Different colors of
Whereas most metals are gray or silvery white, gold is slightly reddish-yellow.
 This color is determined by the frequency of
plasma oscillations among the metal's valence electrons, in the ultraviolet range for most metals but in the visible range for gold due to
relativistic effects affecting the
orbitals around gold atoms.
 Similar effects impart a golden hue to metallic
Common colored gold alloys include the distinctive eighteen-karat
rose gold created by the addition of copper. Alloys containing palladium or nickel are also important in commercial jewelry as these produce white gold alloys. Fourteen-karat gold-copper alloy is nearly identical in color to certain
bronze alloys, and both may be used to produce police and other
badges. White gold alloys can be made with
nickel. Fourteen- and eighteen-karat gold alloys with silver alone appear greenish-yellow and are referred to as
green gold. Blue gold can be made by alloying with
iron, and purple gold can be made by alloying with
aluminium. Less commonly, addition of
indium and other elements can produce more unusual colors of gold for various applications.
Colloidal gold, used by electron-microscopists, is red if the particles are small; larger particles of colloidal gold are blue.
Gold has only one stable
Au, which is also its only naturally occurring isotope, so gold is both a
monoisotopic element. Thirty-six
radioisotopes have been synthesized ranging in
atomic mass from 169 to 205. The most stable of these is 195
Au with a
half-life of 186.1 days. The least stable is 171
Au, which decays by
proton emission with a half-life of 30 µs. Most of gold's radioisotopes with atomic masses below 197 decay by some combination of
α decay, and
β+ decay. The exceptions are 195
Au, which decays by electron capture, and 196
Au, which decays most often by electron capture (93%) with a minor
β− decay path (7%).
 All of gold's radioisotopes with atomic masses above 197 decay by β− decay.
At least 32
nuclear isomers have also been characterized, ranging in atomic mass from 170 to 200. Within that range, only 178
Au, and 188
Au do not have isomers. Gold's most stable isomer is 198m2
Au with a half-life of 2.27 days. Gold's least stable isomer is 177m2
Au with a half-life of only 7 ns. 184m1
Au has three decay paths: β+ decay,
isomeric transition, and alpha decay. No other isomer or isotope of gold has three decay paths.
The production of gold from a more common element, such as
lead, has long been a subject of human inquiry, and the ancient and medieval discipline of
alchemy often focused on it; however, the transmutation of the chemical elements did not become possible until the understanding of nuclear physics in the 20th century. The first synthesis of gold was conducted by Japanese physicist Hantaro Nagaoka, who synthesized gold from
mercury in 1924 by neutron bombardment.
 An American team, working without knowledge of Nagaoka's prior study, conducted the same experiment in 1941, achieving the same result and showing that the
isotopes of gold produced by it were all
Gold can currently be manufactured in a nuclear reactor by
irradiation either of
platinum or mercury.
Only the mercury isotope 196Hg, which occurs with a frequency of 0.15% in natural mercury, can be converted to gold by
neutron capture, and following
electron capture-decay into 197Au with
slow neutrons. Other mercury isotopes are converted when irradiated with slow neutrons into one another, or formed mercury isotopes which
beta decay into
fast neutrons, the mercury isotope 198Hg, which composes 9.97% of natural mercury, can be converted by splitting off a neutron and becoming 197Hg, which then disintegrates to stable gold. This reaction, however, possesses a smaller activation cross-section and is feasible only with un-moderated reactors.
It is also possible to eject several neutrons with very high energy into the other mercury isotopes in order to form 197Hg. However such high-energy neutrons can be produced only by