79
79 is the atomic number of gold. Its colour and its refusal to tarnish have the same cause: at that atomic number, relativistic effects shift gold's light absorption into the blue, so it reflects yellow — and holds its electrons too tightly to corrode.
- Odd
- Prime
Where 79 fits
Why 79 is special
79's real-world identity is entirely down to its element — gold is one of the very few metals with a naturally occurring colour other than grey or silver.
Same number, different meanings
How 79 is understood across different contexts — never implying one meaning because of another.
- Mathematics
79's real-world identity is entirely down to its element — gold is one of the very few metals with a naturally occurring colour other than grey or silver.
- History
Pliny the Younger described the 79 eruption in enough detail that volcanologists named the eruption type after him. A charcoal inscription found in 2018 points to October rather than the traditional 24 August.
- Technology & computing
CWE-79 is cross-site scripting: untrusted input reaching a page without neutralisation, so the browser executes it as part of the site. It has appeared at or near the top of the CWE Top 25 every year the list has been published.
- Science
Gold's colour is a direct consequence of its atomic number being as high as 79. In heavy atoms the innermost electrons move fast enough for relativistic effects to matter, contracting the 6s orbital and narrowing the gap to the 5d orbitals until it falls in the blue part of the spectrum. Gold absorbs blue and reflects the rest, which is why it looks yellow where lighter metals look grey.
79 is part of
Factors & digit properties
- Divisors (2)
- 1, 79
- Prime factorisation
- 79
- Digit sum
- 16
- Digital root
- 7
Representations
- Decimal
- 79
- Scientific notation
- 7.900 × 10^1
Roman numerals
L X X I X
LXXIX uses the subtractive pair IX: one before ten — ten minus one.
Binary
1 0 0 1 1 1 1
1 × 64 + 0 × 32 + 0 × 16 + 1 × 8 + 1 × 4 + 1 × 2 + 1 × 1 = 79
Octal
1 1 7
1 × 64 + 1 × 8 + 7 × 1 = 79
Hexadecimal
4 f
4 × 16 + f × 1 = 79
Read as powers of two: 64 + 8 + 4 + 2 + 1 = 79.
In historical numeral systems
Egyptian
7 hobbles, 9 strokes
7 × 10 + 9 × 1 = 79. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
1 vertical wedge (sixties place), 1 corner wedge + 9 vertical wedges (units place)
1 × 60 + 19 = 79. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
omicron, then theta, marked as a numeral
omicron-theta + keraia
omicron + theta written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
ayin, then tet
ayin-tet
ayin (70) + tet (9) = 79, written in descending order.
Chinese numerals
qī shí jiǔ
qī shí jiǔ
七 before 十 multiplies it: 7 tens. 九 after it adds 9. The written form mirrors the arithmetic — "qī shí jiǔ". The financial (anti-fraud) form is 柒拾玖.
Maya
3 dots (upper, twenties place) over 3 bars + 4 dots (lower, units place)
3 × 20 + 19 = 79. Places stack vertically with the smallest at the bottom, so the upper group is worth twenty times the lower.
Systems that simply cannot reach this value are left out. Where a script needs a font most readers do not have, the reading is given instead of the glyphs — see number systems for each system in full.
Other mathematical patterns
- Deficient number
- Happy number
- Lucky number (mathematical sieve)
Related numbers & connections
79 and 80 are consecutive integers.
View 80 →Excavation at Pompeii began in 1748, revealing a town preserved by the eruption that destroyed it in 79.
View 1748 →History
Pliny the Younger described the 79 eruption in enough detail that volcanologists named the eruption type after him. A charcoal inscription found in 2018 points to October rather than the traditional 24 August.
DocumentedHistoryGeography: ItalyPeriod: 79Gold's long use as currency follows from the same chemistry as its colour: element 79 is highly unreactive, so it does not tarnish or corrode and coins survive burial and circulation largely intact. Combined with scarcity and easy division, that durability made it a store of value across unconnected civilisations long before any of them understood why it behaved that way.
DocumentedHistory
Technology & computing
CWE-79 is cross-site scripting: untrusted input reaching a page without neutralisation, so the browser executes it as part of the site. It has appeared at or near the top of the CWE Top 25 every year the list has been published.
DocumentedTechnology & computingGeography: International — MITRE CWEPeriod: 2006–The same inertness puts element 79 in modern hardware: gold plating on connectors and bonding wires resists the oxide films that would otherwise raise contact resistance. Spacecraft use it differently again — thin gold coatings reflect infrared efficiently, which is why gold films appear on thermal blankets and on the mirrors of some space telescopes.
DocumentedTechnology & computing
Science
Gold's colour is a direct consequence of its atomic number being as high as 79. In heavy atoms the innermost electrons move fast enough for relativistic effects to matter, contracting the 6s orbital and narrowing the gap to the 5d orbitals until it falls in the blue part of the spectrum. Gold absorbs blue and reflects the rest, which is why it looks yellow where lighter metals look grey.
DocumentedScience79 is the atomic number of gold, one of the few metals with a naturally occurring colour other than grey or silver.
DocumentedScience
Mathematics
79 is prime — the 22nd prime number.
DocumentedMathematics
Common questions
Why is gold used in electronics?
Because it does not corrode. Gold is a worse conductor than copper or silver, but a copper contact grows an oxide film that resists current, and gold does not — so a thin gold plating keeps a connector or bonding wire reliable for decades. The quantity is tiny; the property is the point.
Is 79 a prime number?
Yes, 79 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 79?
The factors of 79 are 1, 79.
What is 79 in Roman numerals?
79 in Roman numerals is LXXIX. IX is subtractive: one before ten — ten minus one. Written additively it would be LXXVIIII, which Roman inscriptions used freely — the strict subtractive form is a later convention.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 7 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026